{"id":1170,"date":"2026-04-24T15:28:42","date_gmt":"2026-04-24T15:28:42","guid":{"rendered":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/?post_type=back-matter&p=1170"},"modified":"2026-05-01T12:46:50","modified_gmt":"2026-05-01T12:46:50","slug":"alt-text-long-descriptions","status":"web-only","type":"back-matter","link":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/back-matter\/alt-text-long-descriptions\/","title":{"raw":"Alt Text Long Descriptions","rendered":"Alt Text Long Descriptions"},"content":{"raw":"

Figure 2.3<\/a>: A detailed anatomical diagram of a kangaroo illustrating directional terminology and anatomical planes. The kangaroo is shown in profile, facing left, with three intersecting geometric planes. The sagittal plane is the vertical plane that divides the body into left and right halves. Dorsal plane is the horizontal plane that divides the body into upper (proximal) and lower (distal) planes. Transverse plane is the vertical plane perpendicular to the long axis, dividing the body into front (cranial) and back (caudal) sections. The kangaroo is labeled as follows: head region includes the rostral (toward the nose) and caudal (toward the back of the head). Trunk and tail region includes the dorsal (back\/top), ventral (belly\/bottom), cranial (toward the head), and caudal (toward the tail). The limbs regions include palmar (the \"palm\" side of the front paw) and plantar (the \"sole\" side of the hind foot), cranial (the front of the legs), caudal (back of the legs).<\/p>\r\n

Figure 2.4: <\/a>Two male figures, the figure on the left is in profile, labeled the \"lateral view.\" There is a horizontal bidirectional arrow across the chest labeled \"posterior or dorsal\" on the left side of the arrow, and \"anterior or ventral\" on the right end of the arrow. The vertical bidirectional arrow runs from the ear to hip and is labeled at the top, \"cranial,\" and the bottom, \"caudal.\" The right figure is head-on and labeled the \"anterior view.\" There is a vertical bidirectional arrow from the shoulder labeled \"proximal\" to the wrist, labeled \"distal.\" There is a second bidirectional arrow from the upper thigh, labeled \"proximal\" down to the ankle, labeled \"distal.\" There is a horizontal bidirectional arrow from the left side of the elbow, labeled \"medial\" to the right side, labeled \"lateral.\" Between the figures is a vertical bidirectional arrow labeled \"superior\" at the top and \"inferior\" at the bottom.<\/p>\r\n

Figure 2.6:<\/a> Side by side diagram of a tree, labeled (A) and (B) of vertebrates indicating branches, nodes, roots, and branch tip. The left side tree designates \"root\" as the base horizontal line, \"node\" as the branching point where the lines split into two, specifically Node Y from the fish ancestry and Node X from the mammal, reptiles, and birds\u2019 ancestry. \"Branch\" is the horizontal and vertical lines connecting nodes or nodes to tips. \"Branch tip\" is the terminal end of a branch. From top to bottom, the species are illustrated as shark, bony fish, sheep, lizard, tyrannosaurus rex, and chicken. The diagram on the right is the same set of animals but rearranges the order of the branch tips. The shark, bony fish, and frog remain in the same top positions. The lower branches are rotated: the sheep is now at the bottom, while the T. rex, chicken, and lizard are clustered in the middle.<\/p>\r\n

Figure 2.7:<\/a> Two panels of protostomes. In the top panel, the left begins with a small cluster of cells labeled \"eight-cell stage\" and \"spiral cleavage.\" An arrow points to the next stage, \"gastrulation\" with a larger round cell. The middle is labeled \"archenteron,\" the small blue ovals near the bottom are \"coelum\" and \"mesoderm.\" The divot at the bottom is labeled \"blastopore.\" An arrow points to the last stage, \"protostomes\" with a larger oval shape with two arcs running vertically on each side. The inside of the arcs are labeled \"coelum,\" the outline of the arcs are \"mesoderm.\" The top opening is labeled \"anus,\" and the bottom is labeled \"mouth.\" The label \"blastopore\" from stage two also has a small arrow to the third stage \"mouth\" label.\" The bottom panel begins with a cluster of cells labeled \"radial cleavage.\" An arrow points to the next stage with a larger round shape with the blue ovals now opened and at the top, labeled \"mesoderm\" and \"coelum.\" The divot at the bottom is labeled \"blastopore.\" An arrow points to the third stage, labeled \"deuterostomes\" with the top opening labeled \"mouth\" and the bottom labeled \"anus.\" The middle is labeled \"digestive tube.\" The oval shape has two vertical arcs along the side edges with the inside labeled \"coleum,\" and the outline of the arc labeled \"mesoderm.\" The \"blastopore\" label from stage two has a small black arrow pointing to the third stage label, \"anus.\"<\/p>\r\n

Figure 2.16:<\/a> The illustration at the top shows the tunicate larval stage, which resembles a tadpole, with a post-anal tail at the narrow end. A dorsal hollow nerve cord runs along the upper back, and a notochord runs beneath the nerve cord. The digestive tract starts with the mouth at the front of the animal connected to a stomach. Above the stomach is the anus. The pharyngeal slits, which are located between the stomach and mouth, are connected to an atrial opening at the top of the body. The bottom illustration shows an adult tunicate, which resembles a tree stump anchored to the bottom. The heart, stomach, and gonad are tucked beneath the pharyngeal slits. The top opening is the mouth, surrounded by the branchial siphon. Around the entire diagram is the tunic. The large balloon-like structure in the center is the pharyngeal basket, with small lines throughout, as pharyngeal slits. At the bottom right opening is the anus, surrounded by the atrial siphon.<\/p>\r\n

Figure 2.17:<\/a> An anatomical comparison of Cephalochordate, with a color diagram on top, and a black and white scientific illustration on the bottom. On the top image, is a lateral, cross-sectional color illustration. Key anatomical structures are labeled as follows. The notochord (a stiff rod) runs the length of the body, with the dorsal nerve cord situated directly above it. The head features a rostrum, wheel organ, and an oral hood with tentacles. The large pharynx contains numerous pharyngeal slits, pharyngeal bars, and an endostyle. Further back are the hepatic cecum, ileocolic ring, intestine, and anus. Labeled fins include the dorsal fin, ventral fin, and caudal fin. The atriopore is shown on the ventral side. The segmented muscle blocks called myomeres are visible, along with the gonads. In the bottom image, it highlights the v-shaped muscle segments along the length of the body, labeled with numbers (e.g., my 32, my 42, my 52). Letters point to specific regions like the rostrum (e), notochord (n), gonads (go), and atriopore (at).<\/p>\r\n

Figure 3.2: An illustration of neurogenic placodes. The central diagram is a side-profile line drawing of a developing embryo. Colored patches on the head and neck regions indicate the positions of various placodes. Purple is the Otic (ear) region. Green is the Trigeminal (nerve V) region. Light blue is the eye\/Lens region. Yellow is the olfactory (nose) region. Orange dots are the epibranchial placodes located near the pharyngeal arches. Surrounding the central embryo are detailed callouts showing the structures that arise from these regions. Trigeminal (V) (green) shows the Ophthalmic and Maxillomandibular branches. Epibranchial (orange) shows the development of the Geniculate (VII), Petrosal (IX), and Nodose (X) ganglia. Otic (purple) is a complex diagram of the inner ear structures, including the Cochlear-vestibular (VIII) ganglion. Lens (blue) is a cross-section of the developing eye lens. Olfactory (yellow) is a representation of the developing nasal sensory structures.<\/p>\r\n

Figure 3.5: A scientific diagram illustrating the concept of Hox gene collinearity and the evolutionary conservation of body patterning between Drosophila (fruit flies) and humans. Section A is Genomic Organization. This section shows the linear arrangement of Hox genes on chromosomes, emphasizing that the order of genes on the DNA matches the order of the body parts they regulate. Drosophila: Shows a single cluster of eight genes (including lab, pb, Dfd, Scr, Antp, Ubx, Abd-A, and Abd-B) arranged from the 3' to 5' end. Human Being displays four distinct clusters (HOXA, HOXB, HOXC, HOXD) located on different chromosomes. These are paralogs resulting from genome duplications. The genes are numbered (e.g., A1 through A13) and color-coded to match their counterparts in the fruit fly. Section B is spatial expression. This section features two illustrations\u2014an adult Drosophila and a human embryo\u2014color-coded to correspond with the gene clusters in Section A. Genes at the 3' end of the clusters (pink\/red) are expressed in the head\/anterior regions, while genes at the 5' end (blue\/purple) are expressed in the tail\/posterior regions. In the human embryo, the 5' Hox genes (blue and purple) are also shown mapping to the development of the distal limbs (fingers and toes).<\/p>\r\n

Figure 3.13: A circular infographic representing the geological history of Earth as a 24-hour clock, spanning from its formation to the present day. Time is measured in Ga (billions of years ago) and Ma (millions of years ago). The central ring is divided into color-coded segments representing the major eons of Earth's history, moving clockwise from the top: Hadean (Red, 4.6\u20134.0 Ga) is the earliest period following the formation of the Earth (4550 Ma) and the Formation of the Moon (4527 Ma). Archean (Pink, 4.0\u20132.5 Ga) is noted for the end of the Late Heavy Bombardment and the earliest start of photosynthesis (c. 3200 Ma). Proterozoic (Purple\/Blue, 2.5 Ga\u2013541 Ma) is the longest eon, featuring the first major increase in atmospheric oxygen (2300 Ma) and multiple \"Snowball Earth\" glaciations. Paleozoic is shown in blue, Mesozoic is light green, and Cenozoic is pale green. Major Biological and Environmental Events (Outer Arcs) are shown as colored arcs on the perimeter mark the appearance of life forms and significant climate events. Snowball Earth Events are labeled around 2300 Ma, 716\u2013660 Ma, and 650\u2013635 Ma. Evolution of Life are Prokaryotes & Eukaryotes: arising in the Archean and early Proterozoic. Multicellular Life emerges late in the Proterozoic. The Cambrian Explosion (c. 540 Ma) is a rapid diversification of animal life. Following the Cambrian period are the first Vertebrate Land Animals (c. 380 Ma): Non-avian Dinosaurs (230\u201366 Ma) which span most of the Mesozoic. First Hominins (2 Ma) appear at the very end of the timeline, near the 12 o'clock position.<\/p>\r\n

Figure 3.17: A black-and-white scientific illustration featuring four distinct prehistoric fish-like organisms, stacked vertically, and labeled A through D. Each drawing showcases different anatomical features of early Paleozoic jawless and jawed vertebrates. Image A is a Pteraspidomorph: a streamlined, spindle-shaped fish with a prominent, pointed snout (rostrum). It features large head shields on the top and bottom of the front half, a single dorsal spine, and a body covered in small, diamond-shaped scales leading to a lobed tail. Image B is an osteostracan: a fish with a broad, horseshoe-shaped head shield and small, upward-facing eyes. It possesses distinct, paddle-like pectoral fins and a body covered in vertical, rectangular scales. The tail is heterocercal (upward-turning). Image C is an anaspid: a more slender, elongated fish with a downward-sloping mouth. It lacks a heavy head shield and paired fins but features a series of small spines or scales along its dorsal ridge. Its body is covered in diagonal, ribbon-like scales, and it has a hypocercal (downward-turning) tail. Image D is a climatius: a fish with a large eye and a visible jaw. It is characterized by prominent, stiff spines supporting its dorsal, anal, and pectoral fins. It also features a row of small \"intermediate\" spines along its belly.<\/p>\r\n

Figure 3.26: A collage of 14 photographs showcasing the immense diversity of Actinopterygii (ray-finned fishes), featuring various body shapes, colors, and habitats. Fish species pictured (from top-left to bottom-right). Row 1: An eel swimming among aquatic plants; a large tuna being measured on a deck; and bright red sockeye salmon spawning in a shallow stream. Row 2: A round, silver piranha; a decorative lionfish with striped fins and venomous spines; and a long, camouflaged northern pike in murky water. Row 3: A dark grouper or sea bass; a vibrant blue and orange pygmy angelfish; and a slender, silver herring or sardine. Row 4: A spotted pufferfish underwater; a seahorse clinging to a vertical plant stem; and a bulbous, deep-sea anglerfish against a black background. Row 5: A primitive-looking sturgeon swimming past a rock wall; and a slender, spotted longnose gar hovering near the surface among greenery.<\/p>\r\n

Figure 3.34: A four-panel collage showcasing diverse examples of sarcoptergians. Top-Left (Guiyu oneiros) is an artist's reconstruction of an extinct coelacanth swimming in blue water. It has a robust, silvery-blue body with heavy, plate-like scales on its head and distinct, fleshy lobed fins. It features two dorsal fins and a unique trilobed tail. Top-Right (Latimeria chalumnae) is a photograph of a living coelacanth in its dark, deep-sea habitat. The fish is dark blue or brown with irregular white speckles. Its thick, limb-like fins are clearly visible as it hovers near the ocean floor. Bottom-Left (Neoceratdous forsteri) is a photograph of a modern lungfish resting on a gravelly riverbed. It has an elongated, eel-like body with dark, olive-brown skin. Its pectoral and pelvic fins are notably thin and ribbon-like. Bottom-Right (Panderichthys) is an artistic rendering viewed from above. It has a flattened, crocodile-like head and a pale green, patterned body. The fins are positioned more like limbs, showing the evolutionary transition from water to land.<\/p>\r\n

Figure 3.39: A phylogenetic tree (cladogram) illustrating the evolutionary relationships of the Tetrapoda (four-limbed vertebrates). The diagram uses color-coded branches, representative animal silhouettes, and text labels to categorize different lineages. The tree branches from left to right, starting from ancestral \"Fishapods\" and stem tetrapods at the top, and diversifying into major clades of amphibians, mammals, and reptiles. Major Clades (Top to Bottom) begin with Batrachomorpha (Orange): includes extinct groups like \u2020Temnospondyli and \u2020Lepospondyli, leading to modern Lissamphibia (represented by a frog silhouette). Reptiliomorpha (Black\/Pink\/Purple\/Green) is a large group containing both non-amniotes and the Amniota. Synapsida (Pink) is the lineage leading to mammals. It includes the extinct \u2020Pelycosaurs, \u2020Therapsida, and modern Mammalia (represented by a bear). Reptilia (Purple\/Green) are part of the Amniota group, further divided into (purple): Parareptilia, Squamata, Rhynchocephalia, and Testudines: Turtles (connected by a dashed line indicating phylogenetic uncertainty). Archosauria (Green) includes Pseudosuchia Avemetatarsalia (represented by a chicken). Visual Indicators include dagger symbol used to mark extinct groups, dashed lines to indicate uncertain or debated evolutionary placements for specific groups like turtles and certain early amphibians, and vertical brackets on the right side. Large brackets group these lineages into broader categories: Amniota, Reptiliomorpha, and the overarching Tetrapoda.<\/p>\r\n

Figure 3.41: A scientific comparative diagram titled \"Edopoidea\" against a black background, displaying 8 different dorsal (top-down) skulls from the clade Edopoidea. The skulls are rendered in white with black lines showing the cranial sutures and eye sockets. A small white human hand silhouette and a 10 cm scale bar are placed near the largest skulls to provide a sense of massive size. Top Row (Large Species) features the four largest skulls (labeled 1\u20134). These skulls are broad and robust, with large openings for the eyes. Bottom Row (Small to Medium Species) features four smaller, more elongated skulls (labeled 5\u20138). The diagram includes a numbered key in the bottom right corner corresponding to the following species: Edops craigi (The largest, most massive skull), Adamanterpeton ohioensis, Nigerpeton ricqlesi, Saharastega moradiensis, Chenoprosopus milleri, Chenoprosopus lewisi, Cochleosaurus bohemicus, and Cochleosaurus florensis. The skulls vary significantly in shape, from the wide, U-shaped snout of Edops to the long, narrow, almost gharial-like snouts of the Chenoprosopus species. All skulls show characteristic \"primitive\" tetrapod features, such as a pineal foramen (a small hole for a \"third eye\") located between the main eye sockets.<\/p>\r\n

Figure 3.47: An anatomical diagram showing the lateral (side) view of a prehistoric tetrapod skull and lower jaw. The illustration is a black-and-white line drawing with various bones labeled and shaded to show the complex structure of an early land vertebrate. The upper portion of the image displays the skull, featuring several distinct regions to include the snout, labeled with bones including the Premaxilla (front teeth-bearing bone), Nasal, Maxilla (main upper jawbone), and Lacrimal. The Eye Region includes the orbit (eye socket) and is surrounded by the Prefrontal, Frontal, Postfrontal, and Jugal bones. The Temple and Back of Skull includes the Postorbital, Supratemporal, Squamosal, and Quadrate (the jaw hinge point). The Palate is a small portion of the Pterygoid that is visible beneath the main cheek area. Lower Jaw (Mandible) shows the bottom portion shows the detached lower jaw, which is composed of several interlocking bones including Dentary (the large, front bone that houses the lower teeth), splenial and angular (bones that form the bottom and back-lower edge of the jaw), and surangular and articular (bones at the back of the jaw that facilitate the hinge mechanism with the upper skull).<\/p>\r\n

Figure 3.48: This four-panel collage features different views and species of the prehistoric synapsid Dimetrodon. The image contrasts fossil remains with artistic life reconstructions. Top-Left is a photograph of a mounted Dimetrodon skeleton in a museum. Top-Right is a detailed artistic chart showing several Dimetrodon species (including D. angelensis, D. grandis, and D. milleri) scaled against a silhouette of a human for size comparison. The species vary from dog-sized to over 4 meters in length, with different sail shapes and color patterns. Bottom-Left is a color illustration of a Edaphosaurus pogonia as it may have appeared in life. It is depicted with a lizard-like sprawling gait, a green-and-brown mottled body for camouflage, and a tall, vibrantly colored orange-and-yellow sail. Bottom-Right is another museum photograph of a skeleton. This view highlights the curvature of the ribs and the impressive height and density of the dorsal sail spines.<\/p>\r\n

Figure 3.49: A five-panel collage of artistic reconstructions showcasing the diverse body forms of Therapsids. The image illustrates a wide range of ecological niches, from apex predators to semi-aquatic herbivores. Top-Left (Inostrancevia) is a dramatic scene of a large, saber-toothed Inostrancevia standing over its prey, a green-scaled herbivore. The predator has a pinkish-grey, leathery hide and massive canine teeth designed for a powerful killing bite. Top-Center (Alopecognathus) is a close-up profile of a Alopecognathus, showing a long, narrow snout and specialized teeth. Top-Right (Ostehria) is a portrait of a small, beak-faced therapsid with large, expressive orange eyes. It features a turtle-like beak and two small tusks. Bottom-Left (Moschops) show two massive, heavy-set Moschops walking across a dry landscape. They have thick, barrel-shaped bodies, thick skulls, and a sprawling-to-semi-erect gait. Bottom-Right (Castorocoda) is a reconstruction of a Castorocoda swimming underwater. It has a streamlined, otter-like body and a broad tail.<\/p>\r\n

Figure 3.52: A four-panel collage showcasing extant monotremes. The image features high-resolution photographs and a cutout of these unique animals. Top-Left is a photograph of a platypus swimming at the surface of a dark pond. It shows its distinctive broad, flat tail, waterproof brown fur, and the leathery, duck-like bill that it uses for electrolocation. Top-Right is a photograph of a short-beaked echidna walking on sandy soil. Its body is covered in a dense coat of sturdy, cream-and-black spines. It has a small, dark snout and powerful claws for digging. Bottom-Left is a photograph of a long-beaked echidna foraging in tall green grass. Compared to its short-beaked relative, it has a noticeably longer, downward-curving snout and more visible fur between its lighter-colored spines. Bottom-Right is a clear studio-style cutout of a Western long-beaked echidna against a white background. This image highlights its sturdy, pillar-like legs, large digging claws, and the characteristic elongated \"beak\" used to feed on earthworms.<\/p>\r\n

Figure 3.55: A three-panel collage showcasing a mix of fossil remains and artistic life reconstructions of prehistoric parareptiles.\u00a0 Top is a photograph of a mounted skeleton of a Bradysaurus baini in a museum. The skeleton displays a massive, barrel-shaped body, thick ribs, and a robust skull with visible bony textures. Its sturdy, pillar-like legs are positioned in a semi-sprawling gait, indicating a slow-moving, heavily armored herbivore. Bottom-Left is an artistic reconstruction of a Mesosaurus, a small, aquatic parareptile. It features an elongated, streamlined body with a long neck and a narrow snout filled with fine, needle-like teeth. Its limbs are paddle-like, and it is depicted with a vibrant orange-and-white striped pattern. Bottom-Right (Sclerosaurus armatus) is an artistic rendering, viewed from a rear-angled perspective. This creature is smaller and more lizard-like, with a stout body covered in bumpy, pebbled scales. It features a broad head with small spikes or horns projecting from the back of the skull, likely for defense against predators.<\/p>\r\n

Figure 3.61: A seven-panel collage showcasing a diverse array of Archosaurs. The image mixes modern photography, museum skeletons, and life reconstructions. Top-Left is a photograph of a pair of Mallard ducks standing by the water. Top-Right is a museum mount of a Tyrannosaurus rex skeleton in a walking pose. Middle-Left is an artistic reconstruction of two pterosaurs (flying reptiles) in flight. They have colorful heads, elongated wings, and long tails. Middle-Right is a photograph of a Triceratops skeleton. It shows the iconic three-horned face, large bony frill, and heavy-set quadrupedal body. Bottom-Left is an illustration of a prehistoric Saurosuchus galilei (a crocodile-line archosaur) that looks superficially like a dinosaur but has a different ankle structure and a more sprawling-to-semi-erect gait. Bottom-Right is a photograph of a Nile crocodile with its mouth open in the water. Far-Bottom-Left is a museum display featuring a feathered Deinonychus with it's from limbs stretching out and its mouth open.<\/p>\r\n

Figure 3.63: This phylogenetic tree, titled \u201cArchosauria Evolution,\u201d illustrates the evolutionary relationships and geological timelines of the two major archosaur lineages: the Pseudosuchia (crocodile-line) and the Dinosauria (including birds). The diagram is set against a horizontal timeline spanning from the Permian period (approx. 270 million years ago) to the present day. The diagram uses clean black lines for the tree structure, with species names written in colored text matching their silhouette icons. Light grey vertical bands highlight major geological boundaries and extinction intervals.\u00a0 Pseudosuchia (Top Branch - Blue): This lineage includes early armored forms: Aetosaurus, Riojasuchus, and Desmatosuchus (Triassic), Sphenosuchus and Dibothrosaurus (Jurassic\/Triassic), Dakosaurus (Cretaceous\/Jurassic), and Alligator and Crocodylus (Cretaceous to today). Dinosauria (Middle Branch - Green): This branch details the evolution of non-avian dinosaurs. Dinosaurs include Eoraptor, Plateosaurus, and Coelophysis (Triassic), Dilophosaurus and\u00a0 (Jurassic), Compsognathus (Jurassic\/Cretaceous), Citipati, Velociraptor, Archaeopteryx, and Ichthyornis (Cretaceous\/Paleogene). Neornithes (Bottom Branch - Red): Modern examples shown include Nothura, Gallus, and Geospiza (Cretaceous\/Paleogene, and Neogene periods). Along the bottom of the chart features a color-coded bar representing geological periods: Permian 255-270 million years ago (Red\/Purple), Triassic 200-255 million years ago (Purple), Jurassic 145-200 million years ago (Light Blue), Cretaceous 65-145 million years ago (Green), Paleogene 23-65 million years ago (Orange), and Neogene 0-23 million years ago (Yellow).<\/p>\r\n

Figure 3.64: This image is a collage illustrating the diverse evolutionary history and forms of rauisuchians. Top Left is an illustration of a long, armored Longosuchus meani swimming just below the water's surface. Top Right is a photograph of two Gavialis gangeticus resting on sand, showing their characteristic long, thin snouts. Middle Left is a vibrant illustration of a Dakosaurus maximus leaping out of the water, featuring a streamlined body and a fluke-like tail. Middle Right (Upper) is a depiction of a heavy-set, land-dwelling Rauisuchian with a powerful build and a deep skull, walking through a Triassic forest. Middle Right (Lower) is a slender, long-legged terrestrial Litargosuchus leptorhynchus walking along a fallen log. Bottom Large Panel is an underwater scene showing a Chenanisuches lateroculi with a long snout and paddle-like limbs swimming through a murky blue environment. Bottom Chart is a scientific reconstruction of Postosuchus kirkpatricki shown in profile. It is a large, quadrupedal predator with a deep head and a long tail. A human silhouette is included for scale, showing the creature reaching roughly waist-to-chest height.<\/p>\r\n

Figure 3.66: This image is a four-panel collage focusing on Pterosauria. It highlights their anatomy, fossil record, and varied sizes. Top Left is a colorful paleoart illustration of a small, crested pterosaur in a lush prehistoric forest. It is depicted in mid-flight with its mouth open, pursuing a large, moth-like insect. Top Right is a photograph of a remarkably well-preserved pterosaur fossil embedded in a slab of light-colored limestone. The skeleton is laid out in profile, showing the delicate wing bones, a long neck, and a sharp, elongated skull. Faint impressions of the wing membranes are also visible. Bottom Left is a comparative size chart featuring several pterosaur silhouettes of varying wing spans overlaid against a human silhouette for scale. The silhouettes range from small, bird-sized species to much larger forms with several-meter wingspans. Bottom Right is a specific scale diagram for Simurghia robusta. It shows a black silhouette of the pterosaur in flight next to a human silhouette. A scale bar indicates 1 meter, showing that this species had a wingspan roughly twice the width of a human's height.<\/p>\r\n

Figure 3.68: This image is a six-panel collage showcasing a diverse range of Ornithischian dinosaur skeletons, highlighting the major groups within this lineage. Top Left is a fossil of a small, Heterodontosaurid, preserved in a curled \"death pose\" within a rock matrix. Top Right is a mounted skeleton of a Nipponosaurus. It is shown in a quadrupedal stance with its characteristic toothless beak and high back. Middle Left is the skeleton of a Stegosaurus, featuring the famous upright bony plates along its spine and the \"thagomizer\" spikes on its tail. Middle Right is a low-profile, heavily armored skeleton of an Borealopelta. This \"living tank\" is covered in bony osteoderms and shows a wide, flat body designed for defense. Bottom Left are two skeletons of stegoceras in a dynamic, low-slung pose. These are known for their thick, bony skull caps used for head-butting or display. Bottom Right is a large, mounted skeleton of a Triceratops, showing its massive skull with three horns and a large bony frill at the back of the neck.<\/p>\r\n

Figure 3.72: A six-panel collage highlighting examples of paravian dinosaurs. Top Left is a fossil of a Confusciusornis sanctus preserved in limestone. It shows the distinct impressions of feathers around the wings and tail, alongside a reptilian skeletal structure. Top Right is a mounted skeleton of a Dromaeosaurid. It features sharp teeth, a large \"sickle claw\" on the foot, and a stiffened tail. Middle Left is a fossil slab showing a small troodontid in a flattened \"death pose,\" displaying elongated limbs and carbonized feather traces. Middle Right is a fossil of a Microraptor gui showing the clear outline of the body and limbs, emphasizing the dense covering of integument (feathers\/down). Bottom Left is a photograph of a modern Raven standing in the grass. Bottom Right is a museum display of a small Anchiornis huxleyi skeleton in a dynamic walking pose.<\/p>\r\n

Figure 4.6: Three cross-sectional diagrams illustrating the stages of gastrulation. Throughout the stages, the following layers are color-coded: blue (Ectoderm, the outer layer of cells), yellow (Endoderm, the inner layer of cells), red (Mesoderm, the middle layer that forms during the process). Stage A is Invagination (this initial stage shows the beginning of the inward folding of the embryo.) The blastocoel is a large fluid-filled cavity at the top in blue. Invagination shows the yellow cells at the bottom-right start to buckle inward and the dorsal lip of blastopore (the specific point where cells begin to migrate inside the embryo). Stage B is Involution and Epiboly. Involution show cells (marked in red as Mesoderm) that roll over the edge of the dorsal lip to move into the interior. Epiboly show the blue Ectoderm cells expand to cover the outer surface of the embryo. Blastocoel is a cavity that begins to shrink as it is displaced by new internal structures. Stage C is late Gastrula. Archenteron: a new, large cavity (primitive gut) is formed by the yellow Endoderm. Mesoderm (Red) is now clearly positioned between the Ectoderm and Endoderm. The blastocoel is reduced to a small remnant at the bottom-left.<\/p>\r\n

Figure 4.12: Two scientific diagrams (labeled A and B) illustrating the development of the vertebrate head. Diagram A shows the distribution of Posterior placodes (on the right of the diagram) and Anterior placodes (along the bottom) on the head of a developing embryo. Anterior Placodes include the Olfactory (green), Lens (magenta), and Adenohypophyseal (purple) placodes. Posterior Placodes are a series of colored spots representing the precursors to cranial nerves and sensory organs, including: Trigeminal, V (pink), Geniculate, VII (orange), Otic, VIII (dark blue), Petrosal, IX (red-orange), Nodose, X (deep red). Grey arrows indicate the migratory pathways and expansion of these tissues from the dorsal surface toward the ventral side. Diagram B illustrates the Pharyngeal Arches and Neural crest descendants with a scale at the bottom indicating the ventral-dorsal axis. Craniofacial Skeleton & Mandible are indicated by a light blue highlight along the anterior-dorsal edge, with blue arrows showing migration toward the front of the face. Rhombomeres (r1\u2013r8) show the hindbrain is segmented into regions labeled r1 through r8, colored in shades of purple. Neural Crest-Derived Glia is shown as small green \"S\" shapes clustered within the pharyngeal arches. Pharyngeal Arches are the segmented ridges at the bottom of the head.<\/p>\r\n

Figure 4.14: This scientific diagram illustrates the sonic hedgehog patterning. It highlights how specific signaling gradients dictate the identity of cells within the developing central nervous system. Labeled sections include: surface ectoderm (A green horizontal layer at the top), neural tube (a large blue oval structure in the center. Inside it is the neural canal), roof plate (the top-most section of the neural tube, marked by a yellow triangle with arrows pointing down the neural tube, directly beneath the surface ectoderm), floor plate (the bottom-most section of the neural tube, red arc with arrows pointing up the neural tube), neural crest cells (purple circular cells migrating away from the dorsal part of the neural tube), and Notochord (an orange rod-like structure located directly beneath the floor plate). The right side of the diagram features a \u201cmorphogen gradient\u201d chart that explains how cell identity is determined. BMP (Bone Morphogenetic Protein) is shown as a light green gradient. It is highly concentrated at the dorsal (top) side, secreted by the surface ectoderm and roof plate. Shh (Sonic Hedgehog) is shown as a red gradient. It is highly concentrated at the ventral (bottom) side, secreted by the notochord and floor plate.<\/p>\r\n

Figure 4.16: A scientific diagram illustrating the concept of Hox gene collinearity and the evolutionary conservation of body patterning between Drosophila (fruit flies) and humans. Section A is Genomic Organization. This section shows the linear arrangement of Hox genes on chromosomes, emphasizing that the order of genes on the DNA matches the order of the body parts they regulate. Drosophila shows a single cluster of eight genes (including lab, pb, Dfd, Scr, Antp, Ubx, Abd-A, and Abd-B) arranged from the 3' to 5' end. Human Being displays four distinct clusters (HOXA, HOXB, HOXC, HOXD) located on different chromosomes. These are paralogs resulting from genome duplications. The genes are numbered (e.g., A1 through A13) and color-coded to match their counterparts in the fruit fly. Section B is spatial expression. This section features two illustrations\u2014an adult Drosophila and a human embryo\u2014color-coded to correspond with the gene clusters in Section A. Genes at the 3' end of the clusters (pink\/red) are expressed in the head\/anterior regions, while genes at the 5' end (blue\/purple) are expressed in the tail\/posterior regions. In the human embryo, the 5' Hox genes (blue and purple) are also shown mapping to the development of the distal limbs (fingers and toes).<\/p>\r\n

Figure 4.17: This scientific diagram illustrates the boundaries of Hox gene expression. Rhombomeres (r1\u2013r7) show that the central grey-shaded column represents the hindbrain, divided into seven distinct segments called rhombomeres. Cranial Nerves (V, VII, IX) are on the left side as orange clusters that represent the sensory ganglia of the cranial nerves: V (Trigeminal) that are associated with r2, VII (Facial) that are associated with r4, IX (Glossopharyngeal) that are associated with r6. Branchial Arches (ba1\u2013ba3) are the curved structures on the right that represent the pharyngeal or branchial arches, which eventually form the structures of the face and neck. Neural Crest Cell Migration (ncc) display large green arrows and circles to show neural crest cells migrating from specific rhombomeres into the arches: cells from r1 and r2 migrate into ba1, cells from r4 migrate into ba2, cells from r6 migrate into ba3. Otic Vesicle (OV) is a yellow oval located between ba2 and ba3, which will develop into the inner ear. The right side of the diagram features vertical colored bars representing the expression domains of various Hox genes (Hoxa2, Hoxb2, Hoxa3, etc.).<\/p>\r\n

Figure 4.22: This diagram illustrates the famous Spemann-Mangold organizer experiment, a landmark study in embryology that demonstrated the concept of embryonic induction. The image follows a step-by-step process of transplanting tissue from one amphibian embryo to another to see how it affects development. At the top, a piece of the dorsal lip of the blastopore is taken from a donor embryo and transplanted into the ventral side (the opposite side) of a recipient embryo. The recipient embryo shows a primary invagination on its original dorsal side and the transplanted tissue beginning a secondary invagination on the ventral side. Invagination (Middle Left) shows that as the embryo develops, two separate sites of invagination occur simultaneously: the original \"primary\" site and the \"secondary\" site induced by the transplanted donor tissue. Cross-section (Middle Right) is the transverse section of the developing larva that shows two complete sets of dorsal structures. A neural tube and notochord on one side. A second neural tube and notochord on the opposite side, flanking a shared central endoderm. The final illustration shows a \"conjoined twin\" tadpole. Two distinct heads and bodies are fused along their belly (ventral) side, demonstrating that the transplanted dorsal lip was able to organize a completely new body axis.<\/p>\r\n

Figure 5.1: An anatomical illustration of a human arm and shoulder, featuring three detailed inset diagrams labeled A, B, and C that show the microscopic structures of connective tissues. The central image is a medical illustration of a person's upper torso and arm. The arm is flexed at the elbow, highlighting the biceps muscle, shoulder bones, and the humerus. Three black lines lead from specific parts of the arm to detailed inset boxes. Inset A is the Tendon Structure that shows a hierarchical breakdown of a tendon: the overall tendon contains multiple fascicle bundles, a single bundle contains numerous collagen fibrils, the fibrils are composed of individual triple-helix collagen molecules. Inset B shows the skin layers that displays a cross-section of the integumentary system including epidermis: the top purple layer; dermis: the middle layer, featuring a magnified view of a dense network of thick, wavy collagen fibers and thinner, red star-shaped elastin fibers; fat (Hypodermis): the bottom yellow layer. Inset C is the bone microstructure that is a microscopic view of osseous tissue (bone), showing multiple circular structures. Within the circular structures are lamellae: concentric rings of mineralized matrix; osteocytes: Small, dark, spider-like cells embedded within the rings.<\/p>\r\n

Figure 5.3: Three separate diagrams labeled A, B, and C illustrate how forces work. Panel A: Force and Weight shows an illustration of a purple cat sitting on a dark horizontal platform. Two vertical arrows represent opposing forces: a teal upward arrow labeled with a square F represents a force. A purple downward arrow labeled with a square and w represents the weight of the cat due to gravity. Panel B: Stress and Deformation is a technical diagram showing the deformation of an object. Its original shape is a light purple vertical oval. The deformed shape is a dark horizontal oval showing the object flattened and widened. Internal stress are pink arrows pointing outward from the center, labeled with the Greek letter sigma (\u03c3), representing internal tensile stress. External Pressure (p) are orange arrows pointing inward at the top and bottom, labeled with p, representing compressive pressure. Strain (e) is shown with horizontal double-sided arrows on the left and right labeled e. Panel C: Locomotion Comparison is a side-by-side comparison of movement styles between a hare (rabbit) and a tortoise between a \"START\" and \"FINISH\" line. The hare is a purple rabbit shown next to a dashed path consisting of three high, bouncy parabolic arcs, representing saltatory (hopping) locomotion. The tortoise is a purple tortoise shown next to a dashed path that is mostly flat and low to the ground, representing steady, crawling locomotion.<\/p>\r\n

Figure 5.4: This image is a mathematical and geometric reference chart comparing the Surface Area (SA), Volume (V), and Surface Area to Volume Ratio (SA:V) of three geometric shapes: a cube, a sphere, and a cylinder. Each shape is shown in two sizes to demonstrate how scaling affects these ratios. In a cube, the formulas are SA = 6l^2, V = l^3. The Large Cube (l=2): SA = 24, V=8, SA:V=3. Small Cube (l=1): SA = 6, V = 1, SA:V = 6. Two wireframe cubes are shown with a red line indicating the side length l.2. Sphere formulas are: SA = 4 times pi r^2, V =4\/3 times pi r^3. Large Sphere (r=2): SA = 50.3, V=33.5, SA:V=1.5. Small Sphere (r=1): SA=12.6, V=4.2, SA:V=3. Two spheres are shown with a red radius line r and a horizontal cross-section. The cylinder formulas are: SA = 2 times pi times r times h + 2 times pi times r^2 and V=pi times r^2 times h. The large cylinder (r=2, h=4) is: SA = 75.4, V = 50.3, SA:V=1.5. The small cylinder is (r=1, h=2): SA = 18.9, V = 6.3, SA:V=3. Two vertical cylinders are shown with red lines indicating radius (r) and height (h).<\/p>\r\n

Figure 5.7: This anatomical diagram illustrates the biomechanics of the human arm during two primary movements: extension and flexion, highlighting the antagonistic relationship between the biceps and triceps muscles. Extension (left image) of the arm is straightened out, and the elbow joint is open. The triceps (the muscle on the back of the upper arm) is contracted to pull the forearm down. A thick red arrow labeled Ft indicates a strong contractile force from the triceps. A thinner, smaller red arrow labeled Fb indicates that the biceps is relaxed or elongated. Flexion (right image) shows the arm bent at the elbow. The elbow joint is closed, and the hand moves toward the shoulder. The biceps (the muscle on the front of the upper arm) is contracted and \"bunched up.\" A thick red arrow labeled Fb indicates a strong contractile force from the biceps. A thinner red arrow labeled Ft indicates that the triceps is now the relaxed or elongated muscle.<\/p>\r\n

Figure 5.8: This image explores the mechanics of lever systems. Panel A shows basic lever physics. A cat (the load) sits on one end of a tilted beam, supported by a triangular fulcrum in the middle. A pink downward arrow labeled F-in represents the input force (effort) applied to one end. A teal upward arrow labeled F-out shows the resulting lift on the cat, while a purple downward arrow labeled load represents the cat's weight. Panel B shows comparative Anatomy. Two animal limbs are compared to show how lever lengths affect function. A runner (horse) features a long, thin limb. The effort is applied close to the joint (the fulcrum, represented by an orange triangle). A digger (mole) features a short, robust limb with a wide \"hand.\" The lever arm for the muscle is proportionally longer. Panel C shows classes of levers in the human body. Three anatomical examples demonstrate the different classes of levers: 1st class lever (the head) shows the fulcrum (orange triangle) is between the effort and the load. The neck muscles pull down (effort) to lift the face up (load), pivoting at the base of the skull. 2nd class Lever (the foot) shows the load is between the fulcrum and the effort. Standing on tiptoes uses the ball of the foot as the fulcrum, the body weight as the load, and the calf muscles pulling up as the effort. 3rd class lever (the arm) shows the effort is between the fulcrum and the load. The elbow is the fulcrum, the biceps provide the upward effort in the middle, and the hand\/forearm acts as the load.<\/p>\r\n

Figure 5.9: Five drawings of bones illustrating the different types of loading regime. The first (left) shows tension with red arrows above and below the bone, pointing in opposite directions (reddened bone area at the top and bottom areas). The second bone shows compression with red arrows above and below the bone pointing towards one another (reddened bone area at the top and bottom). The third bone shows shear with a split in the middle of the bone (red around the area near the split) with arrows on each side of the split pointing to one another. The fourth bone shows torsion with redness along the length of the bone and arrows at the top and the bottom curving around the bone, laterally. The last bone shows bending with the bone curving inward, two arrows curving inward (reddened bone along the outside curve).<\/p>\r\n

Figure 5.13: This anatomical diagram compares the skeletal structures and limb orientations of a sprawling alligator and an upright human, illustrating how different body postures affect the arrangement of bones. In all four views, homologous bones (bones that share a common evolutionary origin) are color-coded for comparison: pelvis and shoulder Girdle (purple), humerus \/ Femur (magenta), radius & ulna \/ tibia & fibula (teal). The top left image shows an alligator skeleton viewed from above. The limbs protrude horizontally from the body before bending downward at the \"elbow\" or \"knee.\" The bottom left (Sprawling Model) is a simplified front-view diagram of the alligator's pelvic region. The magenta femur bones extend outward at nearly 90 degrees from the purple pelvis. The right-side image shows a human skeleton in a standing, bipedal position. The limbs are positioned directly underneath the torso. The bottom middle (Upright Model) is a simplified front-view diagram of the human pelvic region. The magenta femur bones are aligned vertically beneath the purple pelvis.<\/p>\r\n

Figure 5.14: This illustration demonstrates forelimbs in various animals. Panel A shows the forelimb skeletons of five different mammals. Each bone group is color-coded to highlight their shared ancestry despite different functions: Beige\/Tan is the Humerus (upper arm), teal is the radius and Ulna (forearm) and Carpals\/Phalanges (hand\/fingers), magenta is the olecranon process (elbow), which acts as a lever arm for the triceps muscle. Orange triangles indicate the fulcrum point of the elbow joint. Flight (bat) has extremely elongated phalanges (fingers) to support a wing membrane. Swimming (whale) has shortened, flattened bones forming a paddle-like flipper. Grasping (human) has a versatile structure with a highly mobile thumb and fingers. Running (horse) has elongated distal bones. Digging (mole) has very short, thick bones with a massive, broad \"hand\" and a large magenta olecranon process. Panel B shows specialized locomotion in the skeleton of a sloth hanging upside down from a branch. Teal Bones show the limbs are exceptionally long, allowing the sloth to reach distant branches while remaining securely suspended. Olecranon Process is a small magenta area at the elbow (marked with an orange triangle) that shows where the muscles attach to maintain this hanging posture.<\/p>\r\n

Figure 7.5: A detailed anatomical diagram of a femur, shown in a longitudinal cross-section to reveal its internal and external structures. The diagram is divided into three primary regions: the epiphyses at the ends, the metaphyses in the transition zones, and the diaphysis making up the long shaft. The Proximal Epiphysis is the upper rounded end of the bone, covered in smooth, blue Articular cartilage. The diaphysis is the long, tubular central shaft of the bone. The distal epiphysis is the lower end of the bone, also capped with articular cartilage. Metaphysis are the regions between the diaphysis and the epiphyses; the proximal metaphysis contains the Epiphyseal line. The Periosteum is a tough, fibrous outer membrane shown partially peeled back. Inside is the compact bone, the dense, hard outer layer that forms the exterior of the diaphysis. The spongy bone is located primarily in the epiphyses, shown with a porous, honeycomb-like structure filled with red bone marrow. The Medullary Cavity is the hollow central chamber of the diaphysis, lined by a thin membrane called the Endosteum. The yellow bone marrow is shown as a fatty, cylindrical mass stored within the medullary cavity. The nutrient artery is a prominent red blood vessel entering the bone through the periosteum to provide nourishment to the internal tissues.<\/p>\r\n

Figure 8.2: This image provides a detailed anterior (front) view of a human skull, with color-coded bones and comprehensive anatomical labels. The frontal bone (pink) forms the forehead and the upper part of the eye sockets (orbits). Parietal bones (tan) located on the sides and roof of the cranium, separated from the frontal bone by the coronal suture. Zygomatic bones (teal) form the lateral walls of the orbits. Maxilla (orange) is the upper jaw bone. Mandible (grey) is the lower jaw bone, featuring the mental foramen and the lower alveolar process. Nasal bones (blue) are the small bones forming the bridge of the nose. The diagram highlights several specialized openings and bones within the orbit. Optic canal is for the passage of the optic nerve. Superior and Inferior orbital fissures are slit-like openings. Lacrimal bone (purple) is a small bone at the inner corner of the eye. Ethmoid and sphenoid bones are deep bones that form the posterior walls of the orbit. Nasal septum is composed of the vomer bone and the perpendicular plate of the ethmoid bone. T middle and inferior nasal conchae are visible as bony plates within the nasal cavity.<\/p>\r\n

Figure 8.4: This scientific illustration provides a lateral (side) view of the internal anatomy of a shark's head and neck region, with major skeletal and respiratory structures color-coded and labeled. The chondrocranium (yellow) is the main braincase of the shark, made of cartilage. It includes the rostral cartilage at the snout, the olfactory capsule for smell, the optic capsule surrounding the eye, and the otic capsule for hearing and balance. The mandibular arch (red) is the first branchial arch, which has evolved into the jaws. It consists of the palatoquadrate (upper jaw) and Meckel\u2019s cartilage (lower jaw). The hyoid arch (blue) is the second branchial arch, which supports the jaw. Key components include the hyomandibula, ceratohyal, and basihyal. Gill arches and slits (Green) are five posterior gill arches support the respiratory system, with vertical gill slits between them for water passage. A smaller opening called the spiracle is located just behind the eye.<\/p>\r\n

Figure 8.5: This diagram provides a lateral (side) view of the head and jaw skeleton of a generalized vertebrate embryo, color-coded by the embryonic germ layers that give rise to specific tissues. The dermatocranium (light blue blocks) is shown as a series of rectangular segments forming the outer \"shell\" of the skull and lower jaw.\u00a0 The neurocranium is the inner braincase, divided into a light blue anterior section and a large orange posterior section (occipital region). It houses the sensory capsules: nasal, optic (eye), and otic (ear). The viscerocranium (light blue arches) is the region that includes the jaws and the branchial (gill) arches. The arches are numbered 1 through 7. Arch 1 forms the primary jaws, while the subsequent arches support the pharynx. The diagram uses a legend in the bottom left to identify the origin of each structure. Blue (Ectoderm) forms the sensory capsules (nasal, optic, otic). Orange (Mesoderm) forms the posterior neurocranium and segments of the dermatocranium. Light Blue (Neural Crest) is a specialized tissue that forms the majority of the facial skeleton, including the jaws and branchial arches. Yellow (Endoderm) forms the lining of the pharynx. Green (Chordamesoderm) forms the notochord, the primitive backbone structure visible on the right.<\/p>\r\n

Figure 8.12: An evolutionary diagram illustrating the divergence of amniote skull types based on the arrangement of four specific skull bones: the Parietal (red), Post-orbital (blue), Squamosal (yellow), and Jugal (green). The diagram starts from a common ancestral skull on the left and branches into two main lineages, which further diversify into five modern or specialized skull configurations on the right. The ancestral form (center-left) is an anapsid-like skull with no temporal fenestrae (holes) behind the eye socket. The upper branch lineage splits toward skulls with two temporal openings. The top-most skull (green sea turtle) shows a clear upper and lower fenestra. The middle-right skull (North American rat snake) shows a highly modified, kinetic structure where several bones are reduced or disconnected. The third skull down (Caiman lizard) shows the loss of the lower temporal bar. The lower branch lineage leads to a skull with a single temporal opening. The bottom-right skull (domestic cat) shows a significantly enlarged parietal bone and a zygomatic arch formed by the jugal and squamosal bones.<\/p>\r\n

Figure 8.13: The image displays a comparative layout of six different reptile and dinosaur skulls, labeled A through F. Each specimen is shown from two angles: a lateral (side) view on top and a dorsal (top-down) view below it. The skulls are rendered as 3D digital scans or high-resolution models against a solid black background. Skull A (Savannah monitor) is long, slender skull with numerous small, sharp teeth. Skull B (Tuatara) is a shorter, robust skull featuring a prominent, dashed white line outlining the temporal fenestra (opening behind the eye socket). Skull C (Tropical rattlesnake) is a highly kinetic skull with thin, delicate bones and a flexible jaw structure, typical of a snake. Skull D (Loggerhead sea turtle) is a smooth, dome-shaped skull with no visible teeth and a beak-like snout. Skull (T-rex) is a massive, deep-jawed skull with large serrated teeth and multiple large openings (fenestrae). Skull F (New Guinea crocodile) is a long, narrow snout with a bumpy, rugose texture and upward-facing nostrils.<\/p>\r\n

Figure 8.14: This image features a collection of 12 distinct avian (bird) skulls, labeled A through L, displayed in a grid against a solid black background. All skulls are shown in a lateral (side) view, rendered as high-resolution 3D digital scans. Skull A (Eurasian sparrowhawk) is robust skull with a short, hooked beak. Skulls B (Tasmanian nativehen) & H (Lesser yellow-headed vulture) are skulls with large, powerful hooked beaks. Skulls C (Kagu) & L (Northern gannet) are skulls with long, thin, straight, and pointed beaks. Skull D (Shoebill) is a specialized skull with a very long, downward-curved tip, like a shorebird or flamingo. Skull E (Bee hummingbird) is a skull with an exceptionally long, needle-thin beak. Skull F (Green-winged teal) is a skull with a wide, flattened beak. Skull G (Ostrich) is a skull with a straight, sturdy beak of moderate length. Skulls I (Yellow-bellied sapsucker) & J (American tree sparrow) are smaller skulls with shorter, generalist beaks. Skull K (Red-tailed hawk) is a skull with a deeply curved, sharp beak.<\/p>\r\n

Figure 8.15: This image is a black-and-white scientific line drawing showing a lateral (side) view of a reptilian skull and lower jaw, with individual bones labeled for anatomical identification. The upper portion of the image displays the cranium with various bones and openings. The snout area includes the Premaxilla (front tip), Maxilla (main tooth-bearing bone), Nasal, and Lacrimal. The eye and forehead include the Prefrontal, Frontal, Postfrontal, and the Jugal bone beneath the eye socket. The rear skull identifies the Postorbital, Supratemporal, Squamosal, and Quadrate (the hinge point for the jaw). Dark shaded areas represent the orbit (eye socket) and temporal fenestrae (openings in the skull for muscle attachment). The lower portion shows the separated mandible, highlighting its complex structure. The front shows the Dentary, which holds the lower teeth, and the Splenial on the inner side. The rear includes the Surangular (top back), Angular (bottom back), and the Articular bone, which connects to the upper skull's quadrate bone to form the jaw joint.<\/p>\r\n

Figure 8.16: This image features a collection of 12 diverse mammalian skulls, labeled A through L, presented in a grid against a solid black background. Each skull is shown in a lateral (side) view and rendered as a high-resolution 3D digital scan. Skulls A (Platypus) & B (Short-beaked echidna are elongated, slender skulls with narrow snouts. Skulls C (Common wombat) & D (Black wallaroo) are skulls with flattened, broad snouts and prominent incisors. Skulls E (Grizzly bear) & F (Fisher cat) are robust, heavy-set skulls with short snouts and strong zygomatic arches (cheekbones). Skulls G (Wolverine) & H (Tiger) are sharp-toothed skulls with prominent canines. Skull I (Chacma baboon) is a specialized skull featuring a massive, downward-pointing upper canine. Skull J (Domestic horse) is a long, low skull with a specialized snout. Skull K (Orca) is unique skull with a long, thin snout filled with numerous small, uniform teeth. Skull L (Vampire bat) is a rounded, high-domed braincase with a short snout and front-facing eye sockets.<\/p>\r\n

Figure 8.17: This image illustrates the evolutionary development of the hard palate across three different stages of synapsid evolution, viewed from both the side (lateral) and the bottom (ventral). The diagram compares three specific groups: Early synapsid shows a primitive arrangement where the palate is largely open, Therapsid shows an intermediate stage with the bones beginning to move toward the midline, and Canis familiaris (Domestic Dog) represents the modern mammalian condition with a fully formed bony secondary palate. Three primary bones are highlighted to show their shifting positions and expansion over time: blue (Premaxilla) is located at the very front of the snout, yellow (Maxilla) is the primary tooth-bearing bone of the upper jaw, which expands inward in mammals, red (Palatine) is located toward the back of the mouth, which also grows inward to complete the roof of the mouth. The lateral view (left column) shows the elongation of the skull and the positioning of the jaw bones from the side. The ventral view (right column) provides a clear look at how the Maxilla and Palatine bones gradually meet at the midline to separate the nasal passage from the oral cavity, a defining characteristic of mammalian evolution that allows for simultaneous eating and breathing.<\/p>\r\n

Figure 8.18: This diagram, titled \"Jaw Evolution Theories,\" presents two major scientific frameworks for how vertebrate jaws originated from ancestral structures. The Gill Arch Theory section illustrates the classical view that jaws evolved from modified gill (branchial) arches. The initial stage shows a primitive jawless head with a series of vertical gill arches. The first three arches are color-coded: blue, orange, and green. The Serial Theory suggests that the first gill arch (orange) migrated forward and enlarged to become the entire jaw, while the second arch (green) became the hyoid support. The Composite Theory proposes a more complex origin where parts of multiple anterior arches (blue and orange) fused together to form the upper and lower jaw structures. The Heterotopic Theory section explores the developmental and genetic origin of jaws based on neural crest cells. The lamprey crest cell pattern displays a jawless lamprey where a specific neural crest cell population (purple) forms a long, singular arched structure. The proposed gnathostome crest cell pattern illustrates the shift in \"jawed\" vertebrates (gnathostomes), where a change in the location of gene expression (heterotopy) caused the neural crest cells to differentiate into distinct upper (yellow) and lower (purple\/pink) jaw elements.<\/p>\r\n

Figure 8.19: This diagram, titled \"Types of Jaw Suspension,\" outlines the evolutionary variations in how the upper jaw attaches to the skull in different vertebrate groups. The structures are color-coded for clarity. Blue is the chondrocranium (the skull\/braincase). Green is the Mandibular Arch (the upper and lower jaw bones). Purple is the Hyomandibula (the bone derived from the second gill arch). Primitive Autostylic is found in early jawed vertebrates. The upper jaw is attached directly to the chondrocranium without any help from the hyomandibula. The Amphistylic, seen in early Chondrichthyes and early bony fishes shows the jaw has a dual attachment: it is supported by both a direct ligamentous connection to the skull and by the hyomandibula. Hyostylic shows derived Chondrichthyes and derived bony fishes. The primary attachment of the jaw is via the hyomandibula, which acts like a swinging hinge. Secondary Autostylic is found in Dipnoans and Tetrapods. The upper jaw is fused directly to the skull. Holostylic is specific to Holocephalans. The upper jaw is completely fused to the chondrocranium, forming a single rigid unit, which is an adaptation for crushing hard-shelled prey.<\/p>\r\n

Figure 8.20: This diagram illustrates jaw articulation shown through four representative skulls. The diagram tracks four specific bones to show their change in size and function: Blue (Dentary) is the primary bone of the lower jaw. Yellow (Squamosal) is part of the skull that eventually forms the socket for the mammalian jaw joint (the glenoid fossa). Pink (Quadrate) is the upper jaw joint. Green (Articular) is part of the lower jaw joint. Basal cynodont displays many bones in the lower jaw and a jaw joint located at the back of the skull. Therapsid shows the expansion of the dentary bone and the reduction of the posterior jaw bones. Basal mammal shows the dentary bone now makes up the majority of the lower jaw, making direct contact with the skull. Didelphis virginianus represents the modern mammalian condition where the jaw joint is formed solely by the dentary and squamosal bones.<\/p>\r\n

Figure 8.21: This diagram illustrates the evolutionary transformation of the jaw joint bones into the mammalian middle ear, viewed from the side (lateral) focusing specifically on the lower jaw and its hinge. The diagram uses consistent colors to track the fate of each bone. Blue (Dentary) is the primary tooth-bearing bone. It expands to become the entire lower jaw in mammals. Red (Angular) becomes the ectotympanic bone, which supports the eardrum in mammals. Yellow (Squamosal) is the part of the skull that articulates with the dentary to form the mammalian jaw joint. Green (Articular) migrates to the ear to become the malleus (hammer). Purple (Quadrate) migrates to the ear to become the incus (anvil). The sequence tracks the reduction and migration of jaw bones through five stages. Dimetrodon is a basal synapsid with a large, multi-boned lower jaw. Basal cynodont shows the dentary bone starting to expand toward the back of the jaw. Therapsid shows the posterior bones become significantly smaller as the dentary continues to grow. Basal mammal shows that the dentary forms a new, direct joint with the skull (squamosal), and the old jaw bones are nearly detached. Didelphis virginianus shows the modern mammalian condition where the old jaw bones have completely migrated to the middle ear.<\/p>\r\n

Figure 8.27: Scientific illustration diagram showing eleven labeled panels (A\u2013K) comparing diverse mammalian tooth types through detailed anatomical line drawings. A (Secodont\/Carnassial Teeth) are comparative drawings of last upper premolars and first lower molars from cat, dog, and bear, with sharp cusps labeled. B (Denticulate) is a multi-cusped tooth from a crab-eater seal with a denticulate crown. C (Triconodont) is a fossil mammal tooth with three cones arranged in a row. D (Trituberculate) is a fossil mammal tooth with three cones in a triangular arrangement. E (Bunodont) is a vertical cross-section of a human or monkey molar showing internal anatomy including enamel, dentine, pulp cavity, cement, neck, and root, with low rounded cusps on the crown. F (Brachydont Selenodont) is a tapir tooth with a small crown, uneven grinding ridges, and visible root. G (Brachydont) is a surface view of a tooth showing crescentic enamel ridge, dentine, and cement. H (Hypsodont Selenodont) shows tall prism-like crowned teeth with uneven grinding ridges of enamel. I (Hypsodont Tooth in cross-section) is a vertical section comparing unworn (left) and worn (right) states, labeling enamel, dentine, cement, and pulp cavity. J (Lophodont) shows three views of teeth with transverse ridges. K (Lophodont of elephant) is a surface view of an elephant molar showing transverse ridges (lophs) and crescentic enamel ridges.<\/p>\r\n

Figure 9.2: An anatomical diagram showing the structure of human vertebrae from two different perspectives, with detailed labels for the bony landmarks and associated neural structures. The illustration on the left shows a single vertebra from a top-down perspective. The Body is a large, circular bony mass located at the Anterior (front) side. The Vertebral Foramen is the central opening that houses the Spinal cord (depicted in yellow). The Vertebral Arch is formed by the Pedicle (sides) and Lamina (roof). The Spinous process points toward the Posterior (back). The transverse processes extend out to the sides. Facets includes the Facet of superior articular process and the Facet for head of rib. The illustration on the right shows a stack of three articulated vertebrae from a side\/back angle. The diagram is labeled with Anterior toward the left (the bodies) and Posterior toward the right (the spinous processes). Intervertebral structures show the Intervertebral discs sandwiched between the vertebral bodies. The Neural Exit highlights a Spinal nerve exiting through the intervertebral foramen, which is the gap between adjacent vertebrae. Joint Connections point out where vertebrae connect, specifically the Inferior articular process of one vertebra meeting the Superior articular process of the one below it.<\/p>\r\n

Figure 9.11: An anatomical diagram of the human rib cage and breastbone, divided into two labeled sections: (a) an isolated anterior view of the sternum and (b) a full anterior view of the skeletal thorax. The sternum image shows the isolated breastbone from the front, oriented with Superior at the top and Inferior at the bottom. The Manubrium is the wide, top portion containing the Jugular notch in the center and Clavicular notches on the sides. The sternal angle is the horizontal joint where the manubrium meets the body. The body is the long, central part of the sternum. The Xiphoid process is the small, pointed tip at the bottom. The view of the skeleton of the thorax image shows the sternum in its anatomical position, connected to the ribs and clavicles. Sternum Connections show the Clavicle (collarbone) attaching to the clavicular notch and the Scapula (shoulder blade) in the background. Ribs are numbered 1 through 12. Ribs 1\u20137 are \"true ribs\" connected directly to the sternum via Costal cartilages. Ribs 8\u201310 are \"false ribs,\" and 11\u201312 are \"floating ribs.\" The Intercostal space are the gaps between the ribs are labeled. The T11 and T12 thoracic vertebrae are visible at the bottom of the rib cage.<\/p>\r\n

Figure 9.14: An anatomical diagram illustrates the first two cervical vertebrae, the atlas (C1) and the axis (C2), showing their unique structures and how they articulate. The superior view of atlas is a top-down view (top left) that of the C1 vertebra shows its ring-like shape, which lacks a traditional vertebral body. It is composed of an Anterior arch and a Posterior arch. Large, oval Superior articular facets are visible, which articulate with the skull. The Transverse processes on the sides contain the Transverse foramen. The Dens (part of the axis) is shown positioned against the anterior arch, held in place by a transverse ligament. The superior view of axis (top right) is a top-down view of the C2 vertebra. The Dens is prominent, finger-like projection that extends upward. The Vertebral Arch includes the Lamina and a bifurcated Spinous process at the rear. Facets shows the Superior articular facets flanking the Dens. The anterior view of axis (bottom right) is a front-facing view of the C2 vertebra. The Dens is the top-most point of the bone. Unlike the atlas, the axis has a central body located below the Dens. Labeled features include the Transverse process and the Inferior articular process, which connects to the C3 vertebra below.<\/p>\r\n

Figure 9.20: An anatomical illustration of the human vertebral column (spine) shown from both a posterior (rear) and right lateral (side) view within the silhouette of a human body. The posterior view (left) shows the spine as a straight vertical column. It highlights various divisions. 7 Cervical vertebrae (C1\u2013C7) are the top-most section located in the neck. 12 Thoracic vertebrae (T1\u2013T12) are the middle section corresponding to the rib cage. 5 Lumbar vertebrae (L1\u2013L5) are the lower back region. Intervertebral disc is the cushioning space between individual vertebrae. The sacrum is a large, triangular bone at the base of the spine. The coccyx is the small \"tailbone\" at the very bottom. The lateral view (right) highlights the natural S-shaped curves of the spine. The cervical curve is a concave (inward) curve of the neck. The thoracic curve is a convex (outward) curve of the upper and middle back. The lumbar curve is a concave (inward) curve of the lower back. The Sacrococcygeal curve is a convex (outward) curve formed by the fused vertebrae of the sacrum and coccyx.<\/p>\r\n

Figure 10.2: An educational diagram comparing two primary evolutionary hypotheses for the origin of paired fins in fish: the Fin Fold Hypothesis and the Gill Arch Hypothesis. Fin Fold Hypothesis (Left Side) uses two fish illustrations to show the transition from continuous folds to distinct fins. The top image shows a primitive fish with continuous longitudinal skin folds along the body. Labels include the Median Fin Fold (blue, along the dorsal side) and Paired Fin Folds (red, along the ventral\/lateral sides). The bottom image shows a more evolved stage where the continuous folds have broken up into localized segments. Labels point to the resulting Pectoral Fin (anterior) and Pelvic Fins (posterior). Gill Arch Hypothesis (Right Side) uses a detailed skeletal diagram of a fish head and thoracic region to suggest a different anatomical origin. It highlights the Gill Arch (yellow) and the Branchial Rays (small red structures extending from the arch), Pectoral girdle and Pectoral fin (along the ventral side).<\/p>\r\n

Figure 10.3: This black-and-white line drawing illustrates the skeletal structure of a shark's pectoral and pelvic fins, showing the internal cartilaginous elements and fin rays. The pectoral fin (left) structure is attached to the Coracoid Bar. It features three basal cartilages that support the rest of the fin labeled with Propterygium: the most anterior (front) basal cartilage. Mesopterygium: the middle basal cartilage. Metapterygium: the most posterior (back) basal cartilage. Radials: a series of rows of small cartilaginous segments extending from the basals. Ceratotrichia: long, slender fibrous fin rays that make up the outer portion of the fin. The pelvic fin structure (right) is attached to the Iliac Process. Its structure is slightly simpler than the pectoral fin labeled with Propterygium: a small anterior basal cartilage. Metapterygium: a long, prominent posterior basal cartilage that supports the majority of the fin.\u00a0 Radials: cartilaginous rods extending outward from the metapterygium. Ceratotrichia: fibrous fin rays forming the trailing edge of the fin.<\/p>\r\n

Figure 10.9: This black-and-white scientific illustration displays the skeletal anatomy of a primitive tetrapod, showcasing the structure of both the pelvic girdle and the pectoral girdle. The top of the image shows a full skeletal reconstruction of the animal, likely an early land-dwelling vertebrate like Ichthyostega. The Pelvic Girdle and Hindlimb (left side) are shown with the three primary bones of the hip that are labeled: the Ilium (upper), the Ischium (posterior), and the femur, pubis, tibia and fibula (anterior). The lower left diagram highlights the Acetabulum, the deep socket where the femur attaches to the hip. It shows the ilium (upper), ischium (posterior), and pubis (anterior). The Pectoral Girdle and Forelimb (Right Side) shows dermal and endoskeletal elements. Labeled parts include the Cleithrum (a tall, blade-like bone), the Clavicle, and the Interclavicle.\u00a0 The lower diagram highlights the Glenoid fossa, the socket where the humerus attaches to the shoulder. The limb includes the Humerus (upper arm), Radius, and Ulna (forearm), leading to the digits of the front foot.<\/p>\r\n

Figure 10.19: This image provides a detailed anatomical view of the human scapula (shoulder blade), showing the bone from three different perspectives with key landmarks labeled. Anterior View (Left) is the front-facing side of the scapula that rests against the rib cage. It includes the Subscapular Fossa: the broad, slightly concave surface that makes up the \"body\" of the bone from this view with the top ridge (Superior border), right ridge (Medial border), and left ridge (Lateral border); Coracoid Process: the hook-like projection at the top that serves as an attachment point for various muscles and ligaments; Acromion Process: the bony tip of the shoulder that articulates with the clavicle (collarbone); Glenoid Cavity: the shallow socket where the head of the humerus (upper arm bone) fits to form the shoulder joint.\u00a0 The Lateral Edge (Center) is side-on view highlighting the thickness of the bone and the alignment of the joints. It includes the Acromion process (bony tip); Coracoid process (hook-like projection); Spine (visible as a ridge protruding from the back); Glenoid Cavity (shown clearly as the cup-like socket for the arm); Lateral and Medial Borders (the outer and inner edges of the bone, respectively). The Posterior View (Right) is the back-facing side of the scapula. It includes the spine (the prominent horizontal ridge that divides the back of the scapula; Supraspinous Fossa (the area above the spine); Infraspinous Fossa (the much larger area below the spine); Acromion and Coracoid Processes (both are visible at the top, showing how they extend outward to protect the shoulder joint).<\/p>\r\n

Figure 11.4: A multi-level anatomical diagram illustrating the hierarchical structure of a skeletal muscle fiber. The top view section shows a single cylindrical muscle fiber (muscle cell) and its internal components including the Sarcolemma (the plasma membrane surrounding the muscle fiber); Sarcoplasm (the cytoplasm of the muscle cell, containing multiple nuclei and mitochondria); Myofibrils (long, rod-like contractile organelles that fill the muscle fiber). One myofibril is shown pulled out to reveal its structure. Striations show the alternating Light I bands and Dark A bands that give skeletal muscle its striped appearance. The bottom is an enlarged view of a single myofibril that includes the segment of a myofibril between two Z discs; thin (actin) filaments shown as light green lines attached to the Z discs; thick (myosin) filaments shown as thick purple lines centered in the sarcomere. Z disc is the boundary of the sarcomere where thin filaments are anchored. M line is the vertical line in the center of the sarcomere that holds thick filaments together. H zone is the central region of the A band where only thick filaments are present (no overlap with thin filaments). A band is the dark region spanning the full length of the thick filaments. I band is the light region containing only thin filaments, spanning across two adjacent sarcomeres. Sarcoplasmic Reticulum is a lacy, yellow network of tubules surrounding the myofibril.<\/p>\r\n

Figure 11.5: An anatomical diagram of the organization of the sarcomere. The image breaks down the relationship between thick and thin filaments and their specific protein structures. The top view is the sarcomere structure, spanning from one Z line to the next. The darker A band is the central region where thick filaments (purple) and thin filaments (green) overlap. The lighter I band is the outer regions containing only thin filaments. The H zone is the center of the A band where only thick filaments are present. The M line is the vertical structure in the very center that anchors the thick filaments. The middle view are the filament overviews. The portion of a thick filament (Left) show a thick purple rod covered in protruding globular structures called Heads. The portion of a thin filament (Right) shows a twisted strand of green actin beads, wrapped with orange Tropomyosin threads, and studded with yellow Troponin complexes. The bottom view shows molecular details. The Myosin Molecule (Left) shows an individual myosin protein consisting of a long Tail, a Flexible hinge region, and two Heads. Each head contains specific Actin-binding sites and an ATP-binding site. The Actin Subunits (Right) show a close-up of the green actin chain. Each actin bead has a dark \"Binding site for myosin,\" which is currently covered by the orange tropomyosin strand in a relaxed state.<\/p>\r\n

Figure 11.7: An anatomical diagram illustrating relative position of the thick and thin filaments during sarcomere contraction. The top view shows the relaxed sarcomere. This section shows the sarcomere at rest, where there is minimal overlap between the filaments. The purple thick (myosin) filaments are centered, while the green thin (actin) filaments are anchored to the Z discs on either side. The lighter I band are wide regions at the ends of the sarcomere containing only thin filaments. The H zone is a wide central region containing only thick filaments. The darker A band is the full length of the thick filaments, including the areas where they overlap with the thin filaments. The M line is the vertical structure in the very center that anchors the thick filaments. The bottom view is the contracted sarcomere. This section shows the changes that occur during contraction, indicated by large black arrows pointing inward from the sides. The thin filaments have been pulled toward the center (M line) by the thick filaments. The distance between the Z discs has decreased, shortening the entire sarcomere. The Lighter I band has narrowed significantly. The H zone has almost entirely disappeared as the thin filaments slide into the center. The width of the Darker A band remains unchanged.<\/p>\r\n

Figure 12.1: This image provides a detailed anatomical breakdown of a skeletal muscle, illustrating its hierarchical structure from the whole organ down to the microscopic level. The diagram uses three levels of \"zoom\" to show how muscle tissue is organized. The top level (whole muscle) includes: skeletal muscle (the entire organ, wrapped in an outer layer of connective tissue called the Epimysium) and the muscle fascicle (a bundle of muscle fibers within the whole muscle, surrounded by a layer called the Perimysium). The middle level (fascicle detail) is shown in cross-section, revealing that it is composed of multiple muscle fibers (individual muscle cells). Each individual muscle fiber is encased in a thin connective tissue layer called the Endomysium. Satellite cells are shown located on the exterior of the muscle fibers. The bottom level (muscle fiber detail) is shown with its plasma membrane, known as the sarcolemma. Inside the fiber are numerous rod-like structures called myofibrils, which are the actual contractile elements of the cell.<\/p>\r\n

Figure 12.4: This medical illustration displays a full-body view of the human muscular system, highlighting seven different fascicle arrangements that determine a muscle's range of motion and power. The central figure is an anterior view of a human with arrows pointing to specific muscle examples. Circular (Orbicularis oris) show fascicles are arranged in concentric rings around the mouth. Multipennate (Deltoid) looks like many feathers side-by-side, with all their quills (tendons) inserting into one large tendon. Convergent (Pectoralis major) shows the muscle has a broad origin, and the fascicles converge toward a single tendon of insertion. This gives the muscle a triangular or fan-like shape. Parallel - Fusiform (Biceps brachii) shows the fascicles run parallel to the long axis of the muscle, which has an expanded midsection (belly) and tapers at each end. Parallel - Non-fusiform (Sartorius) shows the fascicles run parallel to the long axis of the muscle in a strap-like fashion without a central bulge. Unipennate (Extensor digitorum) shows short fascicles attach obliquely to only one side of a central tendon that runs the length of the muscle. Bipennate (Rectus femoris) shows fascicles insert into the tendon from opposite sides, resembling the structure of a feather.<\/p>\r\n

Figure 12.5: This comparative anatomy illustration shows muscle groups between a shark (top\/right) and a cat (bottom\/left). The muscles are color-coded to indicate their developmental origins and functional groups. The diagram uses two views: (a) Lateral view and (b) Ventral view, categorized by the following legend. Extrinsic Eye (Green) are small muscles responsible for moving the eyeball within the socket. Hypobranchial (Red) are located ventrally (on the underside) of the throat. Axial (Blue) are the trunk muscles. In the shark, they form the bulk of the body for swimming. In the cat, they are seen along the spine and ribcage, though they are partially covered by limb muscles. Branchiomeric (Yellow) are muscles associated with the pharyngeal arches. In the shark, these power the jaws and gill arches. In the cat, these have evolved into facial muscles, mastication (chewing) muscles, and throat muscles. Appendicular (Grey) are muscles of the fins or limbs. These are relatively small in the shark but are much more extensive and complex in the cat.<\/p>\r\n

Figure 12.6: This anatomical illustration identifies the extrinsic eye muscles of a shark versus a human eye. The image is divided into two main sections: (a) a dorsal view of the head and (b) detailed views of the individual eye. The dorsal view (a) shows the top-down perspective of the head, illustrating how the muscles originate from the skull's midline and fan out to the eyeballs. The lateral & frontal detail (b) provides a closer look at the \"cross\" pattern formed by the rectus muscles around the eyeball and the specific insertion points of the oblique muscles. The muscles are color-coded and labeled based on their position and function. Superior Rectus (green) is located on the top of the eyeball. Inferior Rectus (orange) is located on the bottom of the eyeball. Medial Rectus (purple) is located on the side of the eye closest to the midline (nose\/brain). Lateral Rectus (blue) is located on the outer side of the eye.\u00a0 Superior Oblique (red) are positioned at the top-front of the eye. Inferior Oblique (yellow) is positioned at the bottom-front of the eye.<\/p>\r\n

Figure 12.8: This anatomical illustration provides a detailed view of the human abdominal wall muscles, highlighting both the superficial and deep layers of the torso. The image features a lateral-anterior view of the male torso, with a \"window\" cut-out on the lower abdomen to reveal the underlying muscle layers. The major upper torso muscles include: Pectoralis major (the large muscle of the chest), Latissimus dorsi (the broad muscle of the back, visible on the side), and Anterior serratus muscles (the finger-like muscle projections along the ribs). The abdominal core includes: the external oblique (the outermost layer of the abdominal wall, with fibers running diagonally downward, Rectus abdominis (the vertical \"six-pack\" muscle, shown enclosed within the Rectus sheath), tendinous intersections (the horizontal bands of connective tissue that divide the rectus abdominis into segments), and Linea alba (the central white line of connective tissue that runs down the midline of the abdomen). The magnified cut-away section shows the layering of the abdominal wall from superficial to deep: external oblique (outermost), internal oblique (middle layer), and transversus abdominis (deepest layer, with fibers running horizontally).<\/p>\r\n

Figure 12.9: This comprehensive anatomical chart displays the major muscles of the human body from both an anterior (front) view and a posterior (back) view. The top diagram illustrates the superficial muscles visible from the front, categorized by body region. The head and neck include the Occipitofrontalis (frontal belly) for facial expression and the Sternocleidomastoid. The torso features large muscles like the Pectoralis major (chest), Rectus abdominis (abs), and Abdominal external oblique. Deep to the chest is the Pectoralis minor and the rib-associated Serratus anterior. The arms show the Deltoid (shoulder), Biceps brachii, and various forearm muscles like the Brachioradialis and Flexor carpi radialis. The legs highlight the Quadriceps group (Rectus femoris, Vastus lateralis, and Vastus medialis), the long Sartorius muscle, and lower leg muscles like the Tibialis anterior. The bottom diagram shows the muscles from the back, including some deeper muscles revealed via dissection. The head and neck show the Occipitofrontalis (occipital belly) and the Splenius capitis. The upper Back and Shoulder features the large, diamond-shaped Trapezius, the Rhomboids, and rotator cuff muscles like the Supraspinatus and Infraspinatus. The Latissimus dorsi covers much of the lower back. The arms focus on the Triceps brachii and the extensor muscles of the forearm, such as the Extensor digitorum. The gluteal region and legs show the Gluteus maximus and Gluteus medius. The back of the thigh features the hamstrings (Biceps femoris, Semitendinosus, and Semimembranosus). The calf is dominated by the Gastrocnemius and Soleus.<\/p>\r\n

Figure 12.10: This medical illustration shows the posterior (rear) view of the human pelvic and gluteal region, detailing the layers of muscle and the location of the sciatic nerve. The image is divided into a superficial view on the left and a deeper, dissected view on the right. The gluteus maximus is shown on the left side of the image and is the largest and most superficial muscle of the buttocks. The gluteus medius revealed on the right side after the gluteus maximus has been removed; it sits deeper and higher on the pelvis. The piriformis is a pear-shaped muscle located deep to the gluteus maximus. The superior gemellus is a small muscle located just below the piriformis. The quadratus femoris is a flat, quadrilateral muscle located further down. The sciatic nerve is depicted as a thick, yellow cord-like structure. It emerges from the pelvis typically just below the piriformis muscle and runs down the back of the leg.<\/p>\r\n

Figure 13.6: A flowchart illustrating the digestive process of various substrates through extracellular and intestinal mucosal enzymes. The chart is organized into three main categories: Carbohydrates, Lipids, and Protein. The first category are the carbohydrates. Soluble alpha-1,4-linked polysaccharides that are broken down by amylolytic enzymes into intermediate products (maltose, isomaltose, and alpha-1,4-linked oligosaccharides). These are then converted by maltase, isomaltase, and alpha-glucosidase into the end product, glucose. Laminarin is broken down by laminarinase into laminaribiose, which is then converted by beta-glucosidase into glucose. Chitin is broken down by chitinase into chitobiose, which is then converted by chitobiase and N-acetyl-beta-D-glucosaminidase into N-acetyl-glucosamine. Trehalose is converted by trehalase directly into glucose. Sucrose is converted by sucrase directly into glucose and fructose. The second category are the lipids. Triglycerides, phospholipids, and waxes are broken down by lipase into monoacylglycerides and fatty acids. The third category, protein, is broken down by pepsin, trypsin, and chymotrypsin into intermediate products (polypeptides and oligopeptides). These intermediates are further broken down by carboxypeptidases into dipeptides. Finally, peptidases convert the dipeptides into the end product, amino acids.<\/p>\r\n

Figure 13.7: Three-panel scientific scatter plot figure comparing intestinal absorption characteristics across vertebrate groups, all using logarithmic axes. Panel A: Log-log plot of nominal intestinal surface area (cm\u00b2) versus body mass (g) comparing birds (open triangles) and non-volant mammals (open circles), with a single regression line showing a strong positive relationship across both groups spanning body masses from roughly 5 g to 100,000 g. Panel B: The same nominal area versus body mass plot comparing bats (inverted open triangles) and non-volant mammals (open circles), again with a regression line. Statistical results are inset: body mass P < 0.001, taxon P < 0.001, and mass \u00d7 taxon interaction P = 0.013, indicating bats have significantly smaller intestinal surface area relative to body mass than non-volant mammals. Panel C: Log-log plot of fractional paracellular absorption versus body mass (g) comparing birds (open circles) and non-volant mammals (X marks). Birds show a steeply declining regression line with higher paracellular absorption at small body sizes, while non-volant mammals show a nearly flat regression line close to zero across all body sizes, suggesting birds rely more heavily on paracellular nutrient absorption, especially at smaller body masses.<\/p>\r\n

Figure 13.8: A scientific infographic comparing gene sequences, physical appearance, and enzyme activity across five different fish species. The data is presented in a horizontal row for each species. Each species has a gene map, a sequence of pointed blocks representing genes on a chromosome. Blue blocks represent amylase genes (amy2A, amy2B). Gray blocks represent flanking genes (col11a1, ntng1). Each species has an Amylase activity in \u03bcmol'min-1'g-1. The top species, C. violaceus, (a stout, mottled gray-green fish photographed on gravel has a gene sequence col11a1, amy2B, amy2A, amy2A, ntng1 with amylase activity 23.17 \u00b1 5.25. Next is X. mucosus (a slender, elongate olive-yellow fish) with a sequence of col11a1, amy2A, amy2A, ntng1 and amylase activity 43.83\u00b131.53. Next is X. atropurpureus (a slender reddish-brown elongate fish) with a sequence of col11a1, amy2A, amy2A, ntng1 and amylase activity of 55.58\u00b123.09. Next is P. chirus (a very slender, uniform olive-brown elongate fish) with a sequence of col11a1, amy2A, ntng1 with amylase activity of 0.74\u00b10.84. Last is A. purpurescens (a brightly colored orange and dark salamander or amphibian photographed on rocks) with a sequence of col11a1, amy2A, ntng1 and amylase activity of 0.23\u00b10.16.<\/p>\r\n

Figure 14.3: A scientific illustration comparing the respiratory systems of four different animals: a fish (Bowfin), a mammal (Eastern gray squirrel), a bird (Blue jay), and an amphibian (Green frog). The image is divided into four vertical columns (A, B, C, D) that zoom in from the organism level down to the microscopic mechanism of blood oxygenation. A color key at the bottom identifies light blue for deoxygenated water\/air, teal for oxygenated water\/air, dark blue for deoxygenated blood, and red for oxygenated blood. Column A (bowfin) starts with the fish, then a gill, secondary lamellae (showing water flowing across thin plates), ending in the countercurrent exchange showing water and blood flow in opposite directions, maintaining a concentration gradient that maximizes oxygen absorption. Column B (eastern gray squirrel), shows mammalian lungs, then the alveoli (appearing as clusters of tiny air sacs), ending in the tidal mechanism that shows that air moves in and out of the same sacs Oxygenated air (teal) enters the sac, and deoxygenated blood (blue) flows around it to become oxygenated (red). Column C (blue jay) shows avian lungs (featuring a complex system of air sacs), parabronchus (shown as a tube with perpendicular vessels), ending in a crosscurrent exchange that shows air flows through the parabronchus in one direction, while blood vessels cross the airflow at angles, allowing for highly efficient oxygen uptake. Column D (green frog) shows the skin (cutaneous respiration), then the skin surface with a dense network of capillaries just beneath the outer layer, and ending with an open mechanism that shows that oxygen from the environment diffuses directly through the moist skin surface into the underlying blood vessels.<\/p>\r\n

Figure 14.6: An anatomical diagram illustrating the development of branching lungs in humans. The process is shown in a clockwise sequence of eight stages. Beginning of fourth week, the process starts with the laryngotracheal tube forming near the pharynx, surrounded by splanchnic mesoderm. At the Tracheal Bud Formation, the tube develops two small protrusions known as tracheal buds. At separation, the diagram shows the respiratory tract separating from the esophagus, with the trachea and tracheal buds becoming more distinct. At the end of fourth week, the tracheal buds have expanded into bronchial buds.\u00a0 As development continues, the trachea bifurcates (splits) clearly into the primary bronchial buds. The bronchial buds undergo further branching to form secondary buds, which will eventually become the lobes of the lungs. The bronchial buds develop into an increasingly complex, tree-like structure of smaller airways. At eight weeks, the final stage shows the fully formed early lungs. The right lung is clearly labeled with three distinct sections: the upper lobe, middle lobe, and lower lobe. The left lung is labeled with two sections: the upper lobe and lower lobe. The internal structure shows a dense, branching network of bronchi within the lung tissue.<\/p>\r\n

Figure 14.8: Scientific illustration comparing egg types across three vertebrate groups on a white background, arranged left to right in evolutionary order. Fish (500 mya): A small circular diagram showing a simple embryo (pink) surrounded by a large yolk sac (yellow) within a minimal membrane, representing the most ancestral condition. Reptiles and birds (300 mya): A larger oval diagram showing the amniotic egg with four extraembryonic membranes labeled the chorion (outermost), allantois (branching structure in green and yellow), amnion (surrounding the embryo), and yolk sac (yellow, labeled at top) with a pink embryo visible within the fluid-filled amnion cavity. Mammals (160 mya): A diagram of the uterine environment showing a similar arrangement of embryonic membranes, but with the chorion replaced by a highly vascularized placenta (shown in red at top) connecting to the uterine wall, replacing the eggshell. Approximate ages of evolutionary origin are noted beneath each group in millions of years ago (mya).<\/p>\r\n

Figure 14.9: A phylogenetic tree titled \"Evolutionary History and Diversity of Respiratory Structures: Part 1,\" which tracks the development of gills, lungs, and gas bladders across various fish lineages. The tree uses a color-coded system for its branches: blue represents respiratory structures for water (gills), yellow represents respiratory structures for air (lungs\/gas bladders), black indicates non-respiratory structures, red text indicates ventilation mechanisms, and green text indicates specific respiratory organs. Lamprey and sharks branch off early, utilizing gills for water respiration and buccal pumping for gill ventilation. The Osteichthyes (Bony Fish) node introduces lungs and buccal pumping for lung ventilation. It splits into two primary groups: Sarcopterygii (Lobe-finned fishes) which maintains the use of lungs (yellow line) and Actinopterygii (Ray-finned fishes) that shows significant diversification of air-breathing structures. Polypterus utilizes recoil aspiration for ventilation. Gar features a physostomous gas bladder used for respiration. The tree shows a transition from respiratory gas bladders to non-respiratory gas bladders, and eventually to physoclistous gas bladders (seen in higher ray-finned fish). The armored catfish uses the intestinal tract for respiration. Mudskippers utilize the buccal cavity. Anabantids possess a specialized labyrinth organ for breathing air.<\/p>\r\n

Figure 14.10: A phylogenetic tree titled \"Evolutionary History and Diversity of Respiratory Structures: Part 2,\" continuing the evolutionary narrative of respiration from bony fish to modern tetrapods. The tree uses a color-coded system for branches: blue for water-based respiration (gills\/skin), yellow for air-based respiration (lungs\/skin), black for non-respiratory structures, red text is the ventilation mechanism, and green text is the specific respiratory organs. Osteichthyes (Bony Fish) are the base of the tree that shows the ancestral state of unicameral lungs. Actinopterygii (Ray-finned fishes) diverge to the left, primarily utilizing gills (blue line). Sarcopterygii (Lobe-finned fishes) diverge to the right, leading toward land-dwelling vertebrates. Lungfish retain the use of both water and air respiration. Tetrapods (Frogs) introduce the use of skin for both water and air respiration (blue and yellow labels) alongside lungs. A major evolutionary shift occurs at the Amniota node, characterized by the loss of gills in adults and the use of axial muscles for exhalation. Mammals branch off with the development of a diaphragm and complex alveolar lungs. Lizards utilize multicameral lungs and costal aspiration (using ribs). Turtles evolved to use abdominal muscles for ventilation due to their rigid shells. Crocodiles feature a specialized hepatic piston mechanism for breathing. Birds represent the most specialized branch, featuring parabronchial lungs and a complex system of air sacs.<\/p>\r\n

Figure 15.2: Medical illustration titled \"The Structure of an Artery Wall\" showing two views of an artery. On the left, a cross-sectional end-on view of the artery depicts the hollow central lumen (shown in purple\/pink) surrounded by concentric layers of the vessel wall, with a boxed region indicating the area magnified on the right. On the right, a detailed magnified lateral cutaway view identifies six labeled structural layers from outermost to innermost: Tunica externa (outermost connective tissue layer), Tunica media (middle layer, labeled at top), Tunica intima (innermost layer, labeled at top), smooth muscle (making up the bulk of the tunica media, shown in deep red fibrous tissue), external elastic membrane (boundary between tunica externa and media), internal elastic membrane (boundary between tunica media and intima), and Endothelium (the innermost cellular lining of the vessel lumen). The illustration uses warm red and burgundy tones to represent the muscular and connective tissue components.<\/p>\r\n

Figure 15.7: Scientific illustration comparing heart development and evolution across four stages, displayed left to right with corresponding ECG traces beneath each. Heart tube (leftmost, gray) shows a simple curved tube representing the earliest embryonic heart, with the outflow tract (oft) at the top and inflow tract (ift) at the bottom, and a simple sine-wave ECG below. Ballooning chambers (second, gray and yellow) show the heart tube has begun to loop and balloon into distinct chambers, with the right ventricle (rv), left ventricle (lv), atrioventricular canal (avc), atrium (a), and outflow tract (oft) labeled, accompanied by a more complex ECG. A formed fish heart (third, blue and yellow) shows a single ventricle (v), atrium (a), atrioventricular canal (avc), and outflow tract (oft), with a corresponding ECG. Formed human heart (fourth, blue and yellow) has a fully formed four-chambered heart with right ventricular outflow tract (rvot), aorta (ao), left atrium (la), left ventricle (lv), and atrioventricular canal (avc) labeled, shown with a full human ECG trace.<\/p>\r\n

Figure 15.12: Comparative schematic diagram showing the evolution of aortic arches and their connection to lungs across seven groups, arranged left to right in a series of color-coded anatomical diagrams. Red vessels represent arteries (oxygenated or outflow), blue vessels represent veins or deoxygenated blood, and pink\/oval shapes represent lungs or associated organs. The first diagram is a fish-like pattern with multiple paired gill arch arteries in red and blue, no lungs, representing a fully aquatic gill-breathing vertebrate. The second diagram is a similar but slightly reduced gill arch pattern, possibly representing a more derived fish. The third diagram is a lungfish form with a white heart outline showing internal division, blue and red vessels, and paired pink lung-like structures with partial cardiac separation. The fourth diagram shows further reduced arches, a more defined heart, and prominent paired pink lungs, representing an amphibian. The fifth diagram is a similar pattern with a white\/gray heart region, representing a mammal heart with partial or incomplete separation. The sixth diagram shows red and blue vessels with a purple element possibly indicating a pulmonary vessel, and pink lungs, representing a more derived reptile or bird. The seventh diagram is the most derived pattern with clearly separated red and blue circulations, a compact heart, and pink lungs, representing a bird with full cardiac separation.<\/p>\r\n

Figure 15.13: A comparative anatomical illustration showing heart morphology across nine vertebrate species arranged in three rows, all drawn as anterior view line diagrams on a white background. A dashed horizontal line on each heart indicates the boundary between atrial and ventricular regions. Labeled chambers and structures vary by species and are abbreviated (VA = ventral aorta, Ven = ventricle, A = atrium, SV = sinus venosus, C = conus arteriosus, B = bulbus arteriosus, RA = right atrium, LA = left atrium, RV = right ventricle, LV = left ventricle, P = pulmonary vessel, T = truncus arteriosus, Cr = cranial, Ca = caudal, R = right, L = left). The top row shows more ancestral vertebrates: hagfish (a simple two-chambered heart with sinus venosus, atrium, and ventricle), shark (a linear four-chambered heart with conus arteriosus), sturgeon (similar to shark with a prominent conus), tuna (a compact heart with bulbus arteriosus), frog (a three-chambered heart with two atria and one ventricle with truncus arteriosus). The middle row shows reptiles: rattlesnake (an incompletely divided heart with right and left atria and a partially divided ventricle), python (similar but with more distinct ventricular subdivision labeled 'RV' and 'LV'), alligator (approaching four-chamber organization with distinct right and left ventricles). The bottom row shows birds and mammals: ostrich (a fully four-chambered bird heart; a central orientation key showing cranial\/caudal and right\/left axes), human (a fully four-chambered mammalian heart with complete separation of right and left sides).<\/p>\r\n

Figure 15.14: Schematic diagram showing six circuit-style diagrams illustrating different vertebrate circulatory system configurations, using color-coded lines and symbols. Each diagram uses a rectangular circuit layout with a heart symbol representing the heart, and colored lines representing vessels: pink\/red for oxygen-rich blood, blue for oxygen-poor blood, and purple for blood with intermediate oxygen enrichment. An orange circle in most diagrams represents the air breathing organ (ABO). The six diagrams progress from simple to complex. Top left is a single-loop circuit with no lung circuit and a simple heart, representing a fish. Top right is a single loop with a small lung circuit branching off, representing a transitional or lungfish-type circulation. Middle left is a partially divided double circuit with a mixed heart (pink and blue), representing an amphibian or primitive reptile with incomplete cardiac separation. Middle right is a similar partially divided circuit with slightly more separation. Bottom left is a more developed double circuit with further cardiac separation. Bottom right is a small legend.<\/p>\r\n

Figure 15.16: Comparative anatomical illustration showing cross-sectional diagrams of hearts from nine vertebrate species arranged in three rows on a white background, with dark gray representing myocardial tissue and white representing internal chambers and lumens. Small red dots indicate specific anatomical landmarks such as valve positions. Structures are labeled with abbreviated anatomical terms. Top row shows more ancestral vertebrates. The hagfish has a simple oval heart with minimal internal division, labeled AVv (atrioventricular valve) and VAv (ventriculo-arterial valve). The shark has a slightly more complex heart with Cv (conus valve) and AVv. The sturgeon-tuna has a heart with multiple internal ridges and Cv and AVv. The frog has a heart with prominent internal trabeculation and Cv and AVv. The middle row shows reptiles. The rattlesnake, python, and Varanid lizard all show partially divided ventricles with right aorta (RAo), left aorta (LAo), pulmonary (p), and AVv labeled, with increasing complexity of ventricular subdivision from rattlesnake to varanid. The bottom row shows more derived vertebrates. The alligator is approaching full four-chamber separation with right and left aortic valves (RAo, LAo), right and left atrioventricular valves (RAVv, LAVv), and pulmonary (p). The ostrich has a fully four-chambered bird heart with RAVv, LAVv, Ao, and p. The human has a fully four-chambered mammalian heart with the same labeling.<\/p>\r\n

Figure 15.17: An anatomical cross-section diagram of an amphibian heart, showing its internal chambers and major vessels. The ventricle is a single, large muscular chamber at the bottom of the heart. The atria are divided into a Right atrium and Left atrium by the Interatrial septum. The atrioventricular valve is located between the atria and the ventricle to regulate blood flow. The conus arteriosus is a thick-walled vessel arising from the ventricle, containing a Spiral valve. Labels indicate the Aortic cavity and Pulmocutaneous cavity within the outflow tract. At the top, the diagram shows paired vessels branching into the carotid artery, systemic artery, and pulmocutaneous artery on both the left and right sides. Labels indicate the entry of pulmonary vein and the entry of sinus venosus. The heart is oriented with \"Right\" and \"Left\" labels at the bottom, corresponding to the animal's perspective.<\/p>\r\n

Figure 15.18: Anatomical illustration showing a frontal cross-sectional view of a reptilian heart depicting blood flow patterns with directional arrows. The heart is rendered in dark red\/brown tones with internal chamber divisions visible. Red arrows indicate the flow of oxygenated blood from the left atrium through the ventricle and out through the systemic and carotid arteries on the left side of the outflow tract. Blue\/purple arrows indicate the flow of deoxygenated blood from the right atrium through the ventricle and out through the pulmonary arteries on the right side. The crossing of red and blue arrows within the single partially divided ventricle illustrates the mixing and partial separation of oxygenated and deoxygenated blood that is characteristic of the reptilian heart. The two atrial chambers are visible as rounded lobes on either side at the top, and multiple outflow vessels are shown exiting at the top with color-coded arrows indicating their respective blood contents.<\/p>\r\n

Figure 15.19: This image consists of two comparative diagrams illustrating cardiac shunting mechanisms in reptiles. The diagrams show how blood can be diverted between the systemic and pulmonary circuits within the ventricular chambers. Each diagram shows a simplified view of the heart's outflow tract. The bottom shows two inflow points: the Right atrium (carrying systemic venous\/deoxygenated blood) and the Left atrium (carrying pulmonary venous\/oxygenated blood). The center shows the ventricular chambers, where the two streams of blood meet. The top shows three major outflow vessels: the Pulmonary artery (left), the Left aorta (middle), and the Right aorta (right). The left-to-right shunt (left side) shows that a significant portion of the oxygenated blood from the left atrium (red arrow) is diverted into the pulmonary artery. The pulmonary artery contains \"Mixed\" blood (purple), while both the left and right aortas receive purely oxygenated blood (red). The right-to-left shunt (right side) shows the deoxygenated blood from the right atrium (blue arrows) is diverted away from the pulmonary artery and into the Left aorta. The pulmonary artery remains purely deoxygenated (blue), but the Left aorta and Right aorta now carry \"Mixed\" blood (purple).<\/p>\r\n

Figure 15.20: Three-panel anatomical line diagram illustrating the crocodilian heart and its unique circulatory adaptations. The left panel is a labeled anatomical diagram of the crocodilian four-chambered heart showing the right aortic arch, left aortic arch, pulmonary artery, right ventricle, left ventricle, and the Foramen of Panizza (a small opening between the left and right aortic arches highlighted in red, unique to crocodilians). The center panel shows the same heart diagram with color-coded arrows showing blood circulation during normal respiration (breathing air). Red arrows indicate oxygenated blood flowing from the left ventricle into the left aortic arch and to the body, while blue arrows indicate deoxygenated blood flowing from the right ventricle into the pulmonary artery toward the lungs. A color key at the bottom identifies blue as deoxygenated and red as oxygenated. The right panel shows the same heart showing blood circulation when underwater. Both red and blue arrows now exit through both aortic arches.<\/p>\r\n

Figure 15.21: Comparative histological illustration contrasting heart morphology between an ectotherm and an endotherm. The left panel is an Ectotherm (Skink), under a blue header that shows a grayscale histological cross-section of a skink heart with a 1mm scale bar, labeled with right atrium (RA), left atrium (LA), and a single undivided ventricle (Ven). The ventricular wall is described as trabecular, with a highly spongy, irregular internal architecture. A small skink silhouette is shown beside the section. Below, a magnified diagram of the wall cross-section shows lumen between trabeculations with a thin compact outer layer, illustrating the spongy trabecular organization. The right panel is an Endotherm (Zebra Finch), under a pink header. It shows a grayscale histological cross-section of a zebra finch heart with a 1mm scale bar, labeled with right atrium (RA), left atrium (LA), right ventricle (RV), and left ventricle (LV), showing complete four-chamber separation. The ventricular wall is described as compact, with a much denser, more uniform myocardial structure. A small zebra finch silhouette is shown beside the section. Below, a magnified diagram shows large free lumens with a thick compact outer layer, contrasting with the skink's trabecular organization.<\/p>\r\n

Figure 15.22: Anatomical illustration showing a frontal cross-sectional view of the human heart with the cardiac conduction system highlighted in yellow against the brown myocardial tissue. Labeled structures trace the pathway of electrical impulse conduction from origin to ventricular muscle: SA node (sinoatrial node, upper right atrial wall), atrial pathways (conducting the impulse across both atria), AV node (atrioventricular node, at the junction of atria and ventricles), AV bundle of His (the bundle connecting the AV node to the ventricular conduction system), right bundle branch (descending along the right side of the interventricular septum), left bundle branch (descending along the left side of the interventricular septum, labeled on the right side of the image), interventricular septum (the wall dividing left and right ventricles), Purkinje fibers (terminal conduction fibers spreading through the ventricular myocardium), and moderator band (a muscular band crossing the right ventricular cavity carrying part of the right bundle branch). The yellow highlighting clearly delineates the sequential pathway of electrical conduction that coordinates the heartbeat.<\/p>\r\n

Figure 15.23: This image provides a comprehensive phylogenetic table illustrating the evolutionary history of the vertebrate heart and circulatory system over the last 600 million years. The left side shows a cladogram of major lineages, from basal chordates to modern endotherms. Key evolutionary milestones are marked. A yellow star represents the water-to-land transition (approx. 380 million years ago). The red stars indicate the independent evolution of endothermy in mammals and birds. Tunicates have a single contractile tube. Hagfish and lampreys have a sinus venosus, one atrium, and one vetricle. Chondrichthyes, ray-rinned fish, Coelacanths, and lungfish have a sinus venosus, one atrium, one ventricle, and a conus arteriosus (the ray-finned fish conus arteriosus is reduced in teleosts). Amphibians have a sinus venosus, two atria, one ventricle, and conus arteriosus. Mammals (red star) have atrialized sinus venosus, two atria, two ventricles, a ventricularized conus arteriosus, and one (left) aortic arch). Lizards and snakes have a sinus venosus, two atria, one ventricle, a ventricularized conus arteriosus and two aortic arches. Turtles have a sinus venosus, two atria, one ventricle, a ventricularized conus arteriosus, and two aortic arches. Crocodilians have a sinus venosus, two atria, 2 ventricles, ventricularized conus arterious and two aortic arches. Birds (red star) have atrialized sinus venosus, two atria, two ventricles, ventricularized conus arteriousus, and one (right) aortic arch.<\/p>\r\n

Figure 15.24: Three-panel scientific illustration comparing alternative interpretations of the secondary vascular system (SVS) in teleost fish, each showing a lateral view of a small fish with color-coded vascular networks overlaid, accompanied by a color-coded legend on the right and two inset diagrams at the bottom. Panel A (Lymphatic interpretation) shows the fish vasculature with arteries (red), blood vascular capillary networks (pink), veins (dark blue), and mammalian-like lymphatic vessels (light blue\/teal), interpreting the secondary vessels as homologous to the lymphatic system of mammals. Panel B (SVS interpretation) shows the same fish with arterial vessels of the SVS (light red\/pink), inter-arterial anastomoses (IAAs) also called arterial-lymphatic conduits (ALCs) (red), and venous vessels of the SVS (light blue). Panel C (Hybrid lymphatic\/SVS interpretation) is a combined interpretation showing mammalian-like lymphatic vessels (teal) alongside hybrid lymphatic\/SVS vessels with variable blood vascular and lymphatic characteristics (light teal), suggesting the fish SVS contains both lymphatic-like and blood vascular-like vessel populations. Two inset diagrams at the bottom compare embryonic lymphatics (showing branching vessels in tan\/beige) with the adult SVS (showing a more organized capillary network in pink and teal), illustrating developmental and structural differences between the two vessel types.<\/p>\r\n

Figure 15.25: Comparative scientific illustration showing the lymphatic systems of two amphibian species. The top left shows Newt Lymphatics (Cynops pyrrhogaster) as a dorsal photograph of a living red-bellied newt with a millimeter scale bar. Below it, a detailed anatomical diagram of the same species maps the lymphatic system onto a dorsal body illustration, with green dots indicating lymphatic vessels or nodes distributed along the body. Three labeled regions are delineated by boxes: Forelimb Lymphatic Territory (front left), Hindlimb Lymphatic Territory (center), and Tail Lymphatic Territory (right\/posterior). Additional labeled structures include LH1 and LH56 landmarks along the lateral body. The right side is a Frog Lymphatics (Xenopus laevis). Panel f shows a photograph of a living frog on a teal surface with a millimeter scale bar. Below it, an anatomical diagram of the frog in lateral view highlights the lymphatic system in color: subcutaneous lymphatic sacs are shown in yellow as large fluid-filled spaces beneath the skin, with pink vessels indicating lymphatic drainage pathways, and green dots marking specific lymphatic structures.<\/p>\r\n

Figure 15.26: Anatomical illustration showing a frontal cross-sectional view of the human heart with blood flow directions indicated by white arrows, color-coded to distinguish oxygenated and deoxygenated blood. Blue regions represent deoxygenated blood and the right side of the heart. Red\/pink regions represent oxygenated blood and the left side of the heart. Labeled structures include, on the heart right side (left side of diagram): superior vena cava (entering top right), right pulmonary arteries, right pulmonary veins, pulmonary semilunar valve, right atrium, tricuspid valve, right ventricle, and inferior vena cava (exiting bottom). On the left side of the heart (right side of diagram): aorta (exiting top), left pulmonary arteries, pulmonary trunk, left atrium, left pulmonary veins, aortic semilunar valve, mitral valve (bicuspid), and left ventricle. White arrows trace the path of blood flow: deoxygenated blood enters the right atrium via the venae cavae, passes through the tricuspid valve into the right ventricle, and exits via the pulmonary trunk to the lungs. Oxygenated blood returns via pulmonary veins to the left atrium, passes through the mitral valve into the left ventricle, and exits via the aorta to the body.<\/p>\r\n

Figure 16.3: A detailed illustration of the work of nephrons in the filtration system. Step 1 begins in the glomerulus that filters small solutes from the blood. This is a small red circle surrounded by a tube that moves up and curves into Step 2, the proximal convoluted tubule, which reabsorbs ions, water, and nutrients and removes toxins and adjusts filtrate pH. The tube then moves down into a U-shape. Moving downward is the Descending loop of Henle where aquaporins allow water to pass from the filtrate into the interstitial fluid. Curving down and moving back up is the Ascending loop of Henle where it reabsorbs Na+ and Cl- from the filtrate into the interstitial fluid. At the tube's recent apex is the Distal tubule that selectively secretes and absorbs different ions to maintain blood pH and electrolyte balance. The tube connects to a stalk-like tube (collecting duct) that reabsorbs solutes and water from the filtrate.<\/p>\r\n

Figure 16.4: A vintage biological illustration showing a transverse (cross-sectional) view of a chick embryo, detailing the early development of nephrons. The Neural Tube (n.Cr.) is located at the top center. This is a large, oval-shaped cluster of cells with a small central cavity. The notochord (N'ch.) is a smaller, distinct circular structure located directly beneath the neural tube, serving as the primitive skeletal support. The Aortae (Ao.) are two small, circular openings located on either side beneath the notochord. The Wolffian Duct (W.D.) & Nephrotome (Neph.) are structures on the left side. The Coelom (Coel.) are large, open cavities on both the left and right sides that will become the main body cavities. The Somatopleure (Som'pl.) & Splanchnopleure (Spl'pl.) are the outer and inner layers of the lateral plate mesoderm, shown on the right side bordering the coelom. The Somite (S. 29) is a segmented block of mesoderm visible to the left of the neural tube.<\/p>\r\n

Figure 16.5: This diagram illustrates the embryonic development of the vertebrate kidney, showing the three successive stages of renal structures: the pronephros, mesonephros, and metanephros. The illustration is divided into three vertical panels showing the progression over time. Stage 1 is the Early Stage (Left) showing the Pronephros. Located at the top, this three-prong branch is connected to a red cluster representing a rudimentary blood supply. The Nephric Duct is a long, teal-colored vertical tube that serves as the drainage system. The Nephrogenic Cord is a shaded green area along the lower portion of the duct where future kidney tissues will develop. Stage 2 is the Intermediate Stage (Center). It begins with the Degenerating Pronephros where the uppermost section fades, as the pronephros is non-functional in most mammals. Below is the Mesonephros, which is a series of teal, hook-like structures associated with red capillary loops. At the bottom is the Cloaca where the nephric duct now extends all the way down to connect to the primitive common chamber for waste. The third stage is the late stage (Right). At the top is the Degenerating Mesonephros, the upper part of the mesonephros begins to fade. Below is the Metanephros (teal, hook-like structures with red \"knot\" structures) that contain two key components: Ureteric Bud (a small teal branch growing out from the duct) and Metanephric Mesenchyme (a concentrated green cluster of cells surrounding the bud.)<\/p>\r\n

Figure 16.7: Circular life cycle diagram of a parasitic lamprey, illustrated against a background divided into two zones: a blue upper zone representing the sea and a tan\/brown lower zone representing river sediment. Blue arrows indicate the forward progression of the life cycle clockwise, and an orange arrow traces the return migration. Starting from the bottom and moving clockwise, the stages and their durations are: Larval life in rivers (4\u00bc years) where small ammocoete larvae are shown burrowed in river sediment; a central circle labels this the sedentary stage. Metamorphosis is when larvae transform into juveniles, transitioning to the free-swimming stage (labeled in the central circle). Downstream migration to sea (\u00bd year) is when juveniles migrate downstream. Parasitic phase at sea (2 years) is when adult lampreys are shown attached to host fish (a large fish and a smaller fish) in the open sea, feeding parasitically. Spawning run (1\u00bc year) shows male (\u2642) and female (\u2640) adults migrate back upstream into rivers. Spawning and death shows adults spawn in gravel nests and die. Migration of adults into rivers is connecting the sea phase back to the spawning event.<\/p>\r\n

Figure 16.12: Two-panel scientific illustration comparing osmoregulation strategies in marine and freshwater fish, using color-coded arrows to indicate the direction of ion and water movement. A legend in each panel identifies red arrows as the direction of ion transfer (Na\u207a, K\u207a, Cl\u207b) and blue arrows as the direction of water transfer. In the top panel, the yellow jack fish actively combats water loss and ion gain in a hypertonic environment. Labeled processes include: drinks seawater (large red arrow entering the mouth), water loss over skin (blue arrows pointing outward from the body surface), active ion depuration through gills (red arrows pointing outward at the gill region), and salty urine containing Mg\u00b2\u207a and SO\u2084\u00b2\u207b (red arrow exiting posteriorly). The bottom panel is a brown trout that actively combats ion loss and water gain in a hypotonic environment. Labeled processes include: food (red arrow entering the mouth), active ion absorption through gills (red arrows pointing inward at the gills), water intake through skin (blue arrows pointing inward over the body surface), and diluted urine (blue arrow exiting posteriorly).<\/p>\r\n

Figure 16.16: A large, sagittal (longitudinal) cut of a right kidney, showing the inner layers, drainage systems, and vasculature. A smaller diagram in the top-left corner shows the kidneys' anatomical position within the human torso, situated against the posterior abdominal wall with the adrenal glands sitting on top. The kidney is labeled with external and entry points: Renal Hilum (the recessed central fissure), renal vein (large blue vessel), renal nerve (yellow branching structures), renal artery (red vessel), capsule (the smooth, thin outer protective layer of the kidney), and ureter (the large tube extending downward). The internal regions include: cortex (the outer layer of the kidney tissue), medulla (the inner region containing the Pyramids), renal column (the tissue between the renal pyramids), papilla (the tip of each pyramid). The drainage system includes: minor calyx (small cup-like structures), major calyx (larger channels), and renal pelvis (the large, funnel-shaped cavity). The blood vessels include: interlobar blood vessels (travel between the pyramids), arcuate blood vessels (arch over the bases of the pyramids at the junction of the cortex and medulla), and cortical blood vessels (small vessels extending into the renal cortex).<\/p>\r\n

Figure 16.17: Detailed anatomical illustration of a single nephron and its associated blood supply, showing the complete structural organization of the kidney's functional unit. The illustration uses red for arterial vessels, blue for venous vessels, and tan for the tubular components of the nephron. Labeled structures include, from top to bottom: the renal corpuscle consisting of the glomerular capsule (Bowman's capsule) and the glomerulus (the capillary tuft), efferent arteriole, juxtaglomerular apparatus, peritubular capillary network (surrounding the tubules), proximal convoluted tubule (with a cross-sectional inset showing cuboidal cells with brush border), distal convoluted tubule (with a cross-sectional inset); collecting tubule (with a cross-sectional inset); descending limb of the nephron loop (Loop of Henle), ascending limb of the nephron loop; vasa recta (long straight capillaries running parallel to the loop of Henle); arcuate vessels (curved vessels at the corticomedullary junction), and interlobar vessels (larger vessels between renal lobes). Cross-sectional insets at key points show the cellular composition of different tubular segments.<\/p>\r\n

Figure 17.2: This is a histological cross-section of an ovary, stained in pink (H&E stain), showing various structures at different stages of the ovarian cycle. A scale bar in the bottom right indicates 2mm. The image is labeled with several key anatomical features beginning with the outer layers that include germinal epithelium (the outermost layer of the ovary) and tunica albuginea (a layer of dense connective tissue located just beneath the germinal epithelium). The ovarian regions include the cortex (the outer region of the ovary); medulla (the central region of the ovary); developing follicles (small, circular structures located within the cortex); mature Graafian follicles (large, fluid-filled sacs); the theca (outer capsule of the follicle) is specifically pointed out on one of these. Corpora Lutea (marked with 'C') are large, solid-looking glandular masses that form from the remains of a follicle after ovulation. There are three prominent corpora lutea visible in this section.<\/p>\r\n

Figure 17.3: An anatomical medical illustration showing a posterior view of the human female reproductive system, specifically the uterus and its associated structures on the right side. The large central body is labeled uterus, leading down into the vagina. The opening at the cervix is identified as the external uterine orifice. The ovary is an almond-shaped organ positioned to the right of the uterus. The uterine tube (Fallopian tube) is a long duct extending from the top of the uterus toward the ovary. The ovarian fimbria are finger-like projections at the end of the uterine tube that hover near the ovary. The Ostium abdominale is the opening of the uterine tube into the abdominal cavity near the fimbriae. The broad ligament is a wide fold of peritoneum that supports the uterus and connects it to the pelvic walls. The ligament of ovary is a fibrous cord connecting the ovary to the lateral side of the uterus. The Epo\u00f6phoron is a small vestigial structure located in the broad ligament between the ovary and the uterine tube. The Ovarian vessels are blood vessels supplying the ovary, shown entering near its lateral pole.<\/p>\r\n

Figure 17.4: A detailed medical diagram showing a cross-section of the human testis and its surrounding structure. The external layers include the spermatic cord which is the structure at the top leading \"into inguinal canal.\" The Cremaster muscle is a muscle layer surrounding the testis. The Tunica vaginalis is the outermost serous membrane covering the testis. The Tunica albuginea is a tough, fibrous inner capsule. It extends inward to form septa, which divide the testis into compartments. The Seminiferous tubule lobules are coiled tubes within the septa where sperm are produced. The straight tubule are small ducts that collect sperm from the seminiferous tubules. The rete testis is a network of delicate canals that receive sperm from the straight tubules. The efferent ductules are small tubes that carry sperm from the rete testis out of the testis and into the epididymis. The Epididymis is a long, coiled tube where sperm mature, divided into three labeled parts including Head of epididymis which is the top portion receiving sperm from the efferent ductules, Body of epididymis which is the middle section running along the side of the testis, tail of epididymis which is the bottom portion where the tube begins to turn upward, and ductus deferens (Vas deferens) which is the thick-walled tube that exits the tail of the epididymis to carry sperm back up toward the pelvic cavity.<\/p>\r\n

Figure 17.6: This flow chart illustrates the process of oogenesis, highlighting the specific points where the biological \"stoplights\" of meiosis occur throughout a female's life. The process begins with diploid stem cells that undergo mitosis to produce primary oocytes (2n). Meiosis I begins as primary oocytes start the first meiotic division. Meiosis is \"paused\" (arrests) in prophase I before the female is born. After puberty Meiosis I resumes. Under hormonal influence, a primary oocyte completes its first division. This results in two haploid (n) cells, a large secondary oocyte and a small first polar body (which may later divide into two second polar bodies). The secondary oocyte begins Meiosis II but arrests at metaphase II. This is the stage at which ovulation occurs. After sperm penetration (green light, oocyte meiosis completes immediately after sperm penetrates the oocyte), the diagram shows a sperm cell approaching the secondary oocyte. The completion of meiosis produces a Mature ovum (n) and another small polar body.<\/p>\r\n

Figure 17.7: This graphic illustrates the process of spermatogenesis through both a cellular flow chart and a histological cross-section of a seminiferous tubule. In spermatogenesis (a), the section outlines the progression of cell division and differentiation. It begins with Spermatogonium (2n) where cells go through mitosis to either primary spermatocyte (2n) or back up to spermatogonium. The cells from the Primary spermatocyte enter Meiosis I, dividing into two (1n). From the secondary spermatocyte (1n), these cells quickly enter Meiosis II. At Spermatid (1n), the four resulting haploid cells form a single primary spermatocyte. Finally, through spermiogenesis, round spermatids develop tails and condensed heads to become Spermatozoa (sperm). Part (b) is the cross-section of the seminiferous tubule leading to a microscopic view (micrograph) showing the physical organization of these cells within the testis. The micrograph shows the Lumen at the center with various parts labeled around the edges. Sertoli (sustentacular) cells are large cells that span the tubule wall. Leydig (interstitial) cells are located in the interstitial tissue outside the tubule. Spermatogonia are located at the very edge. Primary spermatocytes and early spermatids are found in the middle layers. The diagram also points out a lymphatic capillary, arteriole, and peritubular capillary.<\/p>\r\n

Figure 17.8: This diagram illustrates three types of sequential hermaphroditism in fish, where an individual changes its biological sex during its lifetime. Part A is Protogyny. An individual starts its life as a female (indicated by the \u2640 symbol and a smaller, yellow\/grey fish) and later transitions into a male (indicated by the \u2642 symbol and a larger, vibrantly colored blue and green fish). This is a one-way transition (single-headed arrow). Part B is Protandry. An individual starts its life as a male (\u2642) and later transitions into a female (\u2640). The illustration uses Clownfish as the example; in these species, the largest individual in a group typically becomes the female. This is also a one-way transition (single-headed arrow). Part C is the Bidirectional process. An individual has the capacity to change sex in either direction\u2014from female to male or male to female\u2014depending on social or environmental cues. The fish pictured (a goby) appears identical in both states. This is shown with a double-headed arrow, indicating the change is reversible or flexible.<\/p>\r\n

Figure 17.10: This image features an anatomical diagram and histological sections of the urogenital region of a lamprey. Part A (Left) is a three-dimensional cutaway illustration of the posterior body region. The digestive tract (gut) and the body cavity (coelom) are shown. The genital pore is labeled with a white arrow. The Wolffian Duct (WD) is shown as a separate tube above the gut. External features are labeled with the dorsal fin, cloacal labium, connective tissue, and the urogenital papilla. A 1cm scale bar is provided at the bottom left. Parts B & C are histological Sections on the right. These two black-and-white micrographs provide a microscopic view of the genital pore area. Image B shows the physical opening between the coelom and the urinary sinus. Image C is a higher-magnification view of the genital pore. It shows the cellular structure of the opening, including a label for a pycnotic nucleus (a small, dark, condensed nucleus, often indicating cell death or specialized epithelial turnover) at the margin of the pore.<\/p>\r\n

Figure 17.14: A black-and-white biological diagram compares two different arrangements of sperm production: the lobular type (left) and the tubular type (right). A central column of text with downward-pointing arrows tracks the maturation of germ cells from the top to the bottom: Spermatogonium, Spermatocyte, Spermatid, Sperm. The lobular type (left side) shows a structure where the sperm-producing cells are organized into distinct compartments along the wall. The lobular lumen is a central cavity where mature sperm collect. Spermatogonia are at the top and periphery, while mature sperm are released into the wide, lower sperm canal. Sertoli cells are identified as supportive cells within the lobule walls. The tubular type (right) shows a more streamlined, elongated structure where the maturation process happens in horizontal clusters within the tube. The Sertoli cell is labeled near a cluster of spermatids. Mature sperm are shown clustered together in a circular formation before moving downward toward the sperm canal. The bottom of the diagram shows both types converging into a common sperm canal at the base.<\/p>\r\n

Figure 17.17: An anatomical and histological diagram in black and white illustrates the reproductive system and sperm development. The macro anatomy (top left) shows a pair of organs labeled: testicle (an oval structure attached to the kidney), fat body (finger-like projections extending from the top of the testicle), and kidney (elongated, dark organ behind the testicle). The histology of the seminiferous tubule (center) shows a large cross-section surrounded by muscle cells. The epithelium is the inner lining of the tubule where sperm production occurs. The lumen is the central cavity of the tubule, filled with the tails of maturing sperm. The spermatogenesis (bottom and left) is an inset magnification and a series of three circular diagrams (left) show the stages of cell maturation: spermatogonia (the outermost layer of stem cells), spermatocytes (the next layer of cells undergoing division), spermatids (smaller cells moving toward the center), and spermatozoon (mature sperm cells with tails, ready for release into the lumen). The sperm variations (top right) show an arrow from the tubule pointing to six different morphologies of mature sperm, labeled a through f. These variations show different head shapes, ranging from straight and needle-like (a) to curved and hook-shaped (c, d, f) and even a spiral\/corkscrew shape (e).<\/p>\r\n

Figure 17.21: An anatomical line drawing titled \"TURTLE\" displays the reproductive and excretory systems of a male turtle through a longitudinal section and three cross-sections. The longitudinal section at the top (A) shows a side view of the posterior anatomy. The excretory organs include the gut, ureter, bladder, and a cloacal pouch or accessory bladder. The reproductive structures highlight the internal phallus including the bulb of corpus cavernosum, the glans, the corpus fibrosum, and the retractor penis muscle. Cross-section Markers are vertical lines labeled B, C, and D that indicate where the corresponding cross-sectional views are taken. Three circular diagrams (Cross-sections B, C, D) show the internal arrangement of tissues at different points along the body. View B is located furthest back, showing the gut at the top, the urogenital sinus in the center, and the ureter and vas deferens nearby. At the bottom are the coelomic canal and the bulb of corpus cavernosum. View C shows the cloacal pouch extending outward like wings. Below it are the corpus cavernosum, corpus fibrosum, and retractor penis muscle. View D is located near the glans, showing an oval section containing a central seminal groove supported by the corpus fibrosum and the retractor penis muscle.<\/p>\r\n

Figure 17.36: Comprehensive scientific illustration showing the reproductive anatomy in frogs, arranged as a circular sequence of six numbered diagrams (I\u2013VI) surrounding a central large diagram of the ovary. The center is a cross-sectional view of the ovary showing the germinal epithelium on the outer surface, with multiple follicles at various stages of development visible within the ovarian cortex, and blood vessels throughout. Stage I (upper right) is a primordial follicle showing a single follicular cell, nucleus, and Balbiani body. Stage II is an early follicle with developing cytoplasm and initial yolk around a vitelline cover. Stage III is a growing follicle showing increased yolk accumulation, cytoplasm, blood vessels, and surrounding cubic cells. Stage IV is a more advanced follicle with a cavity, theca cell layer, vitello (yolk), and granulosa cells. Stage V is a large pre-ovulatory follicle with a prominent cavity, theca cells, granulosa cells, blood vessels, and an oogonium inset. Stage VI is a nearly mature oocyte showing the oocyte itself, polar body, zona pellucida, vitelline envelope, and surrounding granulosa cells. The upper left is an anatomical diagram of the female reproductive system showing fat body (tentacle-like structures), ovary, oviduct, cloaca, kidney, and granulosa cells with a vitelline cover.<\/p>\r\n

Figure 17.40: Schematic diagram illustrating the reproductive system of an ovary-bearing chicken. At the top, a large white\/gray oval structure represents the ovary, with multiple blue spheres of varying sizes clustered on its surface representing oocytes or follicles at different stages of maturation. The largest blue sphere represents a mature follicle, surrounded by smaller ones, and a cluster of very small blue spheres at the top going into the left ovary (A). Below the ovary, three successive stages are depicted descending vertically from the infundibulum (B): first, a gray crescent-shaped structure with a blue sphere (magnum); second, a gray oval with a central blue sphere in the isthmus (D); and third, a plain gray oval with no blue sphere in the uterus (E). The tube tapers down into the vagina (F) and out the cloaca (G). The large intestine (H) and rudiment of right oviduct (I) are shown to the right of the vagina.<\/p>\r\n

Figure 17.41: Three-panel photograph showing variation in avian vagina morphology, with scale bars in each panel. Anatomical structures are identified by white arrowheads and letter labels. Panel A (upper left) is a dorsal view of the vagina from a pheasant, showing the uterus (U), vagina (V), and cloaca (Cl) labeled, with white arrowheads pointing to junctions between segments. Yellow follicles or eggs are visible at the upper left, and the paired uteri extend upward. Panel B (lower left) is a closer view of the vaginal and cloacal region, labeling the uterus (U), vagina (V), and cloaca (Cl). Asterisks (* and **) mark specific anatomical features of interest and white arrowheads indicate junctions. Panel C (right) is a fully dissected and unfolded reproductive tract laid out on a black background, showing the complete length from uterus (U) at the top to cloaca (Cl) at the bottom. White arrowheads at multiple points indicate regional boundaries or structural features, and a bracket labeled S.S. indicates a specific segment of the oviduct. A scale bar is visible at the right.<\/p>\r\n

Figure 17.44: Comparative anatomical illustration showing the diversity of female reproductive tract organization across mammals, divided into two main groups. On the left, Marsupials are represented by a single diagram labeled \"double uterus\" and \"double vagina,\" showing two separate uteri (yellow) and two separate vaginal canals (pink\/red) that do not fuse, with the urinary bladder (blue) visible below. On the right, Placentates (placental mammals) are represented by three diagrams showing progressive fusion of the reproductive tract from left to right: double uterus (two separate uteri with a single vagina), bicornuate uterus (two uterine horns partially fused at the lower end, with a single vagina), and single uterus (fully fused uterus with a single uterine body). The rightmost diagram labels all structures: fallopian tubes at the top (yellow), connecting to the uterus (red), vagina (pink), urinary bladder (blue), and sinus urogenitalis. A bracket below the three placental diagrams indicates they all share a single vagina, contrasting with the marsupial double vagina.<\/p>\r\n

Figure 17.48: Color diagram showing a developing fetus in the uterus and a detailed inset of the placental interface. The left side shows a fetus in a curled position within the amniotic sac, filled with amniotic fluid. Labels identify the surrounding layers and structures: placenta (the thick organ lining the uterine wall), yolk sac (a small yellow pouch outside the main embryo), amnion and chorion (the inner and outer membranes surrounding the fetus), umbilical cord (connecting the fetus to the placenta), and uterus (the muscular organ containing the pregnancy). An enlarged inset on the right shows the exchange point between maternal and fetal blood systems. Chorionic villi are tree-like branching structures containing fetal blood vessels (blue umbilical arteries and red umbilical veins). These extend into the intervillous space. Maternal Blood Supply are uterine arteries and veins that pump blood into the intervillous space, where it bathes the chorionic villi. The umbilical vein (carrying oxygenated blood to the fetus) and umbilical artery (carrying deoxygenated blood away) are shown traveling through the umbilical cord.<\/p>\r\n

Figure 17.50: This medical diagram provides a comprehensive anatomical view of the female reproductive system, including a central illustration of the organs and two histological (microscopic) insets. The central image shows a frontal view of the uterus, ovaries, and associated structures. The uterine tube (oviduct) consists of three parts: the Infundibulum (with finger-like Fimbriae near the ovary), the Ampulla (middle section), and the Isthmus (the narrow section connecting to the uterus). Labeled parts of the uterus include the Fundus (top rounded portion), the Uterine isthmus (lower narrow portion), and the Cervix (the opening to the vagina). Uterine Wall Layers are labeled in an inset to the right: endometrium (the inner lining), myometrium (the thick middle muscular layer), perimetrium (the outer serous layer.) Ovaries and ligaments show the ovaries held in place by the suspensory ligament, ovarian ligament, and the broad ligament. The vasculature displays the network of the ovarian artery and vein, uterine artery and vein, and vaginal artery. The vagina is the canal leading from the cervix to the exterior of the body. The ovarian section (Left) is a microscopic view of the Ovarian cortex, showing the Tunica albuginea (outer capsule) and the Edge of a follicle. The uterine section (Right) is a microscopic view detailing the transition between the three layers of the uterine wall (Endometrium, Myometrium, and Perimetrium).<\/p>\r\n

Figure 17.51: This medical illustration displays the external and internal anatomy of the human female genitalia, referred to as the vulva. The left panel shows the surface anatomy as viewed from the front. Labels include: Prepuce (the hood of skin covering the clitoris), Glans clitoris (the external, visible portion of the clitoris), labia minora (the smaller, inner folds of skin), labia majora (the larger, outer folds of skin), urethral opening (the small opening through which urine is excreted), vaginal opening (the entrance to the vaginal canal), and anus (labeled at the bottom of the frame, separate from the vulva). The right panel shows the deeper, internal structures that sit beneath the skin: Corpus cavernosum (the internal erectile tissue of the clitoris that extends backward, bulb of vestibule (two masses of erectile tissue situated on either side of the vaginal opening), Bartholin\u2019s glands (small glands located near the vaginal opening), opening of right Bartholin\u2019s gland (a specific label pointing to the duct where the gland's fluid enters the vestibule).<\/p>\r\n

Figure 17.55: Black and white anatomical illustration comparing two pelvic skeletons side by side, viewed from the front, representing the female pelvis (left) and male pelvis (right). Both illustrations have a red oval drawn on them to highlight the pelvic region for comparison. Four structures are labeled with lines pointing to both pelves: Hip bone (large lateral bones forming the sides of the pelvis), Sacrum (the triangular bone at the posterior center), Pelvic brim (the curved ridge outlining the pelvic inlet, traced by the red oval), and Subpubic angle (the angle formed beneath the pubic symphysis at the bottom, indicated by bracket lines spanning both pelves). The female pelvis (left) has a noticeably wider, rounder pelvic inlet and a broader subpubic angle compared to the narrower, more heart-shaped inlet and more acute subpubic angle of the male pelvis (left), illustrating the key anatomical differences between the two.<\/p>\r\n

Figure 18.9: Four-part diagram showing embryonic brain development from primary to secondary brain vesicles, with lateral (side) profile views bookending two labeled cross-sectional illustrations. The far left is a lateral view of a three- to four-week embryo showing the curved, early brain shape. The second illustration (a) is a labeled illustration of the three primary brain vesicles in a three- to four-week embryo, showing three color-coded segments from top to bottom: Prosencephalon (Forebrain) in light gray, Mesencephalon (Midbrain) in gold\/yellow, and Rhombencephalon (Hindbrain) in magenta\/pink, with a brown spinal cord below. The third illustration (b) of the five secondary brain vesicles in a five-week embryo, showing how each primary vesicle subdivides, with arrows indicating the developmental progression. The Prosencephalon becomes the Telencephalon (developing into the Cerebrum) and Diencephalon (developing into the eye cup, Thalamus, hypothalamus, and epithalamus). The Mesencephalon becomes the Mesencephalon (Midbrain). The Rhombencephalon becomes the Metencephalon (developing into the Pons and Cerebellum) and Myelencephalon (developing into the Medulla oblongata). The far right is a lateral view of the 5-week embryo brain showing its more developed, curved shape with distinct regions visible.<\/p>\r\n

Figure 18.10: Diagram showing four illustrations (A\u2013D) of spinal cord anatomy, depicted in gold\/yellow against a white background, representing different developmental stages or species comparisons. Illustration A (red-eared slider) is a long, narrow spinal cord with short, evenly spaced nerve roots branching symmetrically along its entire length, tapering to a fine point at the bottom. Illustration B (harbor seal) is a wider spinal cord with longer, more spread nerve roots that angle downward, also tapering to a point at the bottom. Illustration C (human) shows a spinal cord with very long, sweeping nerve roots that extend far downward and outward, creating a dramatic elongated fringe effect, with the roots converging toward the bottom. Illustration D (common toad) is a shorter, broader spinal cord shown from the top with thick, prominent nerve roots curving outward and downward from the base, showing fewer but larger root bundles.<\/p>\r\n

Figure 18.11: Diagram showing five simplified cross-sectional illustrations (A\u2013E) of spinal cord anatomy, depicted in purple and pink\/lavender against a white background. Each illustration shows the outer white matter (dark purple) surrounding the inner gray matter (light pink\/lavender) in varying shapes across different species. Image A (common toad) is a rounded cross-section with a large, wide pink interior region and a small central canal visible, with minimal gray matter differentiation. Image B (red-eared slider) is a similar rounded shape with a more defined butterfly\/H-shaped gray matter region beginning to emerge. Image C (Hermann's tortoise) is a cross-section with a prominent, well-defined butterfly\/H-shaped gray matter pattern with clear dorsal and ventral horns, and relatively less white matter. Image D (tegu) is a cross-section with a broad, squat H-shaped gray matter region with wide ventral horns, surrounded by proportionally less white matter. Image E (reticulated python) is a large, rounded cross-section with a tall, narrow butterfly-shaped gray matter region and a substantial amount of surrounding white matter.<\/p>\r\n

Figure 18.15: Detailed anatomical illustration of an adult lamprey brain shown from a lateral view, with the brain dissected and labeled to show its key structures. From front (left) to back (right), the labeled regions include: the large olfactory bulb (shown in purple\/violet at the far left, prominent in sharks due to their acute sense of smell), the olfactory nerve connecting it to the brain, the medial pallium (hippocampus) (orange\/tan region), the lateral pallium (cerebral hemisphere) (tan, lower front region), the optic nerve (projecting downward), the adenohypophysis (blue, on the underside), the infundibulum (stalk connecting to the pituitary), the habenula (small dark blue structure on top), the pineal gland (projecting upward from the top), the midbrain (large rounded dorsal structure in the center-right), the oculomotor nerve (projecting from the underside toward the right), and the spinal cord (tapering off to the far right).<\/p>\r\n

Figure 18.21: This image displays four sets of real anatomical specimens of bird brains, labeled a, b, c, and d. Each set includes a ventral view (top row) and a dorsal view (bottom row). The specimens are labeled with several neuroanatomical abbreviations. OB (Olfactory Bulb) is located at the very front (anterior) of the brain. In specimen b, it is labeled as 'OB', suggesting a unique or primitive form in that species. ON (Optic Nerve) is visible in the ventral views as white, cross-like structures (the optic chiasm) where the eyes connect to the brain. OT (Optic Tectum) is highlighted with dashed circles on the sides of the brain. Wulst is a prominent, rounded elevation on the dorsal surface of the forebrain (telencephalon). V (Vallecula) is indicated by a dashed line; this is the furrow or groove that separates the Wulst from the rest of the telencephalon. Specimen \u201ca\u201d features elongated olfactory bulbs and a distinct Wulst. Specimen \u201cb\u201d shows a much broader, more heart-shaped forebrain with very large olfactory structures. Specimen \u201cc\u201d shows a paler specimen with a highly pronounced, rounded Wulst. Specimen \u201cd\u201d displays a more compact brain structure with clearly defined optic tecta.<\/p>\r\n

Figure 18.24: Color-coded anatomical illustration of the human brain in lateral (side) view, showing the four main cerebral lobes and key surface landmarks, each region distinguished by a distinct color. The frontal lobe is shown in pink\/salmon and occupies the anterior (front) portion of the brain. The parietal lobe is shown in purple\/mauve and sits posterior to the frontal lobe. The occipital lobe is shown in green and is located at the rear of the brain. The temporal lobe is shown in dark blue\/slate and runs along the lower lateral surface. Labeled sulci (grooves) and gyri (ridges) include the central sulcus (separating the frontal and parietal lobes), the precentral gyrus (just anterior to the central sulcus), the postcentral gyrus (just posterior to the central sulcus, associated with somatosensory function), the lateral sulcus (separating the temporal lobe from the frontal and parietal lobes), and the parieto-occipital sulcus (marking the boundary between the parietal and occipital lobes). The brainstem and cerebellum are partially visible at the base of the illustration.<\/p>\r\n

Figure 19.2: Diagram comparing the parasympathetic (left) and sympathetic (right) divisions of the autonomic nervous system and their opposing effects on various organs. The central illustration shows a sagittal view of the brain and spinal cord, with the vagus nerve, medulla oblongata, ganglion, solar plexus, and chain of sympathetic ganglia labeled. Lines extend from the central nervous system to illustrated organs on each side, with text describing each division's effect. Parasympathetic effects (left side): the ganglion and medulla oblongata, stimulates flow of saliva. The vagus nerve slows heartbeat, constricts bronchi, stimulates peristalsis and secretion, and stimulates release of bile. The end of the spinal cord has a leader line to contracts bladder. Sympathetic effects (right side): The chain of sympathetic ganglia dilates pupil and inhibits flow of saliva. Multiple leader lines mark from the spine to the various effects: accelerates heartbeat, dilates bronchi, inhibits peristalsis and secretion, triggers conversion of glycogen to glucose, stimulates secretion of adrenaline and noradrenaline, and inhibits bladder contraction.<\/p>\r\n

Figure 19.7: Three-panel anatomical diagram illustrating the evolution of the recurrent laryngeal nerve from various species. Panel A (upper left, shark) is a simplified frontal view of the thoracic cavity showing the spine, ribs, and a red structure representing the heart or major vessels. Numbered labels (1, 2, 3, 9) point to various structures, with yellow structures representing nerve pathways running alongside red vascular structures. Panel B (upper right, mouse) is a more detailed frontal view of the heart and great vessels. Red structures represent the heart and arteries, yellow structures represent nerves, and a purple structure indicates a vessel or nerve loop (likely the recurrent laryngeal nerve loop around the aorta). Numbered labels 1 through 9 identify specific anatomical structures Panel C (lower, giraffe) is a lateral view of an elongated neck and thorax, used to illustrate the impractically long path of the recurrent laryngeal nerve. Yellow nerve pathways (labeled 1 and 4) descend from the brain region (label 10, shown in yellow) down the neck, loop around cardiovascular structures at the base (labels 3, 5, 6, 9), and return upward. Label 11 points to an additional structure along the pathway.<\/p>\r\n

Figure 20.1: Scientific infographic illustrating the electromagnetic spectrum against a dramatic landscape background with mountains, sky, and a satellite visible in the upper right. A wave diagram runs horizontally across the middle of the image, showing waves becoming progressively shorter and more frequent from left to right. A vertical beam of white light splitting into a rainbow (visible light spectrum) is depicted at the center. The spectrum is divided into labeled regions from left to right: Radio Waves (longest wavelength, ~10\u00b2 meters, shown with AM radio and FM radio tower icons), Microwaves (wavelength ~1mm, shown with cell phone\/Wi-Fi and microwave oven icons), Infrared (wavelength ~thickness of paper, shown with a human figure representing body heat and a remote control icon), Visible Light (the narrow band humans can see, shown with an eye icon), Ultraviolet (shown with a sunscreen\/skin icon), X-rays (shown with a medical X-ray image icon), and Gamma Waves (shortest wavelength, ~size of atomic nuclei, shown with a nuclear reactor icon). Above the wave diagram, atmospheric opacity windows are labeled, indicating which wavelengths penetrate Earth's atmosphere, including the Radio Window and Optical Window. Frequency values in Hertz are shown along the top and wavelength values in meters along the bottom.<\/p>\r\n

Figure 20.2: Two-panel anatomical illustration of the primate eye and retina. Panel A (Primate eye) shows a cross-sectional line diagram of the whole eye with a yellow arrow labeled \"LIGHT\" indicating the direction of incoming light through the cornea. Labeled structures include: sclera (outer white coat), choroid (vascular layer), posterior chamber, iris, ciliary body, fovea (point of sharpest vision), vitreous body (gel-filled interior), cornea (transparent front surface), anterior chamber, suspensory ligament, lens, optic nerve, and retina. A red arrow points from the retina to Panel B. Panel B (Retina) is a detailed color illustration of the retinal cell layers shown in cross-section, depicting the cellular organization from the vitreous side (left) to the outer pigment epithelium (right). Labeled cell types and structures from left to right include: ganglion cells (innermost layer, with visible nuclei and axons), bipolar cells (intermediate neurons), cone cells (color photoreceptors), rods (dim-light photoreceptors), connecting stalks, nuclei, discs (membranous photopigment-containing structures), mitochondria, Golgi apparatus, melanin granules, and pigment epithelium (outermost layer).<\/p>\r\n

Figure 20.3: Comparative anatomical illustration showing cross-sectional line diagrams of eyes from six vertebrate groups, arranged in two rows of three, each labeled below. All diagrams show the lens as a prominent central oval structure. Panel A (Lamprey) labels include spectacle, cornea, iris, choroid, retina, protractor lentis muscle, and corneal muscle. Panel B (Shark) is a simpler diagram showing the lens, cornea, choroid, and retina with minimal additional structures. Panel C (Teleost) labels include conjunctiva, iris, suspensory ligament, sclera, choroid, retina, optic nerve, and retractor lentis muscle, with a notably large round lens. Panel D (Amphibian) labels include cornea, protractor lentis muscle, suspensory ligament, optic nerve, sclera, retina, choroid, nictitating membrane, and lower eye lid. Panel E (Lizard) labels include ciliary muscle, papillary cone, optic nerve, iris, cornea, lens, vitreous body, fovea, sclera, retina, choroid, and sclearal ossicle (a ring of small bones in the sclera). Panel F (Bird) labels include ciliary body and muscle, suspensory ligament, optic nerve, iris, cornea, pecten (a pleated vascular structure unique to birds projecting into the vitreous), lens, vitreous body, fovea, sclera, retina, and choroid, and sclearal ossicle.<\/p>\r\n

Figure 20.7: Multi-panel anatomical illustration showing the organization of taste receptors on the human tongue, progressing from gross anatomy to cellular detail. Center left is a dorsal view of the human tongue showing the distribution of different papilla types across the surface, with small boxes indicating the regions magnified in surrounding panels. Top center and right are a circumvallate (vallate) papilla is shown in cross-section, with taste buds labeled along its walls, and a histological photomicrograph (upper right) showing the actual tissue appearance of the circumvallate papilla in pink-stained section, revealing the taste bud-lined trench. The bottom left shows three additional papilla types shown in isolation: fungiform papilla (broad, mushroom-shaped), filiform papilla (narrow, pointed, hair-like), and foliate papilla (leaf-like folds with taste buds labeled along the sides). The bottom right is a detailed cross-sectional diagram of a single taste bud, labeling: taste hairs (microvilli projecting into the taste pore), taste pore (opening at the surface), gustatory cells (the primary sensory receptor cells), basal cells (stem cells at the base), and transitional cells (intermediate cell type).<\/p>\r\n

Figure 20.10: Detailed anatomical illustration showing a cross-sectional view of the human ear, divided into three labeled regions along the bottom: External ear, Middle ear, and Inner ear. The external ear structures labeled are the auricle (the visible outer ear cartilage, shown in brown) and the ear canal (the passage leading inward to the eardrum). The middle ear structures labeled are the tympanic membrane (eardrum, separating the external and middle ear), the three ossicles \u2014 malleus, incus, and stapes (attached to the oval window) \u2014 and the tympanic cavity (the air-filled middle ear space). The inner ear structures labeled are the vestibule, vestibular nerve, cochlear nerve (both shown in yellow as they merge into the vestibulocochlear nerve), round window, cochlea (the snail-shaped hearing organ), and Eustachian tube (connecting the middle ear to the nasopharynx for pressure equalization). The surrounding temporal bone is rendered in beige\/tan with a spongy texture, and the middle ear cavity is shown in red.<\/p>\r\n

Figure 20.11: Anatomical illustration showing a cross-sectional view of the cochlea, with a small inset diagram of the whole cochlear spiral in the upper left indicating the region being magnified. The main image shows a single turn of the cochlea cut in cross-section, revealing its internal compartments and structures. Labeled structures include: bony cochlear wall (the outer osseous shell), scala vestibuli (the upper fluid-filled chamber), cochlear duct (the middle triangular compartment containing endolymph), tectorial membrane (a gelatinous membrane overlying the hair cells), basilar membrane (the floor of the cochlear duct supporting the organ of Corti), scala tympani (the lower fluid-filled chamber), organ of Corti (the sensory epithelium containing hair cells, labeled on the right with a leader line pointing to the structure sitting on the basilar membrane), spiral ganglion (tan dots on the yellow cochlear branches that are the collection of sensory neuron cell bodies at the base), and cochlear branch of N VIII (the cochlear portion of the vestibulocochlear nerve, shown in yellow exiting to the right).<\/p>\r\n

Figure 21.4: A detailed flow chart illustrating the Negative Feedback Loop of hormone regulation, specifically focusing on the release of glucocorticoids. The process is shown in a continuous cycle of four main steps starting from a state of homeostasis. Stage 1, Imbalance: A cross-section of a blood vessel shows a low concentration of blue dots representing glucocorticoids. The hypothalamus perceives low blood concentrations of glucocorticoids via sensors in the blood vessels. Stage 2, Hormone Release: An illustration of the brain's hypothalamus and pituitary gland. The Hypothalamus releases Corticotropin-releasing hormone (CRH). A dashed line shows CRH traveling to start a hormone cascade that triggers the adrenal glands to release glucocorticoid into blood. Stage 3, Correction: An illustration of an adrenal gland (sitting atop a kidney). Blood concentration of glucocorticoids increases. A dotted line runs from the end of this stage (glucocorticoid release) to show another cross-section of a blood vessel with increasing density of blue dots (glucocorticoid levels in the blood increase). Stage 4, Negative Feedback: The hypothalamus illustration returns, but with a red \"X\" over the \"CRH release\" label. As glucocorticoid levels reach a normal concentration, the hypothalamus perceives this \"Correction\" and stops releasing CRH. This negative feedback returns the system to a state of Homeostasis.<\/p>\r\n

Figure 21.6: A flow chart diagram mapping the hormones produced by the pituitary gland and their various target organs in the body. The pituitary is split into two distinct lobes (posterior on the left and anterior on the right). Posterior Pituitary (Left Lobe) releases two primary hormones that act on specific tissues: Antidiuretic hormone and oxytocin that targets an unidentified organ but indicated by an outward arrow). Anterior Pituitary (Right Lobe) produces a wide range of hormones that regulate other endocrine glands and body processes. Prolactin targets the mammary glands and ovaries (to produce estrogens and progesterone). LH (Luteinizing Hormone) targets the ovaries (to produce estrogens and progesterone) and the testes (to produce androgens). FSH (Follicle-Stimulating Hormone) also targets the ovaries (to produce estrogens, progesterone) and testes (to produce androgens). TSH (Thyroid-Stimulating Hormone) targets the Thyroid gland to produce thyroid hormones (T3, T4). MSH (Melanocyte-Stimulating Hormone) targets pigment cells in the skin. ACTH (Adrenocorticotropic Hormone) targets the adrenal gland to produce glucocorticoids. GH (Growth Hormone) targets bones to stimulate growth.<\/p>\r\n

Figure 21.9: A complex scientific diagram illustrating the comparative anatomy and evolution of the pituitary gland across different vertebrate groups. The central element is a phylogenetic wheel, with anatomical cross-sections for specific species radiating outward. The diagram uses a color-coded legend to identify specific regions. grey: brain, brown: Pars nervosa, green: Pars intermedia, pink\/blue\/purple: Pars distalis (divided into Rostral, Medial\/Proximal, and Ventralis), black: Pars tuberalis, orange: median eminence, red lines: vascular system\/blood flow. Comparative Species (Clockwise from top) begins with A (Lamprey\/Hagfish) shows a primitive, elongated structure with distinct blue, pink, brown and green segments, and simple downward blood flow. B (Shark\/Elasmobranch) shows a long structure at the bottom with distinct orange, light blue, pink, brown, and green segments. It features a highly specialized ventralis region (dark blue) extending downward and a complex vascular network. C (Teleost Fish) displays a compact gland where the brain tissue (grey) interdigitates deeply with the pars distalis. D (Lungfish) shows an elongated pars distalis with a prominent orange median eminence. E (Frog\/Amphibian) is a more globular structure where the pars distalis (pink) is situated below the brain, with blood vessels entering through the median eminence. F (Turtle\/Reptile) displays a large, layered pars distalis with clear compartmentalization between the blue and pink regions. G (Mouse\/Mammal) features the classic mammalian structure with a well-defined pars nervosa (brown) and a large, unified pars distalis (pink) fed by a portal system through the median eminence.<\/p>\r\n

Figure 21.15: This diagram illustrates the homeostatic regulation of blood calcium levels (Ca^{2+}), showing the opposing roles of the thyroid and parathyroid glands. The system is depicted as a seesaw balanced on a fulcrum to represent equilibrium. High Blood Calcium (Upper Loop) shows when calcium levels rise above the set point (homeostasis). An imbalance occurs where blood Ca^{2+} is too high, which causes the thyroid gland to release hormones (calcitonin, though not explicitly labeled) to lower calcium. This stimulates bone calcium deposit, where excess calcium is stored in the bone tissue. The result is that blood Ca^{2+} levels decrease (indicated by the downward blue arrow) to return to homeostasis. Low Blood Calcium (Lower Loop) shows when calcium levels fall below the set point. An imbalance occurs where blood Ca^{2+} is too low, thus the Parathyroid Gland (shown as four small nodes) releases Parathyroid Hormone (PTH). PTH stimulates osteoclasts (blue multi-nucleated cells) to break down bone matrix. The result is that calcium is released from the bone into the blood, causing blood Ca^{2+} levels to rise (indicated by the upward blue arrow) back toward homeostasis.<\/p>\r\n

Figure 21.19: A detailed black-and-white anatomical diagram of the adrenal glands, showing their location in the body and their internal cellular structure. Gross Anatomy (Top) have two adrenal glands that are shown sitting directly on top of each kidney. The diagram includes the major blood vessels connecting to the kidneys. Gland Cross-Section (Bottom Left) is a close-up view of a single adrenal gland sliced open to show two main functional areas: Adrenal Medulla (the innermost core of the gland), and Adrenal Cortex (the thick outer layer surrounding the medulla). Microscopic View of the Cortex (Bottom Right) is a highly magnified rectangular inset that shows the three distinct layers (zones) of the adrenal cortex, each responsible for producing different hormones: Zona glomerulosa (the outermost layer, characterized by rounded clusters of cells), Zona fasciculata (the middle and thickest layer, consisting of cells arranged in long, straight columns), and Zona reticularis (the innermost layer of the cortex, featuring a net-like arrangement of cells adjacent to the medulla).<\/p>\r\n

Figure 21.22: This anatomical illustration provides a detailed comparative overview of the male and female reproductive gonads (Testis and Ovary) and their microscopic structures. The left side of the diagram focuses on the male reproductive system. The macro view is a cross-section of the testis that reveals internal compartments containing coiled seminiferous tubules, where sperm production occurs. The microscopic view (Cross section of seminiferous tubule) is circular detail showing the cellular arrangement within a tubule includes spermatogonium with stem cells located at the outer edge of the tubule, Sertoli cells are the large, supportive cells (highlighted in green) and Leydig cells are small clusters of blue cells located in the interstitial space between the tubules. The right side of the diagram illustrates the female reproductive system. The macro view, the ovary is shown containing follicles at various stages of the menstrual cycle including a developing follicle (small circular structures growing within the ovary), mature follicle (a large, fluid-filled follicle ready for ovulation), and Corpus luteum (shown in stages of forming and regressing after the egg has been released). The microscopic view (Cross section of follicle) is a circular detail shows a mature follicle containing oocyte (the central egg cell), granulosa cells (a thick layer of cells, highlighted in purple, surrounding the oocyte), and Theca cells (the outermost layer of cells, highlighted in yellow, that work with granulosa cells to produce estrogen.<\/p>\r\n

Figure 21.24: This anatomical diagram illustrates the Hypothalamic-Pituitary-Gonadal (HPG) axis, the primary hormonal system responsible for regulating reproductive functions in both males and females. The image is structured into three main components. Part 1 is the Hormonal Flowchart (Left) that shows a vertical pathway describing the sequence of hormone signaling. The hypothalamus is the control center that secretes GnRH (Gonadotropin-releasing hormone). Anterior pituitary is stimulated by GnRH to release LH (Luteinizing hormone) and FSH (Follicle-stimulating hormone). Gonad is the target organs (testis or ovary) that receive LH and FSH. Sex steroids are the final products are the sex steroids. Part 2 is the Negative Feedback Loop showing a large, curved arrow pointing from the sex steroids back up to the Anterior pituitary and the Hypothalamus. It is labeled with negative signs (-). Part 3 shows Anatomical Illustrations (Right) in detailed black-and-white line drawings representing the physical organs involved. The top is a cross-section of the brain showing the hypothalamus and pituitary gland. Bottom Left is a cross-section of a testis, showing internal seminiferous tubules. Bottom Right is a cross-section of an ovary, showing follicles at various stages of development and a corpus luteum.<\/p>\r\n

Figure 21.25: This infographic illustrates the menstrual cycle by mapping hormonal fluctuations against the physical changes in the ovary over a 28-day period. Section 1 are the Ovarian Stages (Top Row). The diagram tracks the development of the follicle and the egg across three main phases: Follicular Phase (Days 1\u201313) shows the growth from a small developing follicle to a large, pink mature follicle. Ovulation (Day 14) is a large, irregular white structure represents the ruptured follicle releasing the egg (pink dot). This is labeled as the fertile period. Luteal Phase (Days 15\u201328) shows the transformation of the ruptured follicle into an early corpus luteum (star-shaped), which eventually becomes a smaller regressing corpus luteum as the cycle ends. Section 2 shows Hormone Levels (Middle Section). Colored line graphs represent the concentrations of four key hormones. LH (red) shows a massive spike right before ovulation. FSH (orange) shows a moderate rise during the follicular phase and a small peak alongside LH. Estrogen (green) peaks twice\u2014once just before ovulation to trigger the LH surge, and again during the mid-luteal phase. Progesterone (blue) remains low until after ovulation, where it rises significantly during the luteal phase to support the uterine lining. Section 3 shows the timeline (Bottom Row). The horizontal axis is labeled Days and includes markers for Day 1, 7, 14, 21, and 28 to provide a chronological scale for the biological events.<\/p>","rendered":"

Figure 2.3<\/a>: A detailed anatomical diagram of a kangaroo illustrating directional terminology and anatomical planes. The kangaroo is shown in profile, facing left, with three intersecting geometric planes. The sagittal plane is the vertical plane that divides the body into left and right halves. Dorsal plane is the horizontal plane that divides the body into upper (proximal) and lower (distal) planes. Transverse plane is the vertical plane perpendicular to the long axis, dividing the body into front (cranial) and back (caudal) sections. The kangaroo is labeled as follows: head region includes the rostral (toward the nose) and caudal (toward the back of the head). Trunk and tail region includes the dorsal (back\/top), ventral (belly\/bottom), cranial (toward the head), and caudal (toward the tail). The limbs regions include palmar (the “palm” side of the front paw) and plantar (the “sole” side of the hind foot), cranial (the front of the legs), caudal (back of the legs).<\/p>\n

Figure 2.4: <\/a>Two male figures, the figure on the left is in profile, labeled the “lateral view.” There is a horizontal bidirectional arrow across the chest labeled “posterior or dorsal” on the left side of the arrow, and “anterior or ventral” on the right end of the arrow. The vertical bidirectional arrow runs from the ear to hip and is labeled at the top, “cranial,” and the bottom, “caudal.” The right figure is head-on and labeled the “anterior view.” There is a vertical bidirectional arrow from the shoulder labeled “proximal” to the wrist, labeled “distal.” There is a second bidirectional arrow from the upper thigh, labeled “proximal” down to the ankle, labeled “distal.” There is a horizontal bidirectional arrow from the left side of the elbow, labeled “medial” to the right side, labeled “lateral.” Between the figures is a vertical bidirectional arrow labeled “superior” at the top and “inferior” at the bottom.<\/p>\n

Figure 2.6:<\/a> Side by side diagram of a tree, labeled (A) and (B) of vertebrates indicating branches, nodes, roots, and branch tip. The left side tree designates “root” as the base horizontal line, “node” as the branching point where the lines split into two, specifically Node Y from the fish ancestry and Node X from the mammal, reptiles, and birds\u2019 ancestry. “Branch” is the horizontal and vertical lines connecting nodes or nodes to tips. “Branch tip” is the terminal end of a branch. From top to bottom, the species are illustrated as shark, bony fish, sheep, lizard, tyrannosaurus rex, and chicken. The diagram on the right is the same set of animals but rearranges the order of the branch tips. The shark, bony fish, and frog remain in the same top positions. The lower branches are rotated: the sheep is now at the bottom, while the T. rex, chicken, and lizard are clustered in the middle.<\/p>\n

Figure 2.7:<\/a> Two panels of protostomes. In the top panel, the left begins with a small cluster of cells labeled “eight-cell stage” and “spiral cleavage.” An arrow points to the next stage, “gastrulation” with a larger round cell. The middle is labeled “archenteron,” the small blue ovals near the bottom are “coelum” and “mesoderm.” The divot at the bottom is labeled “blastopore.” An arrow points to the last stage, “protostomes” with a larger oval shape with two arcs running vertically on each side. The inside of the arcs are labeled “coelum,” the outline of the arcs are “mesoderm.” The top opening is labeled “anus,” and the bottom is labeled “mouth.” The label “blastopore” from stage two also has a small arrow to the third stage “mouth” label.” The bottom panel begins with a cluster of cells labeled “radial cleavage.” An arrow points to the next stage with a larger round shape with the blue ovals now opened and at the top, labeled “mesoderm” and “coelum.” The divot at the bottom is labeled “blastopore.” An arrow points to the third stage, labeled “deuterostomes” with the top opening labeled “mouth” and the bottom labeled “anus.” The middle is labeled “digestive tube.” The oval shape has two vertical arcs along the side edges with the inside labeled “coleum,” and the outline of the arc labeled “mesoderm.” The “blastopore” label from stage two has a small black arrow pointing to the third stage label, “anus.”<\/p>\n

Figure 2.16:<\/a> The illustration at the top shows the tunicate larval stage, which resembles a tadpole, with a post-anal tail at the narrow end. A dorsal hollow nerve cord runs along the upper back, and a notochord runs beneath the nerve cord. The digestive tract starts with the mouth at the front of the animal connected to a stomach. Above the stomach is the anus. The pharyngeal slits, which are located between the stomach and mouth, are connected to an atrial opening at the top of the body. The bottom illustration shows an adult tunicate, which resembles a tree stump anchored to the bottom. The heart, stomach, and gonad are tucked beneath the pharyngeal slits. The top opening is the mouth, surrounded by the branchial siphon. Around the entire diagram is the tunic. The large balloon-like structure in the center is the pharyngeal basket, with small lines throughout, as pharyngeal slits. At the bottom right opening is the anus, surrounded by the atrial siphon.<\/p>\n

Figure 2.17:<\/a> An anatomical comparison of Cephalochordate, with a color diagram on top, and a black and white scientific illustration on the bottom. On the top image, is a lateral, cross-sectional color illustration. Key anatomical structures are labeled as follows. The notochord (a stiff rod) runs the length of the body, with the dorsal nerve cord situated directly above it. The head features a rostrum, wheel organ, and an oral hood with tentacles. The large pharynx contains numerous pharyngeal slits, pharyngeal bars, and an endostyle. Further back are the hepatic cecum, ileocolic ring, intestine, and anus. Labeled fins include the dorsal fin, ventral fin, and caudal fin. The atriopore is shown on the ventral side. The segmented muscle blocks called myomeres are visible, along with the gonads. In the bottom image, it highlights the v-shaped muscle segments along the length of the body, labeled with numbers (e.g., my 32, my 42, my 52). Letters point to specific regions like the rostrum (e), notochord (n), gonads (go), and atriopore (at).<\/p>\n

Figure 3.2: An illustration of neurogenic placodes. The central diagram is a side-profile line drawing of a developing embryo. Colored patches on the head and neck regions indicate the positions of various placodes. Purple is the Otic (ear) region. Green is the Trigeminal (nerve V) region. Light blue is the eye\/Lens region. Yellow is the olfactory (nose) region. Orange dots are the epibranchial placodes located near the pharyngeal arches. Surrounding the central embryo are detailed callouts showing the structures that arise from these regions. Trigeminal (V) (green) shows the Ophthalmic and Maxillomandibular branches. Epibranchial (orange) shows the development of the Geniculate (VII), Petrosal (IX), and Nodose (X) ganglia. Otic (purple) is a complex diagram of the inner ear structures, including the Cochlear-vestibular (VIII) ganglion. Lens (blue) is a cross-section of the developing eye lens. Olfactory (yellow) is a representation of the developing nasal sensory structures.<\/p>\n

Figure 3.5: A scientific diagram illustrating the concept of Hox gene collinearity and the evolutionary conservation of body patterning between Drosophila (fruit flies) and humans. Section A is Genomic Organization. This section shows the linear arrangement of Hox genes on chromosomes, emphasizing that the order of genes on the DNA matches the order of the body parts they regulate. Drosophila: Shows a single cluster of eight genes (including lab, pb, Dfd, Scr, Antp, Ubx, Abd-A, and Abd-B) arranged from the 3′ to 5′ end. Human Being displays four distinct clusters (HOXA, HOXB, HOXC, HOXD) located on different chromosomes. These are paralogs resulting from genome duplications. The genes are numbered (e.g., A1 through A13) and color-coded to match their counterparts in the fruit fly. Section B is spatial expression. This section features two illustrations\u2014an adult Drosophila and a human embryo\u2014color-coded to correspond with the gene clusters in Section A. Genes at the 3′ end of the clusters (pink\/red) are expressed in the head\/anterior regions, while genes at the 5′ end (blue\/purple) are expressed in the tail\/posterior regions. In the human embryo, the 5′ Hox genes (blue and purple) are also shown mapping to the development of the distal limbs (fingers and toes).<\/p>\n

Figure 3.13: A circular infographic representing the geological history of Earth as a 24-hour clock, spanning from its formation to the present day. Time is measured in Ga (billions of years ago) and Ma (millions of years ago). The central ring is divided into color-coded segments representing the major eons of Earth’s history, moving clockwise from the top: Hadean (Red, 4.6\u20134.0 Ga) is the earliest period following the formation of the Earth (4550 Ma) and the Formation of the Moon (4527 Ma). Archean (Pink, 4.0\u20132.5 Ga) is noted for the end of the Late Heavy Bombardment and the earliest start of photosynthesis (c. 3200 Ma). Proterozoic (Purple\/Blue, 2.5 Ga\u2013541 Ma) is the longest eon, featuring the first major increase in atmospheric oxygen (2300 Ma) and multiple “Snowball Earth” glaciations. Paleozoic is shown in blue, Mesozoic is light green, and Cenozoic is pale green. Major Biological and Environmental Events (Outer Arcs) are shown as colored arcs on the perimeter mark the appearance of life forms and significant climate events. Snowball Earth Events are labeled around 2300 Ma, 716\u2013660 Ma, and 650\u2013635 Ma. Evolution of Life are Prokaryotes & Eukaryotes: arising in the Archean and early Proterozoic. Multicellular Life emerges late in the Proterozoic. The Cambrian Explosion (c. 540 Ma) is a rapid diversification of animal life. Following the Cambrian period are the first Vertebrate Land Animals (c. 380 Ma): Non-avian Dinosaurs (230\u201366 Ma) which span most of the Mesozoic. First Hominins (2 Ma) appear at the very end of the timeline, near the 12 o’clock position.<\/p>\n

Figure 3.17: A black-and-white scientific illustration featuring four distinct prehistoric fish-like organisms, stacked vertically, and labeled A through D. Each drawing showcases different anatomical features of early Paleozoic jawless and jawed vertebrates. Image A is a Pteraspidomorph: a streamlined, spindle-shaped fish with a prominent, pointed snout (rostrum). It features large head shields on the top and bottom of the front half, a single dorsal spine, and a body covered in small, diamond-shaped scales leading to a lobed tail. Image B is an osteostracan: a fish with a broad, horseshoe-shaped head shield and small, upward-facing eyes. It possesses distinct, paddle-like pectoral fins and a body covered in vertical, rectangular scales. The tail is heterocercal (upward-turning). Image C is an anaspid: a more slender, elongated fish with a downward-sloping mouth. It lacks a heavy head shield and paired fins but features a series of small spines or scales along its dorsal ridge. Its body is covered in diagonal, ribbon-like scales, and it has a hypocercal (downward-turning) tail. Image D is a climatius: a fish with a large eye and a visible jaw. It is characterized by prominent, stiff spines supporting its dorsal, anal, and pectoral fins. It also features a row of small “intermediate” spines along its belly.<\/p>\n

Figure 3.26: A collage of 14 photographs showcasing the immense diversity of Actinopterygii (ray-finned fishes), featuring various body shapes, colors, and habitats. Fish species pictured (from top-left to bottom-right). Row 1: An eel swimming among aquatic plants; a large tuna being measured on a deck; and bright red sockeye salmon spawning in a shallow stream. Row 2: A round, silver piranha; a decorative lionfish with striped fins and venomous spines; and a long, camouflaged northern pike in murky water. Row 3: A dark grouper or sea bass; a vibrant blue and orange pygmy angelfish; and a slender, silver herring or sardine. Row 4: A spotted pufferfish underwater; a seahorse clinging to a vertical plant stem; and a bulbous, deep-sea anglerfish against a black background. Row 5: A primitive-looking sturgeon swimming past a rock wall; and a slender, spotted longnose gar hovering near the surface among greenery.<\/p>\n

Figure 3.34: A four-panel collage showcasing diverse examples of sarcoptergians. Top-Left (Guiyu oneiros) is an artist’s reconstruction of an extinct coelacanth swimming in blue water. It has a robust, silvery-blue body with heavy, plate-like scales on its head and distinct, fleshy lobed fins. It features two dorsal fins and a unique trilobed tail. Top-Right (Latimeria chalumnae) is a photograph of a living coelacanth in its dark, deep-sea habitat. The fish is dark blue or brown with irregular white speckles. Its thick, limb-like fins are clearly visible as it hovers near the ocean floor. Bottom-Left (Neoceratdous forsteri) is a photograph of a modern lungfish resting on a gravelly riverbed. It has an elongated, eel-like body with dark, olive-brown skin. Its pectoral and pelvic fins are notably thin and ribbon-like. Bottom-Right (Panderichthys) is an artistic rendering viewed from above. It has a flattened, crocodile-like head and a pale green, patterned body. The fins are positioned more like limbs, showing the evolutionary transition from water to land.<\/p>\n

Figure 3.39: A phylogenetic tree (cladogram) illustrating the evolutionary relationships of the Tetrapoda (four-limbed vertebrates). The diagram uses color-coded branches, representative animal silhouettes, and text labels to categorize different lineages. The tree branches from left to right, starting from ancestral “Fishapods” and stem tetrapods at the top, and diversifying into major clades of amphibians, mammals, and reptiles. Major Clades (Top to Bottom) begin with Batrachomorpha (Orange): includes extinct groups like \u2020Temnospondyli and \u2020Lepospondyli, leading to modern Lissamphibia (represented by a frog silhouette). Reptiliomorpha (Black\/Pink\/Purple\/Green) is a large group containing both non-amniotes and the Amniota. Synapsida (Pink) is the lineage leading to mammals. It includes the extinct \u2020Pelycosaurs, \u2020Therapsida, and modern Mammalia (represented by a bear). Reptilia (Purple\/Green) are part of the Amniota group, further divided into (purple): Parareptilia, Squamata, Rhynchocephalia, and Testudines: Turtles (connected by a dashed line indicating phylogenetic uncertainty). Archosauria (Green) includes Pseudosuchia Avemetatarsalia (represented by a chicken). Visual Indicators include dagger symbol used to mark extinct groups, dashed lines to indicate uncertain or debated evolutionary placements for specific groups like turtles and certain early amphibians, and vertical brackets on the right side. Large brackets group these lineages into broader categories: Amniota, Reptiliomorpha, and the overarching Tetrapoda.<\/p>\n

Figure 3.41: A scientific comparative diagram titled “Edopoidea” against a black background, displaying 8 different dorsal (top-down) skulls from the clade Edopoidea. The skulls are rendered in white with black lines showing the cranial sutures and eye sockets. A small white human hand silhouette and a 10 cm scale bar are placed near the largest skulls to provide a sense of massive size. Top Row (Large Species) features the four largest skulls (labeled 1\u20134). These skulls are broad and robust, with large openings for the eyes. Bottom Row (Small to Medium Species) features four smaller, more elongated skulls (labeled 5\u20138). The diagram includes a numbered key in the bottom right corner corresponding to the following species: Edops craigi (The largest, most massive skull), Adamanterpeton ohioensis, Nigerpeton ricqlesi, Saharastega moradiensis, Chenoprosopus milleri, Chenoprosopus lewisi, Cochleosaurus bohemicus, and Cochleosaurus florensis. The skulls vary significantly in shape, from the wide, U-shaped snout of Edops to the long, narrow, almost gharial-like snouts of the Chenoprosopus species. All skulls show characteristic “primitive” tetrapod features, such as a pineal foramen (a small hole for a “third eye”) located between the main eye sockets.<\/p>\n

Figure 3.47: An anatomical diagram showing the lateral (side) view of a prehistoric tetrapod skull and lower jaw. The illustration is a black-and-white line drawing with various bones labeled and shaded to show the complex structure of an early land vertebrate. The upper portion of the image displays the skull, featuring several distinct regions to include the snout, labeled with bones including the Premaxilla (front teeth-bearing bone), Nasal, Maxilla (main upper jawbone), and Lacrimal. The Eye Region includes the orbit (eye socket) and is surrounded by the Prefrontal, Frontal, Postfrontal, and Jugal bones. The Temple and Back of Skull includes the Postorbital, Supratemporal, Squamosal, and Quadrate (the jaw hinge point). The Palate is a small portion of the Pterygoid that is visible beneath the main cheek area. Lower Jaw (Mandible) shows the bottom portion shows the detached lower jaw, which is composed of several interlocking bones including Dentary (the large, front bone that houses the lower teeth), splenial and angular (bones that form the bottom and back-lower edge of the jaw), and surangular and articular (bones at the back of the jaw that facilitate the hinge mechanism with the upper skull).<\/p>\n

Figure 3.48: This four-panel collage features different views and species of the prehistoric synapsid Dimetrodon. The image contrasts fossil remains with artistic life reconstructions. Top-Left is a photograph of a mounted Dimetrodon skeleton in a museum. Top-Right is a detailed artistic chart showing several Dimetrodon species (including D. angelensis, D. grandis, and D. milleri) scaled against a silhouette of a human for size comparison. The species vary from dog-sized to over 4 meters in length, with different sail shapes and color patterns. Bottom-Left is a color illustration of a Edaphosaurus pogonia as it may have appeared in life. It is depicted with a lizard-like sprawling gait, a green-and-brown mottled body for camouflage, and a tall, vibrantly colored orange-and-yellow sail. Bottom-Right is another museum photograph of a skeleton. This view highlights the curvature of the ribs and the impressive height and density of the dorsal sail spines.<\/p>\n

Figure 3.49: A five-panel collage of artistic reconstructions showcasing the diverse body forms of Therapsids. The image illustrates a wide range of ecological niches, from apex predators to semi-aquatic herbivores. Top-Left (Inostrancevia) is a dramatic scene of a large, saber-toothed Inostrancevia standing over its prey, a green-scaled herbivore. The predator has a pinkish-grey, leathery hide and massive canine teeth designed for a powerful killing bite. Top-Center (Alopecognathus) is a close-up profile of a Alopecognathus, showing a long, narrow snout and specialized teeth. Top-Right (Ostehria) is a portrait of a small, beak-faced therapsid with large, expressive orange eyes. It features a turtle-like beak and two small tusks. Bottom-Left (Moschops) show two massive, heavy-set Moschops walking across a dry landscape. They have thick, barrel-shaped bodies, thick skulls, and a sprawling-to-semi-erect gait. Bottom-Right (Castorocoda) is a reconstruction of a Castorocoda swimming underwater. It has a streamlined, otter-like body and a broad tail.<\/p>\n

Figure 3.52: A four-panel collage showcasing extant monotremes. The image features high-resolution photographs and a cutout of these unique animals. Top-Left is a photograph of a platypus swimming at the surface of a dark pond. It shows its distinctive broad, flat tail, waterproof brown fur, and the leathery, duck-like bill that it uses for electrolocation. Top-Right is a photograph of a short-beaked echidna walking on sandy soil. Its body is covered in a dense coat of sturdy, cream-and-black spines. It has a small, dark snout and powerful claws for digging. Bottom-Left is a photograph of a long-beaked echidna foraging in tall green grass. Compared to its short-beaked relative, it has a noticeably longer, downward-curving snout and more visible fur between its lighter-colored spines. Bottom-Right is a clear studio-style cutout of a Western long-beaked echidna against a white background. This image highlights its sturdy, pillar-like legs, large digging claws, and the characteristic elongated “beak” used to feed on earthworms.<\/p>\n

Figure 3.55: A three-panel collage showcasing a mix of fossil remains and artistic life reconstructions of prehistoric parareptiles.\u00a0 Top is a photograph of a mounted skeleton of a Bradysaurus baini in a museum. The skeleton displays a massive, barrel-shaped body, thick ribs, and a robust skull with visible bony textures. Its sturdy, pillar-like legs are positioned in a semi-sprawling gait, indicating a slow-moving, heavily armored herbivore. Bottom-Left is an artistic reconstruction of a Mesosaurus, a small, aquatic parareptile. It features an elongated, streamlined body with a long neck and a narrow snout filled with fine, needle-like teeth. Its limbs are paddle-like, and it is depicted with a vibrant orange-and-white striped pattern. Bottom-Right (Sclerosaurus armatus) is an artistic rendering, viewed from a rear-angled perspective. This creature is smaller and more lizard-like, with a stout body covered in bumpy, pebbled scales. It features a broad head with small spikes or horns projecting from the back of the skull, likely for defense against predators.<\/p>\n

Figure 3.61: A seven-panel collage showcasing a diverse array of Archosaurs. The image mixes modern photography, museum skeletons, and life reconstructions. Top-Left is a photograph of a pair of Mallard ducks standing by the water. Top-Right is a museum mount of a Tyrannosaurus rex skeleton in a walking pose. Middle-Left is an artistic reconstruction of two pterosaurs (flying reptiles) in flight. They have colorful heads, elongated wings, and long tails. Middle-Right is a photograph of a Triceratops skeleton. It shows the iconic three-horned face, large bony frill, and heavy-set quadrupedal body. Bottom-Left is an illustration of a prehistoric Saurosuchus galilei (a crocodile-line archosaur) that looks superficially like a dinosaur but has a different ankle structure and a more sprawling-to-semi-erect gait. Bottom-Right is a photograph of a Nile crocodile with its mouth open in the water. Far-Bottom-Left is a museum display featuring a feathered Deinonychus with it’s from limbs stretching out and its mouth open.<\/p>\n

Figure 3.63: This phylogenetic tree, titled \u201cArchosauria Evolution,\u201d illustrates the evolutionary relationships and geological timelines of the two major archosaur lineages: the Pseudosuchia (crocodile-line) and the Dinosauria (including birds). The diagram is set against a horizontal timeline spanning from the Permian period (approx. 270 million years ago) to the present day. The diagram uses clean black lines for the tree structure, with species names written in colored text matching their silhouette icons. Light grey vertical bands highlight major geological boundaries and extinction intervals.\u00a0 Pseudosuchia (Top Branch – Blue): This lineage includes early armored forms: Aetosaurus, Riojasuchus, and Desmatosuchus (Triassic), Sphenosuchus and Dibothrosaurus (Jurassic\/Triassic), Dakosaurus (Cretaceous\/Jurassic), and Alligator and Crocodylus (Cretaceous to today). Dinosauria (Middle Branch – Green): This branch details the evolution of non-avian dinosaurs. Dinosaurs include Eoraptor, Plateosaurus, and Coelophysis (Triassic), Dilophosaurus and\u00a0 (Jurassic), Compsognathus (Jurassic\/Cretaceous), Citipati, Velociraptor, Archaeopteryx, and Ichthyornis (Cretaceous\/Paleogene). Neornithes (Bottom Branch – Red): Modern examples shown include Nothura, Gallus, and Geospiza (Cretaceous\/Paleogene, and Neogene periods). Along the bottom of the chart features a color-coded bar representing geological periods: Permian 255-270 million years ago (Red\/Purple), Triassic 200-255 million years ago (Purple), Jurassic 145-200 million years ago (Light Blue), Cretaceous 65-145 million years ago (Green), Paleogene 23-65 million years ago (Orange), and Neogene 0-23 million years ago (Yellow).<\/p>\n

Figure 3.64: This image is a collage illustrating the diverse evolutionary history and forms of rauisuchians. Top Left is an illustration of a long, armored Longosuchus meani swimming just below the water’s surface. Top Right is a photograph of two Gavialis gangeticus resting on sand, showing their characteristic long, thin snouts. Middle Left is a vibrant illustration of a Dakosaurus maximus leaping out of the water, featuring a streamlined body and a fluke-like tail. Middle Right (Upper) is a depiction of a heavy-set, land-dwelling Rauisuchian with a powerful build and a deep skull, walking through a Triassic forest. Middle Right (Lower) is a slender, long-legged terrestrial Litargosuchus leptorhynchus walking along a fallen log. Bottom Large Panel is an underwater scene showing a Chenanisuches lateroculi with a long snout and paddle-like limbs swimming through a murky blue environment. Bottom Chart is a scientific reconstruction of Postosuchus kirkpatricki shown in profile. It is a large, quadrupedal predator with a deep head and a long tail. A human silhouette is included for scale, showing the creature reaching roughly waist-to-chest height.<\/p>\n

Figure 3.66: This image is a four-panel collage focusing on Pterosauria. It highlights their anatomy, fossil record, and varied sizes. Top Left is a colorful paleoart illustration of a small, crested pterosaur in a lush prehistoric forest. It is depicted in mid-flight with its mouth open, pursuing a large, moth-like insect. Top Right is a photograph of a remarkably well-preserved pterosaur fossil embedded in a slab of light-colored limestone. The skeleton is laid out in profile, showing the delicate wing bones, a long neck, and a sharp, elongated skull. Faint impressions of the wing membranes are also visible. Bottom Left is a comparative size chart featuring several pterosaur silhouettes of varying wing spans overlaid against a human silhouette for scale. The silhouettes range from small, bird-sized species to much larger forms with several-meter wingspans. Bottom Right is a specific scale diagram for Simurghia robusta. It shows a black silhouette of the pterosaur in flight next to a human silhouette. A scale bar indicates 1 meter, showing that this species had a wingspan roughly twice the width of a human’s height.<\/p>\n

Figure 3.68: This image is a six-panel collage showcasing a diverse range of Ornithischian dinosaur skeletons, highlighting the major groups within this lineage. Top Left is a fossil of a small, Heterodontosaurid, preserved in a curled “death pose” within a rock matrix. Top Right is a mounted skeleton of a Nipponosaurus. It is shown in a quadrupedal stance with its characteristic toothless beak and high back. Middle Left is the skeleton of a Stegosaurus, featuring the famous upright bony plates along its spine and the “thagomizer” spikes on its tail. Middle Right is a low-profile, heavily armored skeleton of an Borealopelta. This “living tank” is covered in bony osteoderms and shows a wide, flat body designed for defense. Bottom Left are two skeletons of stegoceras in a dynamic, low-slung pose. These are known for their thick, bony skull caps used for head-butting or display. Bottom Right is a large, mounted skeleton of a Triceratops, showing its massive skull with three horns and a large bony frill at the back of the neck.<\/p>\n

Figure 3.72: A six-panel collage highlighting examples of paravian dinosaurs. Top Left is a fossil of a Confusciusornis sanctus preserved in limestone. It shows the distinct impressions of feathers around the wings and tail, alongside a reptilian skeletal structure. Top Right is a mounted skeleton of a Dromaeosaurid. It features sharp teeth, a large “sickle claw” on the foot, and a stiffened tail. Middle Left is a fossil slab showing a small troodontid in a flattened “death pose,” displaying elongated limbs and carbonized feather traces. Middle Right is a fossil of a Microraptor gui showing the clear outline of the body and limbs, emphasizing the dense covering of integument (feathers\/down). Bottom Left is a photograph of a modern Raven standing in the grass. Bottom Right is a museum display of a small Anchiornis huxleyi skeleton in a dynamic walking pose.<\/p>\n

Figure 4.6: Three cross-sectional diagrams illustrating the stages of gastrulation. Throughout the stages, the following layers are color-coded: blue (Ectoderm, the outer layer of cells), yellow (Endoderm, the inner layer of cells), red (Mesoderm, the middle layer that forms during the process). Stage A is Invagination (this initial stage shows the beginning of the inward folding of the embryo.) The blastocoel is a large fluid-filled cavity at the top in blue. Invagination shows the yellow cells at the bottom-right start to buckle inward and the dorsal lip of blastopore (the specific point where cells begin to migrate inside the embryo). Stage B is Involution and Epiboly. Involution show cells (marked in red as Mesoderm) that roll over the edge of the dorsal lip to move into the interior. Epiboly show the blue Ectoderm cells expand to cover the outer surface of the embryo. Blastocoel is a cavity that begins to shrink as it is displaced by new internal structures. Stage C is late Gastrula. Archenteron: a new, large cavity (primitive gut) is formed by the yellow Endoderm. Mesoderm (Red) is now clearly positioned between the Ectoderm and Endoderm. The blastocoel is reduced to a small remnant at the bottom-left.<\/p>\n

Figure 4.12: Two scientific diagrams (labeled A and B) illustrating the development of the vertebrate head. Diagram A shows the distribution of Posterior placodes (on the right of the diagram) and Anterior placodes (along the bottom) on the head of a developing embryo. Anterior Placodes include the Olfactory (green), Lens (magenta), and Adenohypophyseal (purple) placodes. Posterior Placodes are a series of colored spots representing the precursors to cranial nerves and sensory organs, including: Trigeminal, V (pink), Geniculate, VII (orange), Otic, VIII (dark blue), Petrosal, IX (red-orange), Nodose, X (deep red). Grey arrows indicate the migratory pathways and expansion of these tissues from the dorsal surface toward the ventral side. Diagram B illustrates the Pharyngeal Arches and Neural crest descendants with a scale at the bottom indicating the ventral-dorsal axis. Craniofacial Skeleton & Mandible are indicated by a light blue highlight along the anterior-dorsal edge, with blue arrows showing migration toward the front of the face. Rhombomeres (r1\u2013r8) show the hindbrain is segmented into regions labeled r1 through r8, colored in shades of purple. Neural Crest-Derived Glia is shown as small green “S” shapes clustered within the pharyngeal arches. Pharyngeal Arches are the segmented ridges at the bottom of the head.<\/p>\n

Figure 4.14: This scientific diagram illustrates the sonic hedgehog patterning. It highlights how specific signaling gradients dictate the identity of cells within the developing central nervous system. Labeled sections include: surface ectoderm (A green horizontal layer at the top), neural tube (a large blue oval structure in the center. Inside it is the neural canal), roof plate (the top-most section of the neural tube, marked by a yellow triangle with arrows pointing down the neural tube, directly beneath the surface ectoderm), floor plate (the bottom-most section of the neural tube, red arc with arrows pointing up the neural tube), neural crest cells (purple circular cells migrating away from the dorsal part of the neural tube), and Notochord (an orange rod-like structure located directly beneath the floor plate). The right side of the diagram features a \u201cmorphogen gradient\u201d chart that explains how cell identity is determined. BMP (Bone Morphogenetic Protein) is shown as a light green gradient. It is highly concentrated at the dorsal (top) side, secreted by the surface ectoderm and roof plate. Shh (Sonic Hedgehog) is shown as a red gradient. It is highly concentrated at the ventral (bottom) side, secreted by the notochord and floor plate.<\/p>\n

Figure 4.16: A scientific diagram illustrating the concept of Hox gene collinearity and the evolutionary conservation of body patterning between Drosophila (fruit flies) and humans. Section A is Genomic Organization. This section shows the linear arrangement of Hox genes on chromosomes, emphasizing that the order of genes on the DNA matches the order of the body parts they regulate. Drosophila shows a single cluster of eight genes (including lab, pb, Dfd, Scr, Antp, Ubx, Abd-A, and Abd-B) arranged from the 3′ to 5′ end. Human Being displays four distinct clusters (HOXA, HOXB, HOXC, HOXD) located on different chromosomes. These are paralogs resulting from genome duplications. The genes are numbered (e.g., A1 through A13) and color-coded to match their counterparts in the fruit fly. Section B is spatial expression. This section features two illustrations\u2014an adult Drosophila and a human embryo\u2014color-coded to correspond with the gene clusters in Section A. Genes at the 3′ end of the clusters (pink\/red) are expressed in the head\/anterior regions, while genes at the 5′ end (blue\/purple) are expressed in the tail\/posterior regions. In the human embryo, the 5′ Hox genes (blue and purple) are also shown mapping to the development of the distal limbs (fingers and toes).<\/p>\n

Figure 4.17: This scientific diagram illustrates the boundaries of Hox gene expression. Rhombomeres (r1\u2013r7) show that the central grey-shaded column represents the hindbrain, divided into seven distinct segments called rhombomeres. Cranial Nerves (V, VII, IX) are on the left side as orange clusters that represent the sensory ganglia of the cranial nerves: V (Trigeminal) that are associated with r2, VII (Facial) that are associated with r4, IX (Glossopharyngeal) that are associated with r6. Branchial Arches (ba1\u2013ba3) are the curved structures on the right that represent the pharyngeal or branchial arches, which eventually form the structures of the face and neck. Neural Crest Cell Migration (ncc) display large green arrows and circles to show neural crest cells migrating from specific rhombomeres into the arches: cells from r1 and r2 migrate into ba1, cells from r4 migrate into ba2, cells from r6 migrate into ba3. Otic Vesicle (OV) is a yellow oval located between ba2 and ba3, which will develop into the inner ear. The right side of the diagram features vertical colored bars representing the expression domains of various Hox genes (Hoxa2, Hoxb2, Hoxa3, etc.).<\/p>\n

Figure 4.22: This diagram illustrates the famous Spemann-Mangold organizer experiment, a landmark study in embryology that demonstrated the concept of embryonic induction. The image follows a step-by-step process of transplanting tissue from one amphibian embryo to another to see how it affects development. At the top, a piece of the dorsal lip of the blastopore is taken from a donor embryo and transplanted into the ventral side (the opposite side) of a recipient embryo. The recipient embryo shows a primary invagination on its original dorsal side and the transplanted tissue beginning a secondary invagination on the ventral side. Invagination (Middle Left) shows that as the embryo develops, two separate sites of invagination occur simultaneously: the original “primary” site and the “secondary” site induced by the transplanted donor tissue. Cross-section (Middle Right) is the transverse section of the developing larva that shows two complete sets of dorsal structures. A neural tube and notochord on one side. A second neural tube and notochord on the opposite side, flanking a shared central endoderm. The final illustration shows a “conjoined twin” tadpole. Two distinct heads and bodies are fused along their belly (ventral) side, demonstrating that the transplanted dorsal lip was able to organize a completely new body axis.<\/p>\n

Figure 5.1: An anatomical illustration of a human arm and shoulder, featuring three detailed inset diagrams labeled A, B, and C that show the microscopic structures of connective tissues. The central image is a medical illustration of a person’s upper torso and arm. The arm is flexed at the elbow, highlighting the biceps muscle, shoulder bones, and the humerus. Three black lines lead from specific parts of the arm to detailed inset boxes. Inset A is the Tendon Structure that shows a hierarchical breakdown of a tendon: the overall tendon contains multiple fascicle bundles, a single bundle contains numerous collagen fibrils, the fibrils are composed of individual triple-helix collagen molecules. Inset B shows the skin layers that displays a cross-section of the integumentary system including epidermis: the top purple layer; dermis: the middle layer, featuring a magnified view of a dense network of thick, wavy collagen fibers and thinner, red star-shaped elastin fibers; fat (Hypodermis): the bottom yellow layer. Inset C is the bone microstructure that is a microscopic view of osseous tissue (bone), showing multiple circular structures. Within the circular structures are lamellae: concentric rings of mineralized matrix; osteocytes: Small, dark, spider-like cells embedded within the rings.<\/p>\n

Figure 5.3: Three separate diagrams labeled A, B, and C illustrate how forces work. Panel A: Force and Weight shows an illustration of a purple cat sitting on a dark horizontal platform. Two vertical arrows represent opposing forces: a teal upward arrow labeled with a square F represents a force. A purple downward arrow labeled with a square and w represents the weight of the cat due to gravity. Panel B: Stress and Deformation is a technical diagram showing the deformation of an object. Its original shape is a light purple vertical oval. The deformed shape is a dark horizontal oval showing the object flattened and widened. Internal stress are pink arrows pointing outward from the center, labeled with the Greek letter sigma (\u03c3), representing internal tensile stress. External Pressure (p) are orange arrows pointing inward at the top and bottom, labeled with p, representing compressive pressure. Strain (e) is shown with horizontal double-sided arrows on the left and right labeled e. Panel C: Locomotion Comparison is a side-by-side comparison of movement styles between a hare (rabbit) and a tortoise between a “START” and “FINISH” line. The hare is a purple rabbit shown next to a dashed path consisting of three high, bouncy parabolic arcs, representing saltatory (hopping) locomotion. The tortoise is a purple tortoise shown next to a dashed path that is mostly flat and low to the ground, representing steady, crawling locomotion.<\/p>\n

Figure 5.4: This image is a mathematical and geometric reference chart comparing the Surface Area (SA), Volume (V), and Surface Area to Volume Ratio (SA:V) of three geometric shapes: a cube, a sphere, and a cylinder. Each shape is shown in two sizes to demonstrate how scaling affects these ratios. In a cube, the formulas are SA = 6l^2, V = l^3. The Large Cube (l=2): SA = 24, V=8, SA:V=3. Small Cube (l=1): SA = 6, V = 1, SA:V = 6. Two wireframe cubes are shown with a red line indicating the side length l.2. Sphere formulas are: SA = 4 times pi r^2, V =4\/3 times pi r^3. Large Sphere (r=2): SA = 50.3, V=33.5, SA:V=1.5. Small Sphere (r=1): SA=12.6, V=4.2, SA:V=3. Two spheres are shown with a red radius line r and a horizontal cross-section. The cylinder formulas are: SA = 2 times pi times r times h + 2 times pi times r^2 and V=pi times r^2 times h. The large cylinder (r=2, h=4) is: SA = 75.4, V = 50.3, SA:V=1.5. The small cylinder is (r=1, h=2): SA = 18.9, V = 6.3, SA:V=3. Two vertical cylinders are shown with red lines indicating radius (r) and height (h).<\/p>\n

Figure 5.7: This anatomical diagram illustrates the biomechanics of the human arm during two primary movements: extension and flexion, highlighting the antagonistic relationship between the biceps and triceps muscles. Extension (left image) of the arm is straightened out, and the elbow joint is open. The triceps (the muscle on the back of the upper arm) is contracted to pull the forearm down. A thick red arrow labeled Ft indicates a strong contractile force from the triceps. A thinner, smaller red arrow labeled Fb indicates that the biceps is relaxed or elongated. Flexion (right image) shows the arm bent at the elbow. The elbow joint is closed, and the hand moves toward the shoulder. The biceps (the muscle on the front of the upper arm) is contracted and “bunched up.” A thick red arrow labeled Fb indicates a strong contractile force from the biceps. A thinner red arrow labeled Ft indicates that the triceps is now the relaxed or elongated muscle.<\/p>\n

Figure 5.8: This image explores the mechanics of lever systems. Panel A shows basic lever physics. A cat (the load) sits on one end of a tilted beam, supported by a triangular fulcrum in the middle. A pink downward arrow labeled F-in represents the input force (effort) applied to one end. A teal upward arrow labeled F-out shows the resulting lift on the cat, while a purple downward arrow labeled load represents the cat’s weight. Panel B shows comparative Anatomy. Two animal limbs are compared to show how lever lengths affect function. A runner (horse) features a long, thin limb. The effort is applied close to the joint (the fulcrum, represented by an orange triangle). A digger (mole) features a short, robust limb with a wide “hand.” The lever arm for the muscle is proportionally longer. Panel C shows classes of levers in the human body. Three anatomical examples demonstrate the different classes of levers: 1st class lever (the head) shows the fulcrum (orange triangle) is between the effort and the load. The neck muscles pull down (effort) to lift the face up (load), pivoting at the base of the skull. 2nd class Lever (the foot) shows the load is between the fulcrum and the effort. Standing on tiptoes uses the ball of the foot as the fulcrum, the body weight as the load, and the calf muscles pulling up as the effort. 3rd class lever (the arm) shows the effort is between the fulcrum and the load. The elbow is the fulcrum, the biceps provide the upward effort in the middle, and the hand\/forearm acts as the load.<\/p>\n

Figure 5.9: Five drawings of bones illustrating the different types of loading regime. The first (left) shows tension with red arrows above and below the bone, pointing in opposite directions (reddened bone area at the top and bottom areas). The second bone shows compression with red arrows above and below the bone pointing towards one another (reddened bone area at the top and bottom). The third bone shows shear with a split in the middle of the bone (red around the area near the split) with arrows on each side of the split pointing to one another. The fourth bone shows torsion with redness along the length of the bone and arrows at the top and the bottom curving around the bone, laterally. The last bone shows bending with the bone curving inward, two arrows curving inward (reddened bone along the outside curve).<\/p>\n

Figure 5.13: This anatomical diagram compares the skeletal structures and limb orientations of a sprawling alligator and an upright human, illustrating how different body postures affect the arrangement of bones. In all four views, homologous bones (bones that share a common evolutionary origin) are color-coded for comparison: pelvis and shoulder Girdle (purple), humerus \/ Femur (magenta), radius & ulna \/ tibia & fibula (teal). The top left image shows an alligator skeleton viewed from above. The limbs protrude horizontally from the body before bending downward at the “elbow” or “knee.” The bottom left (Sprawling Model) is a simplified front-view diagram of the alligator’s pelvic region. The magenta femur bones extend outward at nearly 90 degrees from the purple pelvis. The right-side image shows a human skeleton in a standing, bipedal position. The limbs are positioned directly underneath the torso. The bottom middle (Upright Model) is a simplified front-view diagram of the human pelvic region. The magenta femur bones are aligned vertically beneath the purple pelvis.<\/p>\n

Figure 5.14: This illustration demonstrates forelimbs in various animals. Panel A shows the forelimb skeletons of five different mammals. Each bone group is color-coded to highlight their shared ancestry despite different functions: Beige\/Tan is the Humerus (upper arm), teal is the radius and Ulna (forearm) and Carpals\/Phalanges (hand\/fingers), magenta is the olecranon process (elbow), which acts as a lever arm for the triceps muscle. Orange triangles indicate the fulcrum point of the elbow joint. Flight (bat) has extremely elongated phalanges (fingers) to support a wing membrane. Swimming (whale) has shortened, flattened bones forming a paddle-like flipper. Grasping (human) has a versatile structure with a highly mobile thumb and fingers. Running (horse) has elongated distal bones. Digging (mole) has very short, thick bones with a massive, broad “hand” and a large magenta olecranon process. Panel B shows specialized locomotion in the skeleton of a sloth hanging upside down from a branch. Teal Bones show the limbs are exceptionally long, allowing the sloth to reach distant branches while remaining securely suspended. Olecranon Process is a small magenta area at the elbow (marked with an orange triangle) that shows where the muscles attach to maintain this hanging posture.<\/p>\n

Figure 7.5: A detailed anatomical diagram of a femur, shown in a longitudinal cross-section to reveal its internal and external structures. The diagram is divided into three primary regions: the epiphyses at the ends, the metaphyses in the transition zones, and the diaphysis making up the long shaft. The Proximal Epiphysis is the upper rounded end of the bone, covered in smooth, blue Articular cartilage. The diaphysis is the long, tubular central shaft of the bone. The distal epiphysis is the lower end of the bone, also capped with articular cartilage. Metaphysis are the regions between the diaphysis and the epiphyses; the proximal metaphysis contains the Epiphyseal line. The Periosteum is a tough, fibrous outer membrane shown partially peeled back. Inside is the compact bone, the dense, hard outer layer that forms the exterior of the diaphysis. The spongy bone is located primarily in the epiphyses, shown with a porous, honeycomb-like structure filled with red bone marrow. The Medullary Cavity is the hollow central chamber of the diaphysis, lined by a thin membrane called the Endosteum. The yellow bone marrow is shown as a fatty, cylindrical mass stored within the medullary cavity. The nutrient artery is a prominent red blood vessel entering the bone through the periosteum to provide nourishment to the internal tissues.<\/p>\n

Figure 8.2: This image provides a detailed anterior (front) view of a human skull, with color-coded bones and comprehensive anatomical labels. The frontal bone (pink) forms the forehead and the upper part of the eye sockets (orbits). Parietal bones (tan) located on the sides and roof of the cranium, separated from the frontal bone by the coronal suture. Zygomatic bones (teal) form the lateral walls of the orbits. Maxilla (orange) is the upper jaw bone. Mandible (grey) is the lower jaw bone, featuring the mental foramen and the lower alveolar process. Nasal bones (blue) are the small bones forming the bridge of the nose. The diagram highlights several specialized openings and bones within the orbit. Optic canal is for the passage of the optic nerve. Superior and Inferior orbital fissures are slit-like openings. Lacrimal bone (purple) is a small bone at the inner corner of the eye. Ethmoid and sphenoid bones are deep bones that form the posterior walls of the orbit. Nasal septum is composed of the vomer bone and the perpendicular plate of the ethmoid bone. T middle and inferior nasal conchae are visible as bony plates within the nasal cavity.<\/p>\n

Figure 8.4: This scientific illustration provides a lateral (side) view of the internal anatomy of a shark’s head and neck region, with major skeletal and respiratory structures color-coded and labeled. The chondrocranium (yellow) is the main braincase of the shark, made of cartilage. It includes the rostral cartilage at the snout, the olfactory capsule for smell, the optic capsule surrounding the eye, and the otic capsule for hearing and balance. The mandibular arch (red) is the first branchial arch, which has evolved into the jaws. It consists of the palatoquadrate (upper jaw) and Meckel\u2019s cartilage (lower jaw). The hyoid arch (blue) is the second branchial arch, which supports the jaw. Key components include the hyomandibula, ceratohyal, and basihyal. Gill arches and slits (Green) are five posterior gill arches support the respiratory system, with vertical gill slits between them for water passage. A smaller opening called the spiracle is located just behind the eye.<\/p>\n

Figure 8.5: This diagram provides a lateral (side) view of the head and jaw skeleton of a generalized vertebrate embryo, color-coded by the embryonic germ layers that give rise to specific tissues. The dermatocranium (light blue blocks) is shown as a series of rectangular segments forming the outer “shell” of the skull and lower jaw.\u00a0 The neurocranium is the inner braincase, divided into a light blue anterior section and a large orange posterior section (occipital region). It houses the sensory capsules: nasal, optic (eye), and otic (ear). The viscerocranium (light blue arches) is the region that includes the jaws and the branchial (gill) arches. The arches are numbered 1 through 7. Arch 1 forms the primary jaws, while the subsequent arches support the pharynx. The diagram uses a legend in the bottom left to identify the origin of each structure. Blue (Ectoderm) forms the sensory capsules (nasal, optic, otic). Orange (Mesoderm) forms the posterior neurocranium and segments of the dermatocranium. Light Blue (Neural Crest) is a specialized tissue that forms the majority of the facial skeleton, including the jaws and branchial arches. Yellow (Endoderm) forms the lining of the pharynx. Green (Chordamesoderm) forms the notochord, the primitive backbone structure visible on the right.<\/p>\n

Figure 8.12: An evolutionary diagram illustrating the divergence of amniote skull types based on the arrangement of four specific skull bones: the Parietal (red), Post-orbital (blue), Squamosal (yellow), and Jugal (green). The diagram starts from a common ancestral skull on the left and branches into two main lineages, which further diversify into five modern or specialized skull configurations on the right. The ancestral form (center-left) is an anapsid-like skull with no temporal fenestrae (holes) behind the eye socket. The upper branch lineage splits toward skulls with two temporal openings. The top-most skull (green sea turtle) shows a clear upper and lower fenestra. The middle-right skull (North American rat snake) shows a highly modified, kinetic structure where several bones are reduced or disconnected. The third skull down (Caiman lizard) shows the loss of the lower temporal bar. The lower branch lineage leads to a skull with a single temporal opening. The bottom-right skull (domestic cat) shows a significantly enlarged parietal bone and a zygomatic arch formed by the jugal and squamosal bones.<\/p>\n

Figure 8.13: The image displays a comparative layout of six different reptile and dinosaur skulls, labeled A through F. Each specimen is shown from two angles: a lateral (side) view on top and a dorsal (top-down) view below it. The skulls are rendered as 3D digital scans or high-resolution models against a solid black background. Skull A (Savannah monitor) is long, slender skull with numerous small, sharp teeth. Skull B (Tuatara) is a shorter, robust skull featuring a prominent, dashed white line outlining the temporal fenestra (opening behind the eye socket). Skull C (Tropical rattlesnake) is a highly kinetic skull with thin, delicate bones and a flexible jaw structure, typical of a snake. Skull D (Loggerhead sea turtle) is a smooth, dome-shaped skull with no visible teeth and a beak-like snout. Skull (T-rex) is a massive, deep-jawed skull with large serrated teeth and multiple large openings (fenestrae). Skull F (New Guinea crocodile) is a long, narrow snout with a bumpy, rugose texture and upward-facing nostrils.<\/p>\n

Figure 8.14: This image features a collection of 12 distinct avian (bird) skulls, labeled A through L, displayed in a grid against a solid black background. All skulls are shown in a lateral (side) view, rendered as high-resolution 3D digital scans. Skull A (Eurasian sparrowhawk) is robust skull with a short, hooked beak. Skulls B (Tasmanian nativehen) & H (Lesser yellow-headed vulture) are skulls with large, powerful hooked beaks. Skulls C (Kagu) & L (Northern gannet) are skulls with long, thin, straight, and pointed beaks. Skull D (Shoebill) is a specialized skull with a very long, downward-curved tip, like a shorebird or flamingo. Skull E (Bee hummingbird) is a skull with an exceptionally long, needle-thin beak. Skull F (Green-winged teal) is a skull with a wide, flattened beak. Skull G (Ostrich) is a skull with a straight, sturdy beak of moderate length. Skulls I (Yellow-bellied sapsucker) & J (American tree sparrow) are smaller skulls with shorter, generalist beaks. Skull K (Red-tailed hawk) is a skull with a deeply curved, sharp beak.<\/p>\n

Figure 8.15: This image is a black-and-white scientific line drawing showing a lateral (side) view of a reptilian skull and lower jaw, with individual bones labeled for anatomical identification. The upper portion of the image displays the cranium with various bones and openings. The snout area includes the Premaxilla (front tip), Maxilla (main tooth-bearing bone), Nasal, and Lacrimal. The eye and forehead include the Prefrontal, Frontal, Postfrontal, and the Jugal bone beneath the eye socket. The rear skull identifies the Postorbital, Supratemporal, Squamosal, and Quadrate (the hinge point for the jaw). Dark shaded areas represent the orbit (eye socket) and temporal fenestrae (openings in the skull for muscle attachment). The lower portion shows the separated mandible, highlighting its complex structure. The front shows the Dentary, which holds the lower teeth, and the Splenial on the inner side. The rear includes the Surangular (top back), Angular (bottom back), and the Articular bone, which connects to the upper skull’s quadrate bone to form the jaw joint.<\/p>\n

Figure 8.16: This image features a collection of 12 diverse mammalian skulls, labeled A through L, presented in a grid against a solid black background. Each skull is shown in a lateral (side) view and rendered as a high-resolution 3D digital scan. Skulls A (Platypus) & B (Short-beaked echidna are elongated, slender skulls with narrow snouts. Skulls C (Common wombat) & D (Black wallaroo) are skulls with flattened, broad snouts and prominent incisors. Skulls E (Grizzly bear) & F (Fisher cat) are robust, heavy-set skulls with short snouts and strong zygomatic arches (cheekbones). Skulls G (Wolverine) & H (Tiger) are sharp-toothed skulls with prominent canines. Skull I (Chacma baboon) is a specialized skull featuring a massive, downward-pointing upper canine. Skull J (Domestic horse) is a long, low skull with a specialized snout. Skull K (Orca) is unique skull with a long, thin snout filled with numerous small, uniform teeth. Skull L (Vampire bat) is a rounded, high-domed braincase with a short snout and front-facing eye sockets.<\/p>\n

Figure 8.17: This image illustrates the evolutionary development of the hard palate across three different stages of synapsid evolution, viewed from both the side (lateral) and the bottom (ventral). The diagram compares three specific groups: Early synapsid shows a primitive arrangement where the palate is largely open, Therapsid shows an intermediate stage with the bones beginning to move toward the midline, and Canis familiaris (Domestic Dog) represents the modern mammalian condition with a fully formed bony secondary palate. Three primary bones are highlighted to show their shifting positions and expansion over time: blue (Premaxilla) is located at the very front of the snout, yellow (Maxilla) is the primary tooth-bearing bone of the upper jaw, which expands inward in mammals, red (Palatine) is located toward the back of the mouth, which also grows inward to complete the roof of the mouth. The lateral view (left column) shows the elongation of the skull and the positioning of the jaw bones from the side. The ventral view (right column) provides a clear look at how the Maxilla and Palatine bones gradually meet at the midline to separate the nasal passage from the oral cavity, a defining characteristic of mammalian evolution that allows for simultaneous eating and breathing.<\/p>\n

Figure 8.18: This diagram, titled “Jaw Evolution Theories,” presents two major scientific frameworks for how vertebrate jaws originated from ancestral structures. The Gill Arch Theory section illustrates the classical view that jaws evolved from modified gill (branchial) arches. The initial stage shows a primitive jawless head with a series of vertical gill arches. The first three arches are color-coded: blue, orange, and green. The Serial Theory suggests that the first gill arch (orange) migrated forward and enlarged to become the entire jaw, while the second arch (green) became the hyoid support. The Composite Theory proposes a more complex origin where parts of multiple anterior arches (blue and orange) fused together to form the upper and lower jaw structures. The Heterotopic Theory section explores the developmental and genetic origin of jaws based on neural crest cells. The lamprey crest cell pattern displays a jawless lamprey where a specific neural crest cell population (purple) forms a long, singular arched structure. The proposed gnathostome crest cell pattern illustrates the shift in “jawed” vertebrates (gnathostomes), where a change in the location of gene expression (heterotopy) caused the neural crest cells to differentiate into distinct upper (yellow) and lower (purple\/pink) jaw elements.<\/p>\n

Figure 8.19: This diagram, titled “Types of Jaw Suspension,” outlines the evolutionary variations in how the upper jaw attaches to the skull in different vertebrate groups. The structures are color-coded for clarity. Blue is the chondrocranium (the skull\/braincase). Green is the Mandibular Arch (the upper and lower jaw bones). Purple is the Hyomandibula (the bone derived from the second gill arch). Primitive Autostylic is found in early jawed vertebrates. The upper jaw is attached directly to the chondrocranium without any help from the hyomandibula. The Amphistylic, seen in early Chondrichthyes and early bony fishes shows the jaw has a dual attachment: it is supported by both a direct ligamentous connection to the skull and by the hyomandibula. Hyostylic shows derived Chondrichthyes and derived bony fishes. The primary attachment of the jaw is via the hyomandibula, which acts like a swinging hinge. Secondary Autostylic is found in Dipnoans and Tetrapods. The upper jaw is fused directly to the skull. Holostylic is specific to Holocephalans. The upper jaw is completely fused to the chondrocranium, forming a single rigid unit, which is an adaptation for crushing hard-shelled prey.<\/p>\n

Figure 8.20: This diagram illustrates jaw articulation shown through four representative skulls. The diagram tracks four specific bones to show their change in size and function: Blue (Dentary) is the primary bone of the lower jaw. Yellow (Squamosal) is part of the skull that eventually forms the socket for the mammalian jaw joint (the glenoid fossa). Pink (Quadrate) is the upper jaw joint. Green (Articular) is part of the lower jaw joint. Basal cynodont displays many bones in the lower jaw and a jaw joint located at the back of the skull. Therapsid shows the expansion of the dentary bone and the reduction of the posterior jaw bones. Basal mammal shows the dentary bone now makes up the majority of the lower jaw, making direct contact with the skull. Didelphis virginianus represents the modern mammalian condition where the jaw joint is formed solely by the dentary and squamosal bones.<\/p>\n

Figure 8.21: This diagram illustrates the evolutionary transformation of the jaw joint bones into the mammalian middle ear, viewed from the side (lateral) focusing specifically on the lower jaw and its hinge. The diagram uses consistent colors to track the fate of each bone. Blue (Dentary) is the primary tooth-bearing bone. It expands to become the entire lower jaw in mammals. Red (Angular) becomes the ectotympanic bone, which supports the eardrum in mammals. Yellow (Squamosal) is the part of the skull that articulates with the dentary to form the mammalian jaw joint. Green (Articular) migrates to the ear to become the malleus (hammer). Purple (Quadrate) migrates to the ear to become the incus (anvil). The sequence tracks the reduction and migration of jaw bones through five stages. Dimetrodon is a basal synapsid with a large, multi-boned lower jaw. Basal cynodont shows the dentary bone starting to expand toward the back of the jaw. Therapsid shows the posterior bones become significantly smaller as the dentary continues to grow. Basal mammal shows that the dentary forms a new, direct joint with the skull (squamosal), and the old jaw bones are nearly detached. Didelphis virginianus shows the modern mammalian condition where the old jaw bones have completely migrated to the middle ear.<\/p>\n

Figure 8.27: Scientific illustration diagram showing eleven labeled panels (A\u2013K) comparing diverse mammalian tooth types through detailed anatomical line drawings. A (Secodont\/Carnassial Teeth) are comparative drawings of last upper premolars and first lower molars from cat, dog, and bear, with sharp cusps labeled. B (Denticulate) is a multi-cusped tooth from a crab-eater seal with a denticulate crown. C (Triconodont) is a fossil mammal tooth with three cones arranged in a row. D (Trituberculate) is a fossil mammal tooth with three cones in a triangular arrangement. E (Bunodont) is a vertical cross-section of a human or monkey molar showing internal anatomy including enamel, dentine, pulp cavity, cement, neck, and root, with low rounded cusps on the crown. F (Brachydont Selenodont) is a tapir tooth with a small crown, uneven grinding ridges, and visible root. G (Brachydont) is a surface view of a tooth showing crescentic enamel ridge, dentine, and cement. H (Hypsodont Selenodont) shows tall prism-like crowned teeth with uneven grinding ridges of enamel. I (Hypsodont Tooth in cross-section) is a vertical section comparing unworn (left) and worn (right) states, labeling enamel, dentine, cement, and pulp cavity. J (Lophodont) shows three views of teeth with transverse ridges. K (Lophodont of elephant) is a surface view of an elephant molar showing transverse ridges (lophs) and crescentic enamel ridges.<\/p>\n

Figure 9.2: An anatomical diagram showing the structure of human vertebrae from two different perspectives, with detailed labels for the bony landmarks and associated neural structures. The illustration on the left shows a single vertebra from a top-down perspective. The Body is a large, circular bony mass located at the Anterior (front) side. The Vertebral Foramen is the central opening that houses the Spinal cord (depicted in yellow). The Vertebral Arch is formed by the Pedicle (sides) and Lamina (roof). The Spinous process points toward the Posterior (back). The transverse processes extend out to the sides. Facets includes the Facet of superior articular process and the Facet for head of rib. The illustration on the right shows a stack of three articulated vertebrae from a side\/back angle. The diagram is labeled with Anterior toward the left (the bodies) and Posterior toward the right (the spinous processes). Intervertebral structures show the Intervertebral discs sandwiched between the vertebral bodies. The Neural Exit highlights a Spinal nerve exiting through the intervertebral foramen, which is the gap between adjacent vertebrae. Joint Connections point out where vertebrae connect, specifically the Inferior articular process of one vertebra meeting the Superior articular process of the one below it.<\/p>\n

Figure 9.11: An anatomical diagram of the human rib cage and breastbone, divided into two labeled sections: (a) an isolated anterior view of the sternum and (b) a full anterior view of the skeletal thorax. The sternum image shows the isolated breastbone from the front, oriented with Superior at the top and Inferior at the bottom. The Manubrium is the wide, top portion containing the Jugular notch in the center and Clavicular notches on the sides. The sternal angle is the horizontal joint where the manubrium meets the body. The body is the long, central part of the sternum. The Xiphoid process is the small, pointed tip at the bottom. The view of the skeleton of the thorax image shows the sternum in its anatomical position, connected to the ribs and clavicles. Sternum Connections show the Clavicle (collarbone) attaching to the clavicular notch and the Scapula (shoulder blade) in the background. Ribs are numbered 1 through 12. Ribs 1\u20137 are “true ribs” connected directly to the sternum via Costal cartilages. Ribs 8\u201310 are “false ribs,” and 11\u201312 are “floating ribs.” The Intercostal space are the gaps between the ribs are labeled. The T11 and T12 thoracic vertebrae are visible at the bottom of the rib cage.<\/p>\n

Figure 9.14: An anatomical diagram illustrates the first two cervical vertebrae, the atlas (C1) and the axis (C2), showing their unique structures and how they articulate. The superior view of atlas is a top-down view (top left) that of the C1 vertebra shows its ring-like shape, which lacks a traditional vertebral body. It is composed of an Anterior arch and a Posterior arch. Large, oval Superior articular facets are visible, which articulate with the skull. The Transverse processes on the sides contain the Transverse foramen. The Dens (part of the axis) is shown positioned against the anterior arch, held in place by a transverse ligament. The superior view of axis (top right) is a top-down view of the C2 vertebra. The Dens is prominent, finger-like projection that extends upward. The Vertebral Arch includes the Lamina and a bifurcated Spinous process at the rear. Facets shows the Superior articular facets flanking the Dens. The anterior view of axis (bottom right) is a front-facing view of the C2 vertebra. The Dens is the top-most point of the bone. Unlike the atlas, the axis has a central body located below the Dens. Labeled features include the Transverse process and the Inferior articular process, which connects to the C3 vertebra below.<\/p>\n

Figure 9.20: An anatomical illustration of the human vertebral column (spine) shown from both a posterior (rear) and right lateral (side) view within the silhouette of a human body. The posterior view (left) shows the spine as a straight vertical column. It highlights various divisions. 7 Cervical vertebrae (C1\u2013C7) are the top-most section located in the neck. 12 Thoracic vertebrae (T1\u2013T12) are the middle section corresponding to the rib cage. 5 Lumbar vertebrae (L1\u2013L5) are the lower back region. Intervertebral disc is the cushioning space between individual vertebrae. The sacrum is a large, triangular bone at the base of the spine. The coccyx is the small “tailbone” at the very bottom. The lateral view (right) highlights the natural S-shaped curves of the spine. The cervical curve is a concave (inward) curve of the neck. The thoracic curve is a convex (outward) curve of the upper and middle back. The lumbar curve is a concave (inward) curve of the lower back. The Sacrococcygeal curve is a convex (outward) curve formed by the fused vertebrae of the sacrum and coccyx.<\/p>\n

Figure 10.2: An educational diagram comparing two primary evolutionary hypotheses for the origin of paired fins in fish: the Fin Fold Hypothesis and the Gill Arch Hypothesis. Fin Fold Hypothesis (Left Side) uses two fish illustrations to show the transition from continuous folds to distinct fins. The top image shows a primitive fish with continuous longitudinal skin folds along the body. Labels include the Median Fin Fold (blue, along the dorsal side) and Paired Fin Folds (red, along the ventral\/lateral sides). The bottom image shows a more evolved stage where the continuous folds have broken up into localized segments. Labels point to the resulting Pectoral Fin (anterior) and Pelvic Fins (posterior). Gill Arch Hypothesis (Right Side) uses a detailed skeletal diagram of a fish head and thoracic region to suggest a different anatomical origin. It highlights the Gill Arch (yellow) and the Branchial Rays (small red structures extending from the arch), Pectoral girdle and Pectoral fin (along the ventral side).<\/p>\n

Figure 10.3: This black-and-white line drawing illustrates the skeletal structure of a shark’s pectoral and pelvic fins, showing the internal cartilaginous elements and fin rays. The pectoral fin (left) structure is attached to the Coracoid Bar. It features three basal cartilages that support the rest of the fin labeled with Propterygium: the most anterior (front) basal cartilage. Mesopterygium: the middle basal cartilage. Metapterygium: the most posterior (back) basal cartilage. Radials: a series of rows of small cartilaginous segments extending from the basals. Ceratotrichia: long, slender fibrous fin rays that make up the outer portion of the fin. The pelvic fin structure (right) is attached to the Iliac Process. Its structure is slightly simpler than the pectoral fin labeled with Propterygium: a small anterior basal cartilage. Metapterygium: a long, prominent posterior basal cartilage that supports the majority of the fin.\u00a0 Radials: cartilaginous rods extending outward from the metapterygium. Ceratotrichia: fibrous fin rays forming the trailing edge of the fin.<\/p>\n

Figure 10.9: This black-and-white scientific illustration displays the skeletal anatomy of a primitive tetrapod, showcasing the structure of both the pelvic girdle and the pectoral girdle. The top of the image shows a full skeletal reconstruction of the animal, likely an early land-dwelling vertebrate like Ichthyostega. The Pelvic Girdle and Hindlimb (left side) are shown with the three primary bones of the hip that are labeled: the Ilium (upper), the Ischium (posterior), and the femur, pubis, tibia and fibula (anterior). The lower left diagram highlights the Acetabulum, the deep socket where the femur attaches to the hip. It shows the ilium (upper), ischium (posterior), and pubis (anterior). The Pectoral Girdle and Forelimb (Right Side) shows dermal and endoskeletal elements. Labeled parts include the Cleithrum (a tall, blade-like bone), the Clavicle, and the Interclavicle.\u00a0 The lower diagram highlights the Glenoid fossa, the socket where the humerus attaches to the shoulder. The limb includes the Humerus (upper arm), Radius, and Ulna (forearm), leading to the digits of the front foot.<\/p>\n

Figure 10.19: This image provides a detailed anatomical view of the human scapula (shoulder blade), showing the bone from three different perspectives with key landmarks labeled. Anterior View (Left) is the front-facing side of the scapula that rests against the rib cage. It includes the Subscapular Fossa: the broad, slightly concave surface that makes up the “body” of the bone from this view with the top ridge (Superior border), right ridge (Medial border), and left ridge (Lateral border); Coracoid Process: the hook-like projection at the top that serves as an attachment point for various muscles and ligaments; Acromion Process: the bony tip of the shoulder that articulates with the clavicle (collarbone); Glenoid Cavity: the shallow socket where the head of the humerus (upper arm bone) fits to form the shoulder joint.\u00a0 The Lateral Edge (Center) is side-on view highlighting the thickness of the bone and the alignment of the joints. It includes the Acromion process (bony tip); Coracoid process (hook-like projection); Spine (visible as a ridge protruding from the back); Glenoid Cavity (shown clearly as the cup-like socket for the arm); Lateral and Medial Borders (the outer and inner edges of the bone, respectively). The Posterior View (Right) is the back-facing side of the scapula. It includes the spine (the prominent horizontal ridge that divides the back of the scapula; Supraspinous Fossa (the area above the spine); Infraspinous Fossa (the much larger area below the spine); Acromion and Coracoid Processes (both are visible at the top, showing how they extend outward to protect the shoulder joint).<\/p>\n

Figure 11.4: A multi-level anatomical diagram illustrating the hierarchical structure of a skeletal muscle fiber. The top view section shows a single cylindrical muscle fiber (muscle cell) and its internal components including the Sarcolemma (the plasma membrane surrounding the muscle fiber); Sarcoplasm (the cytoplasm of the muscle cell, containing multiple nuclei and mitochondria); Myofibrils (long, rod-like contractile organelles that fill the muscle fiber). One myofibril is shown pulled out to reveal its structure. Striations show the alternating Light I bands and Dark A bands that give skeletal muscle its striped appearance. The bottom is an enlarged view of a single myofibril that includes the segment of a myofibril between two Z discs; thin (actin) filaments shown as light green lines attached to the Z discs; thick (myosin) filaments shown as thick purple lines centered in the sarcomere. Z disc is the boundary of the sarcomere where thin filaments are anchored. M line is the vertical line in the center of the sarcomere that holds thick filaments together. H zone is the central region of the A band where only thick filaments are present (no overlap with thin filaments). A band is the dark region spanning the full length of the thick filaments. I band is the light region containing only thin filaments, spanning across two adjacent sarcomeres. Sarcoplasmic Reticulum is a lacy, yellow network of tubules surrounding the myofibril.<\/p>\n

Figure 11.5: An anatomical diagram of the organization of the sarcomere. The image breaks down the relationship between thick and thin filaments and their specific protein structures. The top view is the sarcomere structure, spanning from one Z line to the next. The darker A band is the central region where thick filaments (purple) and thin filaments (green) overlap. The lighter I band is the outer regions containing only thin filaments. The H zone is the center of the A band where only thick filaments are present. The M line is the vertical structure in the very center that anchors the thick filaments. The middle view are the filament overviews. The portion of a thick filament (Left) show a thick purple rod covered in protruding globular structures called Heads. The portion of a thin filament (Right) shows a twisted strand of green actin beads, wrapped with orange Tropomyosin threads, and studded with yellow Troponin complexes. The bottom view shows molecular details. The Myosin Molecule (Left) shows an individual myosin protein consisting of a long Tail, a Flexible hinge region, and two Heads. Each head contains specific Actin-binding sites and an ATP-binding site. The Actin Subunits (Right) show a close-up of the green actin chain. Each actin bead has a dark “Binding site for myosin,” which is currently covered by the orange tropomyosin strand in a relaxed state.<\/p>\n

Figure 11.7: An anatomical diagram illustrating relative position of the thick and thin filaments during sarcomere contraction. The top view shows the relaxed sarcomere. This section shows the sarcomere at rest, where there is minimal overlap between the filaments. The purple thick (myosin) filaments are centered, while the green thin (actin) filaments are anchored to the Z discs on either side. The lighter I band are wide regions at the ends of the sarcomere containing only thin filaments. The H zone is a wide central region containing only thick filaments. The darker A band is the full length of the thick filaments, including the areas where they overlap with the thin filaments. The M line is the vertical structure in the very center that anchors the thick filaments. The bottom view is the contracted sarcomere. This section shows the changes that occur during contraction, indicated by large black arrows pointing inward from the sides. The thin filaments have been pulled toward the center (M line) by the thick filaments. The distance between the Z discs has decreased, shortening the entire sarcomere. The Lighter I band has narrowed significantly. The H zone has almost entirely disappeared as the thin filaments slide into the center. The width of the Darker A band remains unchanged.<\/p>\n

Figure 12.1: This image provides a detailed anatomical breakdown of a skeletal muscle, illustrating its hierarchical structure from the whole organ down to the microscopic level. The diagram uses three levels of “zoom” to show how muscle tissue is organized. The top level (whole muscle) includes: skeletal muscle (the entire organ, wrapped in an outer layer of connective tissue called the Epimysium) and the muscle fascicle (a bundle of muscle fibers within the whole muscle, surrounded by a layer called the Perimysium). The middle level (fascicle detail) is shown in cross-section, revealing that it is composed of multiple muscle fibers (individual muscle cells). Each individual muscle fiber is encased in a thin connective tissue layer called the Endomysium. Satellite cells are shown located on the exterior of the muscle fibers. The bottom level (muscle fiber detail) is shown with its plasma membrane, known as the sarcolemma. Inside the fiber are numerous rod-like structures called myofibrils, which are the actual contractile elements of the cell.<\/p>\n

Figure 12.4: This medical illustration displays a full-body view of the human muscular system, highlighting seven different fascicle arrangements that determine a muscle’s range of motion and power. The central figure is an anterior view of a human with arrows pointing to specific muscle examples. Circular (Orbicularis oris) show fascicles are arranged in concentric rings around the mouth. Multipennate (Deltoid) looks like many feathers side-by-side, with all their quills (tendons) inserting into one large tendon. Convergent (Pectoralis major) shows the muscle has a broad origin, and the fascicles converge toward a single tendon of insertion. This gives the muscle a triangular or fan-like shape. Parallel – Fusiform (Biceps brachii) shows the fascicles run parallel to the long axis of the muscle, which has an expanded midsection (belly) and tapers at each end. Parallel – Non-fusiform (Sartorius) shows the fascicles run parallel to the long axis of the muscle in a strap-like fashion without a central bulge. Unipennate (Extensor digitorum) shows short fascicles attach obliquely to only one side of a central tendon that runs the length of the muscle. Bipennate (Rectus femoris) shows fascicles insert into the tendon from opposite sides, resembling the structure of a feather.<\/p>\n

Figure 12.5: This comparative anatomy illustration shows muscle groups between a shark (top\/right) and a cat (bottom\/left). The muscles are color-coded to indicate their developmental origins and functional groups. The diagram uses two views: (a) Lateral view and (b) Ventral view, categorized by the following legend. Extrinsic Eye (Green) are small muscles responsible for moving the eyeball within the socket. Hypobranchial (Red) are located ventrally (on the underside) of the throat. Axial (Blue) are the trunk muscles. In the shark, they form the bulk of the body for swimming. In the cat, they are seen along the spine and ribcage, though they are partially covered by limb muscles. Branchiomeric (Yellow) are muscles associated with the pharyngeal arches. In the shark, these power the jaws and gill arches. In the cat, these have evolved into facial muscles, mastication (chewing) muscles, and throat muscles. Appendicular (Grey) are muscles of the fins or limbs. These are relatively small in the shark but are much more extensive and complex in the cat.<\/p>\n

Figure 12.6: This anatomical illustration identifies the extrinsic eye muscles of a shark versus a human eye. The image is divided into two main sections: (a) a dorsal view of the head and (b) detailed views of the individual eye. The dorsal view (a) shows the top-down perspective of the head, illustrating how the muscles originate from the skull’s midline and fan out to the eyeballs. The lateral & frontal detail (b) provides a closer look at the “cross” pattern formed by the rectus muscles around the eyeball and the specific insertion points of the oblique muscles. The muscles are color-coded and labeled based on their position and function. Superior Rectus (green) is located on the top of the eyeball. Inferior Rectus (orange) is located on the bottom of the eyeball. Medial Rectus (purple) is located on the side of the eye closest to the midline (nose\/brain). Lateral Rectus (blue) is located on the outer side of the eye.\u00a0 Superior Oblique (red) are positioned at the top-front of the eye. Inferior Oblique (yellow) is positioned at the bottom-front of the eye.<\/p>\n

Figure 12.8: This anatomical illustration provides a detailed view of the human abdominal wall muscles, highlighting both the superficial and deep layers of the torso. The image features a lateral-anterior view of the male torso, with a “window” cut-out on the lower abdomen to reveal the underlying muscle layers. The major upper torso muscles include: Pectoralis major (the large muscle of the chest), Latissimus dorsi (the broad muscle of the back, visible on the side), and Anterior serratus muscles (the finger-like muscle projections along the ribs). The abdominal core includes: the external oblique (the outermost layer of the abdominal wall, with fibers running diagonally downward, Rectus abdominis (the vertical “six-pack” muscle, shown enclosed within the Rectus sheath), tendinous intersections (the horizontal bands of connective tissue that divide the rectus abdominis into segments), and Linea alba (the central white line of connective tissue that runs down the midline of the abdomen). The magnified cut-away section shows the layering of the abdominal wall from superficial to deep: external oblique (outermost), internal oblique (middle layer), and transversus abdominis (deepest layer, with fibers running horizontally).<\/p>\n

Figure 12.9: This comprehensive anatomical chart displays the major muscles of the human body from both an anterior (front) view and a posterior (back) view. The top diagram illustrates the superficial muscles visible from the front, categorized by body region. The head and neck include the Occipitofrontalis (frontal belly) for facial expression and the Sternocleidomastoid. The torso features large muscles like the Pectoralis major (chest), Rectus abdominis (abs), and Abdominal external oblique. Deep to the chest is the Pectoralis minor and the rib-associated Serratus anterior. The arms show the Deltoid (shoulder), Biceps brachii, and various forearm muscles like the Brachioradialis and Flexor carpi radialis. The legs highlight the Quadriceps group (Rectus femoris, Vastus lateralis, and Vastus medialis), the long Sartorius muscle, and lower leg muscles like the Tibialis anterior. The bottom diagram shows the muscles from the back, including some deeper muscles revealed via dissection. The head and neck show the Occipitofrontalis (occipital belly) and the Splenius capitis. The upper Back and Shoulder features the large, diamond-shaped Trapezius, the Rhomboids, and rotator cuff muscles like the Supraspinatus and Infraspinatus. The Latissimus dorsi covers much of the lower back. The arms focus on the Triceps brachii and the extensor muscles of the forearm, such as the Extensor digitorum. The gluteal region and legs show the Gluteus maximus and Gluteus medius. The back of the thigh features the hamstrings (Biceps femoris, Semitendinosus, and Semimembranosus). The calf is dominated by the Gastrocnemius and Soleus.<\/p>\n

Figure 12.10: This medical illustration shows the posterior (rear) view of the human pelvic and gluteal region, detailing the layers of muscle and the location of the sciatic nerve. The image is divided into a superficial view on the left and a deeper, dissected view on the right. The gluteus maximus is shown on the left side of the image and is the largest and most superficial muscle of the buttocks. The gluteus medius revealed on the right side after the gluteus maximus has been removed; it sits deeper and higher on the pelvis. The piriformis is a pear-shaped muscle located deep to the gluteus maximus. The superior gemellus is a small muscle located just below the piriformis. The quadratus femoris is a flat, quadrilateral muscle located further down. The sciatic nerve is depicted as a thick, yellow cord-like structure. It emerges from the pelvis typically just below the piriformis muscle and runs down the back of the leg.<\/p>\n

Figure 13.6: A flowchart illustrating the digestive process of various substrates through extracellular and intestinal mucosal enzymes. The chart is organized into three main categories: Carbohydrates, Lipids, and Protein. The first category are the carbohydrates. Soluble alpha-1,4-linked polysaccharides that are broken down by amylolytic enzymes into intermediate products (maltose, isomaltose, and alpha-1,4-linked oligosaccharides). These are then converted by maltase, isomaltase, and alpha-glucosidase into the end product, glucose. Laminarin is broken down by laminarinase into laminaribiose, which is then converted by beta-glucosidase into glucose. Chitin is broken down by chitinase into chitobiose, which is then converted by chitobiase and N-acetyl-beta-D-glucosaminidase into N-acetyl-glucosamine. Trehalose is converted by trehalase directly into glucose. Sucrose is converted by sucrase directly into glucose and fructose. The second category are the lipids. Triglycerides, phospholipids, and waxes are broken down by lipase into monoacylglycerides and fatty acids. The third category, protein, is broken down by pepsin, trypsin, and chymotrypsin into intermediate products (polypeptides and oligopeptides). These intermediates are further broken down by carboxypeptidases into dipeptides. Finally, peptidases convert the dipeptides into the end product, amino acids.<\/p>\n

Figure 13.7: Three-panel scientific scatter plot figure comparing intestinal absorption characteristics across vertebrate groups, all using logarithmic axes. Panel A: Log-log plot of nominal intestinal surface area (cm\u00b2) versus body mass (g) comparing birds (open triangles) and non-volant mammals (open circles), with a single regression line showing a strong positive relationship across both groups spanning body masses from roughly 5 g to 100,000 g. Panel B: The same nominal area versus body mass plot comparing bats (inverted open triangles) and non-volant mammals (open circles), again with a regression line. Statistical results are inset: body mass P < 0.001, taxon P < 0.001, and mass \u00d7 taxon interaction P = 0.013, indicating bats have significantly smaller intestinal surface area relative to body mass than non-volant mammals. Panel C: Log-log plot of fractional paracellular absorption versus body mass (g) comparing birds (open circles) and non-volant mammals (X marks). Birds show a steeply declining regression line with higher paracellular absorption at small body sizes, while non-volant mammals show a nearly flat regression line close to zero across all body sizes, suggesting birds rely more heavily on paracellular nutrient absorption, especially at smaller body masses.<\/p>\n

Figure 13.8: A scientific infographic comparing gene sequences, physical appearance, and enzyme activity across five different fish species. The data is presented in a horizontal row for each species. Each species has a gene map, a sequence of pointed blocks representing genes on a chromosome. Blue blocks represent amylase genes (amy2A, amy2B). Gray blocks represent flanking genes (col11a1, ntng1). Each species has an Amylase activity in \u03bcmol’min-1’g-1. The top species, C. violaceus, (a stout, mottled gray-green fish photographed on gravel has a gene sequence col11a1, amy2B, amy2A, amy2A, ntng1 with amylase activity 23.17 \u00b1 5.25. Next is X. mucosus (a slender, elongate olive-yellow fish) with a sequence of col11a1, amy2A, amy2A, ntng1 and amylase activity 43.83\u00b131.53. Next is X. atropurpureus (a slender reddish-brown elongate fish) with a sequence of col11a1, amy2A, amy2A, ntng1 and amylase activity of 55.58\u00b123.09. Next is P. chirus (a very slender, uniform olive-brown elongate fish) with a sequence of col11a1, amy2A, ntng1 with amylase activity of 0.74\u00b10.84. Last is A. purpurescens (a brightly colored orange and dark salamander or amphibian photographed on rocks) with a sequence of col11a1, amy2A, ntng1 and amylase activity of 0.23\u00b10.16.<\/p>\n

Figure 14.3: A scientific illustration comparing the respiratory systems of four different animals: a fish (Bowfin), a mammal (Eastern gray squirrel), a bird (Blue jay), and an amphibian (Green frog). The image is divided into four vertical columns (A, B, C, D) that zoom in from the organism level down to the microscopic mechanism of blood oxygenation. A color key at the bottom identifies light blue for deoxygenated water\/air, teal for oxygenated water\/air, dark blue for deoxygenated blood, and red for oxygenated blood. Column A (bowfin) starts with the fish, then a gill, secondary lamellae (showing water flowing across thin plates), ending in the countercurrent exchange showing water and blood flow in opposite directions, maintaining a concentration gradient that maximizes oxygen absorption. Column B (eastern gray squirrel), shows mammalian lungs, then the alveoli (appearing as clusters of tiny air sacs), ending in the tidal mechanism that shows that air moves in and out of the same sacs Oxygenated air (teal) enters the sac, and deoxygenated blood (blue) flows around it to become oxygenated (red). Column C (blue jay) shows avian lungs (featuring a complex system of air sacs), parabronchus (shown as a tube with perpendicular vessels), ending in a crosscurrent exchange that shows air flows through the parabronchus in one direction, while blood vessels cross the airflow at angles, allowing for highly efficient oxygen uptake. Column D (green frog) shows the skin (cutaneous respiration), then the skin surface with a dense network of capillaries just beneath the outer layer, and ending with an open mechanism that shows that oxygen from the environment diffuses directly through the moist skin surface into the underlying blood vessels.<\/p>\n

Figure 14.6: An anatomical diagram illustrating the development of branching lungs in humans. The process is shown in a clockwise sequence of eight stages. Beginning of fourth week, the process starts with the laryngotracheal tube forming near the pharynx, surrounded by splanchnic mesoderm. At the Tracheal Bud Formation, the tube develops two small protrusions known as tracheal buds. At separation, the diagram shows the respiratory tract separating from the esophagus, with the trachea and tracheal buds becoming more distinct. At the end of fourth week, the tracheal buds have expanded into bronchial buds.\u00a0 As development continues, the trachea bifurcates (splits) clearly into the primary bronchial buds. The bronchial buds undergo further branching to form secondary buds, which will eventually become the lobes of the lungs. The bronchial buds develop into an increasingly complex, tree-like structure of smaller airways. At eight weeks, the final stage shows the fully formed early lungs. The right lung is clearly labeled with three distinct sections: the upper lobe, middle lobe, and lower lobe. The left lung is labeled with two sections: the upper lobe and lower lobe. The internal structure shows a dense, branching network of bronchi within the lung tissue.<\/p>\n

Figure 14.8: Scientific illustration comparing egg types across three vertebrate groups on a white background, arranged left to right in evolutionary order. Fish (500 mya): A small circular diagram showing a simple embryo (pink) surrounded by a large yolk sac (yellow) within a minimal membrane, representing the most ancestral condition. Reptiles and birds (300 mya): A larger oval diagram showing the amniotic egg with four extraembryonic membranes labeled the chorion (outermost), allantois (branching structure in green and yellow), amnion (surrounding the embryo), and yolk sac (yellow, labeled at top) with a pink embryo visible within the fluid-filled amnion cavity. Mammals (160 mya): A diagram of the uterine environment showing a similar arrangement of embryonic membranes, but with the chorion replaced by a highly vascularized placenta (shown in red at top) connecting to the uterine wall, replacing the eggshell. Approximate ages of evolutionary origin are noted beneath each group in millions of years ago (mya).<\/p>\n

Figure 14.9: A phylogenetic tree titled “Evolutionary History and Diversity of Respiratory Structures: Part 1,” which tracks the development of gills, lungs, and gas bladders across various fish lineages. The tree uses a color-coded system for its branches: blue represents respiratory structures for water (gills), yellow represents respiratory structures for air (lungs\/gas bladders), black indicates non-respiratory structures, red text indicates ventilation mechanisms, and green text indicates specific respiratory organs. Lamprey and sharks branch off early, utilizing gills for water respiration and buccal pumping for gill ventilation. The Osteichthyes (Bony Fish) node introduces lungs and buccal pumping for lung ventilation. It splits into two primary groups: Sarcopterygii (Lobe-finned fishes) which maintains the use of lungs (yellow line) and Actinopterygii (Ray-finned fishes) that shows significant diversification of air-breathing structures. Polypterus utilizes recoil aspiration for ventilation. Gar features a physostomous gas bladder used for respiration. The tree shows a transition from respiratory gas bladders to non-respiratory gas bladders, and eventually to physoclistous gas bladders (seen in higher ray-finned fish). The armored catfish uses the intestinal tract for respiration. Mudskippers utilize the buccal cavity. Anabantids possess a specialized labyrinth organ for breathing air.<\/p>\n

Figure 14.10: A phylogenetic tree titled “Evolutionary History and Diversity of Respiratory Structures: Part 2,” continuing the evolutionary narrative of respiration from bony fish to modern tetrapods. The tree uses a color-coded system for branches: blue for water-based respiration (gills\/skin), yellow for air-based respiration (lungs\/skin), black for non-respiratory structures, red text is the ventilation mechanism, and green text is the specific respiratory organs. Osteichthyes (Bony Fish) are the base of the tree that shows the ancestral state of unicameral lungs. Actinopterygii (Ray-finned fishes) diverge to the left, primarily utilizing gills (blue line). Sarcopterygii (Lobe-finned fishes) diverge to the right, leading toward land-dwelling vertebrates. Lungfish retain the use of both water and air respiration. Tetrapods (Frogs) introduce the use of skin for both water and air respiration (blue and yellow labels) alongside lungs. A major evolutionary shift occurs at the Amniota node, characterized by the loss of gills in adults and the use of axial muscles for exhalation. Mammals branch off with the development of a diaphragm and complex alveolar lungs. Lizards utilize multicameral lungs and costal aspiration (using ribs). Turtles evolved to use abdominal muscles for ventilation due to their rigid shells. Crocodiles feature a specialized hepatic piston mechanism for breathing. Birds represent the most specialized branch, featuring parabronchial lungs and a complex system of air sacs.<\/p>\n

Figure 15.2: Medical illustration titled “The Structure of an Artery Wall” showing two views of an artery. On the left, a cross-sectional end-on view of the artery depicts the hollow central lumen (shown in purple\/pink) surrounded by concentric layers of the vessel wall, with a boxed region indicating the area magnified on the right. On the right, a detailed magnified lateral cutaway view identifies six labeled structural layers from outermost to innermost: Tunica externa (outermost connective tissue layer), Tunica media (middle layer, labeled at top), Tunica intima (innermost layer, labeled at top), smooth muscle (making up the bulk of the tunica media, shown in deep red fibrous tissue), external elastic membrane (boundary between tunica externa and media), internal elastic membrane (boundary between tunica media and intima), and Endothelium (the innermost cellular lining of the vessel lumen). The illustration uses warm red and burgundy tones to represent the muscular and connective tissue components.<\/p>\n

Figure 15.7: Scientific illustration comparing heart development and evolution across four stages, displayed left to right with corresponding ECG traces beneath each. Heart tube (leftmost, gray) shows a simple curved tube representing the earliest embryonic heart, with the outflow tract (oft) at the top and inflow tract (ift) at the bottom, and a simple sine-wave ECG below. Ballooning chambers (second, gray and yellow) show the heart tube has begun to loop and balloon into distinct chambers, with the right ventricle (rv), left ventricle (lv), atrioventricular canal (avc), atrium (a), and outflow tract (oft) labeled, accompanied by a more complex ECG. A formed fish heart (third, blue and yellow) shows a single ventricle (v), atrium (a), atrioventricular canal (avc), and outflow tract (oft), with a corresponding ECG. Formed human heart (fourth, blue and yellow) has a fully formed four-chambered heart with right ventricular outflow tract (rvot), aorta (ao), left atrium (la), left ventricle (lv), and atrioventricular canal (avc) labeled, shown with a full human ECG trace.<\/p>\n

Figure 15.12: Comparative schematic diagram showing the evolution of aortic arches and their connection to lungs across seven groups, arranged left to right in a series of color-coded anatomical diagrams. Red vessels represent arteries (oxygenated or outflow), blue vessels represent veins or deoxygenated blood, and pink\/oval shapes represent lungs or associated organs. The first diagram is a fish-like pattern with multiple paired gill arch arteries in red and blue, no lungs, representing a fully aquatic gill-breathing vertebrate. The second diagram is a similar but slightly reduced gill arch pattern, possibly representing a more derived fish. The third diagram is a lungfish form with a white heart outline showing internal division, blue and red vessels, and paired pink lung-like structures with partial cardiac separation. The fourth diagram shows further reduced arches, a more defined heart, and prominent paired pink lungs, representing an amphibian. The fifth diagram is a similar pattern with a white\/gray heart region, representing a mammal heart with partial or incomplete separation. The sixth diagram shows red and blue vessels with a purple element possibly indicating a pulmonary vessel, and pink lungs, representing a more derived reptile or bird. The seventh diagram is the most derived pattern with clearly separated red and blue circulations, a compact heart, and pink lungs, representing a bird with full cardiac separation.<\/p>\n

Figure 15.13: A comparative anatomical illustration showing heart morphology across nine vertebrate species arranged in three rows, all drawn as anterior view line diagrams on a white background. A dashed horizontal line on each heart indicates the boundary between atrial and ventricular regions. Labeled chambers and structures vary by species and are abbreviated (VA = ventral aorta, Ven = ventricle, A = atrium, SV = sinus venosus, C = conus arteriosus, B = bulbus arteriosus, RA = right atrium, LA = left atrium, RV = right ventricle, LV = left ventricle, P = pulmonary vessel, T = truncus arteriosus, Cr = cranial, Ca = caudal, R = right, L = left). The top row shows more ancestral vertebrates: hagfish (a simple two-chambered heart with sinus venosus, atrium, and ventricle), shark (a linear four-chambered heart with conus arteriosus), sturgeon (similar to shark with a prominent conus), tuna (a compact heart with bulbus arteriosus), frog (a three-chambered heart with two atria and one ventricle with truncus arteriosus). The middle row shows reptiles: rattlesnake (an incompletely divided heart with right and left atria and a partially divided ventricle), python (similar but with more distinct ventricular subdivision labeled ‘RV’ and ‘LV’), alligator (approaching four-chamber organization with distinct right and left ventricles). The bottom row shows birds and mammals: ostrich (a fully four-chambered bird heart; a central orientation key showing cranial\/caudal and right\/left axes), human (a fully four-chambered mammalian heart with complete separation of right and left sides).<\/p>\n

Figure 15.14: Schematic diagram showing six circuit-style diagrams illustrating different vertebrate circulatory system configurations, using color-coded lines and symbols. Each diagram uses a rectangular circuit layout with a heart symbol representing the heart, and colored lines representing vessels: pink\/red for oxygen-rich blood, blue for oxygen-poor blood, and purple for blood with intermediate oxygen enrichment. An orange circle in most diagrams represents the air breathing organ (ABO). The six diagrams progress from simple to complex. Top left is a single-loop circuit with no lung circuit and a simple heart, representing a fish. Top right is a single loop with a small lung circuit branching off, representing a transitional or lungfish-type circulation. Middle left is a partially divided double circuit with a mixed heart (pink and blue), representing an amphibian or primitive reptile with incomplete cardiac separation. Middle right is a similar partially divided circuit with slightly more separation. Bottom left is a more developed double circuit with further cardiac separation. Bottom right is a small legend.<\/p>\n

Figure 15.16: Comparative anatomical illustration showing cross-sectional diagrams of hearts from nine vertebrate species arranged in three rows on a white background, with dark gray representing myocardial tissue and white representing internal chambers and lumens. Small red dots indicate specific anatomical landmarks such as valve positions. Structures are labeled with abbreviated anatomical terms. Top row shows more ancestral vertebrates. The hagfish has a simple oval heart with minimal internal division, labeled AVv (atrioventricular valve) and VAv (ventriculo-arterial valve). The shark has a slightly more complex heart with Cv (conus valve) and AVv. The sturgeon-tuna has a heart with multiple internal ridges and Cv and AVv. The frog has a heart with prominent internal trabeculation and Cv and AVv. The middle row shows reptiles. The rattlesnake, python, and Varanid lizard all show partially divided ventricles with right aorta (RAo), left aorta (LAo), pulmonary (p), and AVv labeled, with increasing complexity of ventricular subdivision from rattlesnake to varanid. The bottom row shows more derived vertebrates. The alligator is approaching full four-chamber separation with right and left aortic valves (RAo, LAo), right and left atrioventricular valves (RAVv, LAVv), and pulmonary (p). The ostrich has a fully four-chambered bird heart with RAVv, LAVv, Ao, and p. The human has a fully four-chambered mammalian heart with the same labeling.<\/p>\n

Figure 15.17: An anatomical cross-section diagram of an amphibian heart, showing its internal chambers and major vessels. The ventricle is a single, large muscular chamber at the bottom of the heart. The atria are divided into a Right atrium and Left atrium by the Interatrial septum. The atrioventricular valve is located between the atria and the ventricle to regulate blood flow. The conus arteriosus is a thick-walled vessel arising from the ventricle, containing a Spiral valve. Labels indicate the Aortic cavity and Pulmocutaneous cavity within the outflow tract. At the top, the diagram shows paired vessels branching into the carotid artery, systemic artery, and pulmocutaneous artery on both the left and right sides. Labels indicate the entry of pulmonary vein and the entry of sinus venosus. The heart is oriented with “Right” and “Left” labels at the bottom, corresponding to the animal’s perspective.<\/p>\n

Figure 15.18: Anatomical illustration showing a frontal cross-sectional view of a reptilian heart depicting blood flow patterns with directional arrows. The heart is rendered in dark red\/brown tones with internal chamber divisions visible. Red arrows indicate the flow of oxygenated blood from the left atrium through the ventricle and out through the systemic and carotid arteries on the left side of the outflow tract. Blue\/purple arrows indicate the flow of deoxygenated blood from the right atrium through the ventricle and out through the pulmonary arteries on the right side. The crossing of red and blue arrows within the single partially divided ventricle illustrates the mixing and partial separation of oxygenated and deoxygenated blood that is characteristic of the reptilian heart. The two atrial chambers are visible as rounded lobes on either side at the top, and multiple outflow vessels are shown exiting at the top with color-coded arrows indicating their respective blood contents.<\/p>\n

Figure 15.19: This image consists of two comparative diagrams illustrating cardiac shunting mechanisms in reptiles. The diagrams show how blood can be diverted between the systemic and pulmonary circuits within the ventricular chambers. Each diagram shows a simplified view of the heart’s outflow tract. The bottom shows two inflow points: the Right atrium (carrying systemic venous\/deoxygenated blood) and the Left atrium (carrying pulmonary venous\/oxygenated blood). The center shows the ventricular chambers, where the two streams of blood meet. The top shows three major outflow vessels: the Pulmonary artery (left), the Left aorta (middle), and the Right aorta (right). The left-to-right shunt (left side) shows that a significant portion of the oxygenated blood from the left atrium (red arrow) is diverted into the pulmonary artery. The pulmonary artery contains “Mixed” blood (purple), while both the left and right aortas receive purely oxygenated blood (red). The right-to-left shunt (right side) shows the deoxygenated blood from the right atrium (blue arrows) is diverted away from the pulmonary artery and into the Left aorta. The pulmonary artery remains purely deoxygenated (blue), but the Left aorta and Right aorta now carry “Mixed” blood (purple).<\/p>\n

Figure 15.20: Three-panel anatomical line diagram illustrating the crocodilian heart and its unique circulatory adaptations. The left panel is a labeled anatomical diagram of the crocodilian four-chambered heart showing the right aortic arch, left aortic arch, pulmonary artery, right ventricle, left ventricle, and the Foramen of Panizza (a small opening between the left and right aortic arches highlighted in red, unique to crocodilians). The center panel shows the same heart diagram with color-coded arrows showing blood circulation during normal respiration (breathing air). Red arrows indicate oxygenated blood flowing from the left ventricle into the left aortic arch and to the body, while blue arrows indicate deoxygenated blood flowing from the right ventricle into the pulmonary artery toward the lungs. A color key at the bottom identifies blue as deoxygenated and red as oxygenated. The right panel shows the same heart showing blood circulation when underwater. Both red and blue arrows now exit through both aortic arches.<\/p>\n

Figure 15.21: Comparative histological illustration contrasting heart morphology between an ectotherm and an endotherm. The left panel is an Ectotherm (Skink), under a blue header that shows a grayscale histological cross-section of a skink heart with a 1mm scale bar, labeled with right atrium (RA), left atrium (LA), and a single undivided ventricle (Ven). The ventricular wall is described as trabecular, with a highly spongy, irregular internal architecture. A small skink silhouette is shown beside the section. Below, a magnified diagram of the wall cross-section shows lumen between trabeculations with a thin compact outer layer, illustrating the spongy trabecular organization. The right panel is an Endotherm (Zebra Finch), under a pink header. It shows a grayscale histological cross-section of a zebra finch heart with a 1mm scale bar, labeled with right atrium (RA), left atrium (LA), right ventricle (RV), and left ventricle (LV), showing complete four-chamber separation. The ventricular wall is described as compact, with a much denser, more uniform myocardial structure. A small zebra finch silhouette is shown beside the section. Below, a magnified diagram shows large free lumens with a thick compact outer layer, contrasting with the skink’s trabecular organization.<\/p>\n

Figure 15.22: Anatomical illustration showing a frontal cross-sectional view of the human heart with the cardiac conduction system highlighted in yellow against the brown myocardial tissue. Labeled structures trace the pathway of electrical impulse conduction from origin to ventricular muscle: SA node (sinoatrial node, upper right atrial wall), atrial pathways (conducting the impulse across both atria), AV node (atrioventricular node, at the junction of atria and ventricles), AV bundle of His (the bundle connecting the AV node to the ventricular conduction system), right bundle branch (descending along the right side of the interventricular septum), left bundle branch (descending along the left side of the interventricular septum, labeled on the right side of the image), interventricular septum (the wall dividing left and right ventricles), Purkinje fibers (terminal conduction fibers spreading through the ventricular myocardium), and moderator band (a muscular band crossing the right ventricular cavity carrying part of the right bundle branch). The yellow highlighting clearly delineates the sequential pathway of electrical conduction that coordinates the heartbeat.<\/p>\n

Figure 15.23: This image provides a comprehensive phylogenetic table illustrating the evolutionary history of the vertebrate heart and circulatory system over the last 600 million years. The left side shows a cladogram of major lineages, from basal chordates to modern endotherms. Key evolutionary milestones are marked. A yellow star represents the water-to-land transition (approx. 380 million years ago). The red stars indicate the independent evolution of endothermy in mammals and birds. Tunicates have a single contractile tube. Hagfish and lampreys have a sinus venosus, one atrium, and one vetricle. Chondrichthyes, ray-rinned fish, Coelacanths, and lungfish have a sinus venosus, one atrium, one ventricle, and a conus arteriosus (the ray-finned fish conus arteriosus is reduced in teleosts). Amphibians have a sinus venosus, two atria, one ventricle, and conus arteriosus. Mammals (red star) have atrialized sinus venosus, two atria, two ventricles, a ventricularized conus arteriosus, and one (left) aortic arch). Lizards and snakes have a sinus venosus, two atria, one ventricle, a ventricularized conus arteriosus and two aortic arches. Turtles have a sinus venosus, two atria, one ventricle, a ventricularized conus arteriosus, and two aortic arches. Crocodilians have a sinus venosus, two atria, 2 ventricles, ventricularized conus arterious and two aortic arches. Birds (red star) have atrialized sinus venosus, two atria, two ventricles, ventricularized conus arteriousus, and one (right) aortic arch.<\/p>\n

Figure 15.24: Three-panel scientific illustration comparing alternative interpretations of the secondary vascular system (SVS) in teleost fish, each showing a lateral view of a small fish with color-coded vascular networks overlaid, accompanied by a color-coded legend on the right and two inset diagrams at the bottom. Panel A (Lymphatic interpretation) shows the fish vasculature with arteries (red), blood vascular capillary networks (pink), veins (dark blue), and mammalian-like lymphatic vessels (light blue\/teal), interpreting the secondary vessels as homologous to the lymphatic system of mammals. Panel B (SVS interpretation) shows the same fish with arterial vessels of the SVS (light red\/pink), inter-arterial anastomoses (IAAs) also called arterial-lymphatic conduits (ALCs) (red), and venous vessels of the SVS (light blue). Panel C (Hybrid lymphatic\/SVS interpretation) is a combined interpretation showing mammalian-like lymphatic vessels (teal) alongside hybrid lymphatic\/SVS vessels with variable blood vascular and lymphatic characteristics (light teal), suggesting the fish SVS contains both lymphatic-like and blood vascular-like vessel populations. Two inset diagrams at the bottom compare embryonic lymphatics (showing branching vessels in tan\/beige) with the adult SVS (showing a more organized capillary network in pink and teal), illustrating developmental and structural differences between the two vessel types.<\/p>\n

Figure 15.25: Comparative scientific illustration showing the lymphatic systems of two amphibian species. The top left shows Newt Lymphatics (Cynops pyrrhogaster) as a dorsal photograph of a living red-bellied newt with a millimeter scale bar. Below it, a detailed anatomical diagram of the same species maps the lymphatic system onto a dorsal body illustration, with green dots indicating lymphatic vessels or nodes distributed along the body. Three labeled regions are delineated by boxes: Forelimb Lymphatic Territory (front left), Hindlimb Lymphatic Territory (center), and Tail Lymphatic Territory (right\/posterior). Additional labeled structures include LH1 and LH56 landmarks along the lateral body. The right side is a Frog Lymphatics (Xenopus laevis). Panel f shows a photograph of a living frog on a teal surface with a millimeter scale bar. Below it, an anatomical diagram of the frog in lateral view highlights the lymphatic system in color: subcutaneous lymphatic sacs are shown in yellow as large fluid-filled spaces beneath the skin, with pink vessels indicating lymphatic drainage pathways, and green dots marking specific lymphatic structures.<\/p>\n

Figure 15.26: Anatomical illustration showing a frontal cross-sectional view of the human heart with blood flow directions indicated by white arrows, color-coded to distinguish oxygenated and deoxygenated blood. Blue regions represent deoxygenated blood and the right side of the heart. Red\/pink regions represent oxygenated blood and the left side of the heart. Labeled structures include, on the heart right side (left side of diagram): superior vena cava (entering top right), right pulmonary arteries, right pulmonary veins, pulmonary semilunar valve, right atrium, tricuspid valve, right ventricle, and inferior vena cava (exiting bottom). On the left side of the heart (right side of diagram): aorta (exiting top), left pulmonary arteries, pulmonary trunk, left atrium, left pulmonary veins, aortic semilunar valve, mitral valve (bicuspid), and left ventricle. White arrows trace the path of blood flow: deoxygenated blood enters the right atrium via the venae cavae, passes through the tricuspid valve into the right ventricle, and exits via the pulmonary trunk to the lungs. Oxygenated blood returns via pulmonary veins to the left atrium, passes through the mitral valve into the left ventricle, and exits via the aorta to the body.<\/p>\n

Figure 16.3: A detailed illustration of the work of nephrons in the filtration system. Step 1 begins in the glomerulus that filters small solutes from the blood. This is a small red circle surrounded by a tube that moves up and curves into Step 2, the proximal convoluted tubule, which reabsorbs ions, water, and nutrients and removes toxins and adjusts filtrate pH. The tube then moves down into a U-shape. Moving downward is the Descending loop of Henle where aquaporins allow water to pass from the filtrate into the interstitial fluid. Curving down and moving back up is the Ascending loop of Henle where it reabsorbs Na+ and Cl- from the filtrate into the interstitial fluid. At the tube’s recent apex is the Distal tubule that selectively secretes and absorbs different ions to maintain blood pH and electrolyte balance. The tube connects to a stalk-like tube (collecting duct) that reabsorbs solutes and water from the filtrate.<\/p>\n

Figure 16.4: A vintage biological illustration showing a transverse (cross-sectional) view of a chick embryo, detailing the early development of nephrons. The Neural Tube (n.Cr.) is located at the top center. This is a large, oval-shaped cluster of cells with a small central cavity. The notochord (N’ch.) is a smaller, distinct circular structure located directly beneath the neural tube, serving as the primitive skeletal support. The Aortae (Ao.) are two small, circular openings located on either side beneath the notochord. The Wolffian Duct (W.D.) & Nephrotome (Neph.) are structures on the left side. The Coelom (Coel.) are large, open cavities on both the left and right sides that will become the main body cavities. The Somatopleure (Som’pl.) & Splanchnopleure (Spl’pl.) are the outer and inner layers of the lateral plate mesoderm, shown on the right side bordering the coelom. The Somite (S. 29) is a segmented block of mesoderm visible to the left of the neural tube.<\/p>\n

Figure 16.5: This diagram illustrates the embryonic development of the vertebrate kidney, showing the three successive stages of renal structures: the pronephros, mesonephros, and metanephros. The illustration is divided into three vertical panels showing the progression over time. Stage 1 is the Early Stage (Left) showing the Pronephros. Located at the top, this three-prong branch is connected to a red cluster representing a rudimentary blood supply. The Nephric Duct is a long, teal-colored vertical tube that serves as the drainage system. The Nephrogenic Cord is a shaded green area along the lower portion of the duct where future kidney tissues will develop. Stage 2 is the Intermediate Stage (Center). It begins with the Degenerating Pronephros where the uppermost section fades, as the pronephros is non-functional in most mammals. Below is the Mesonephros, which is a series of teal, hook-like structures associated with red capillary loops. At the bottom is the Cloaca where the nephric duct now extends all the way down to connect to the primitive common chamber for waste. The third stage is the late stage (Right). At the top is the Degenerating Mesonephros, the upper part of the mesonephros begins to fade. Below is the Metanephros (teal, hook-like structures with red “knot” structures) that contain two key components: Ureteric Bud (a small teal branch growing out from the duct) and Metanephric Mesenchyme (a concentrated green cluster of cells surrounding the bud.)<\/p>\n

Figure 16.7: Circular life cycle diagram of a parasitic lamprey, illustrated against a background divided into two zones: a blue upper zone representing the sea and a tan\/brown lower zone representing river sediment. Blue arrows indicate the forward progression of the life cycle clockwise, and an orange arrow traces the return migration. Starting from the bottom and moving clockwise, the stages and their durations are: Larval life in rivers (4\u00bc years) where small ammocoete larvae are shown burrowed in river sediment; a central circle labels this the sedentary stage. Metamorphosis is when larvae transform into juveniles, transitioning to the free-swimming stage (labeled in the central circle). Downstream migration to sea (\u00bd year) is when juveniles migrate downstream. Parasitic phase at sea (2 years) is when adult lampreys are shown attached to host fish (a large fish and a smaller fish) in the open sea, feeding parasitically. Spawning run (1\u00bc year) shows male (\u2642) and female (\u2640) adults migrate back upstream into rivers. Spawning and death shows adults spawn in gravel nests and die. Migration of adults into rivers is connecting the sea phase back to the spawning event.<\/p>\n

Figure 16.12: Two-panel scientific illustration comparing osmoregulation strategies in marine and freshwater fish, using color-coded arrows to indicate the direction of ion and water movement. A legend in each panel identifies red arrows as the direction of ion transfer (Na\u207a, K\u207a, Cl\u207b) and blue arrows as the direction of water transfer. In the top panel, the yellow jack fish actively combats water loss and ion gain in a hypertonic environment. Labeled processes include: drinks seawater (large red arrow entering the mouth), water loss over skin (blue arrows pointing outward from the body surface), active ion depuration through gills (red arrows pointing outward at the gill region), and salty urine containing Mg\u00b2\u207a and SO\u2084\u00b2\u207b (red arrow exiting posteriorly). The bottom panel is a brown trout that actively combats ion loss and water gain in a hypotonic environment. Labeled processes include: food (red arrow entering the mouth), active ion absorption through gills (red arrows pointing inward at the gills), water intake through skin (blue arrows pointing inward over the body surface), and diluted urine (blue arrow exiting posteriorly).<\/p>\n

Figure 16.16: A large, sagittal (longitudinal) cut of a right kidney, showing the inner layers, drainage systems, and vasculature. A smaller diagram in the top-left corner shows the kidneys’ anatomical position within the human torso, situated against the posterior abdominal wall with the adrenal glands sitting on top. The kidney is labeled with external and entry points: Renal Hilum (the recessed central fissure), renal vein (large blue vessel), renal nerve (yellow branching structures), renal artery (red vessel), capsule (the smooth, thin outer protective layer of the kidney), and ureter (the large tube extending downward). The internal regions include: cortex (the outer layer of the kidney tissue), medulla (the inner region containing the Pyramids), renal column (the tissue between the renal pyramids), papilla (the tip of each pyramid). The drainage system includes: minor calyx (small cup-like structures), major calyx (larger channels), and renal pelvis (the large, funnel-shaped cavity). The blood vessels include: interlobar blood vessels (travel between the pyramids), arcuate blood vessels (arch over the bases of the pyramids at the junction of the cortex and medulla), and cortical blood vessels (small vessels extending into the renal cortex).<\/p>\n

Figure 16.17: Detailed anatomical illustration of a single nephron and its associated blood supply, showing the complete structural organization of the kidney’s functional unit. The illustration uses red for arterial vessels, blue for venous vessels, and tan for the tubular components of the nephron. Labeled structures include, from top to bottom: the renal corpuscle consisting of the glomerular capsule (Bowman’s capsule) and the glomerulus (the capillary tuft), efferent arteriole, juxtaglomerular apparatus, peritubular capillary network (surrounding the tubules), proximal convoluted tubule (with a cross-sectional inset showing cuboidal cells with brush border), distal convoluted tubule (with a cross-sectional inset); collecting tubule (with a cross-sectional inset); descending limb of the nephron loop (Loop of Henle), ascending limb of the nephron loop; vasa recta (long straight capillaries running parallel to the loop of Henle); arcuate vessels (curved vessels at the corticomedullary junction), and interlobar vessels (larger vessels between renal lobes). Cross-sectional insets at key points show the cellular composition of different tubular segments.<\/p>\n

Figure 17.2: This is a histological cross-section of an ovary, stained in pink (H&E stain), showing various structures at different stages of the ovarian cycle. A scale bar in the bottom right indicates 2mm. The image is labeled with several key anatomical features beginning with the outer layers that include germinal epithelium (the outermost layer of the ovary) and tunica albuginea (a layer of dense connective tissue located just beneath the germinal epithelium). The ovarian regions include the cortex (the outer region of the ovary); medulla (the central region of the ovary); developing follicles (small, circular structures located within the cortex); mature Graafian follicles (large, fluid-filled sacs); the theca (outer capsule of the follicle) is specifically pointed out on one of these. Corpora Lutea (marked with ‘C’) are large, solid-looking glandular masses that form from the remains of a follicle after ovulation. There are three prominent corpora lutea visible in this section.<\/p>\n

Figure 17.3: An anatomical medical illustration showing a posterior view of the human female reproductive system, specifically the uterus and its associated structures on the right side. The large central body is labeled uterus, leading down into the vagina. The opening at the cervix is identified as the external uterine orifice. The ovary is an almond-shaped organ positioned to the right of the uterus. The uterine tube (Fallopian tube) is a long duct extending from the top of the uterus toward the ovary. The ovarian fimbria are finger-like projections at the end of the uterine tube that hover near the ovary. The Ostium abdominale is the opening of the uterine tube into the abdominal cavity near the fimbriae. The broad ligament is a wide fold of peritoneum that supports the uterus and connects it to the pelvic walls. The ligament of ovary is a fibrous cord connecting the ovary to the lateral side of the uterus. The Epo\u00f6phoron is a small vestigial structure located in the broad ligament between the ovary and the uterine tube. The Ovarian vessels are blood vessels supplying the ovary, shown entering near its lateral pole.<\/p>\n

Figure 17.4: A detailed medical diagram showing a cross-section of the human testis and its surrounding structure. The external layers include the spermatic cord which is the structure at the top leading “into inguinal canal.” The Cremaster muscle is a muscle layer surrounding the testis. The Tunica vaginalis is the outermost serous membrane covering the testis. The Tunica albuginea is a tough, fibrous inner capsule. It extends inward to form septa, which divide the testis into compartments. The Seminiferous tubule lobules are coiled tubes within the septa where sperm are produced. The straight tubule are small ducts that collect sperm from the seminiferous tubules. The rete testis is a network of delicate canals that receive sperm from the straight tubules. The efferent ductules are small tubes that carry sperm from the rete testis out of the testis and into the epididymis. The Epididymis is a long, coiled tube where sperm mature, divided into three labeled parts including Head of epididymis which is the top portion receiving sperm from the efferent ductules, Body of epididymis which is the middle section running along the side of the testis, tail of epididymis which is the bottom portion where the tube begins to turn upward, and ductus deferens (Vas deferens) which is the thick-walled tube that exits the tail of the epididymis to carry sperm back up toward the pelvic cavity.<\/p>\n

Figure 17.6: This flow chart illustrates the process of oogenesis, highlighting the specific points where the biological “stoplights” of meiosis occur throughout a female’s life. The process begins with diploid stem cells that undergo mitosis to produce primary oocytes (2n). Meiosis I begins as primary oocytes start the first meiotic division. Meiosis is “paused” (arrests) in prophase I before the female is born. After puberty Meiosis I resumes. Under hormonal influence, a primary oocyte completes its first division. This results in two haploid (n) cells, a large secondary oocyte and a small first polar body (which may later divide into two second polar bodies). The secondary oocyte begins Meiosis II but arrests at metaphase II. This is the stage at which ovulation occurs. After sperm penetration (green light, oocyte meiosis completes immediately after sperm penetrates the oocyte), the diagram shows a sperm cell approaching the secondary oocyte. The completion of meiosis produces a Mature ovum (n) and another small polar body.<\/p>\n

Figure 17.7: This graphic illustrates the process of spermatogenesis through both a cellular flow chart and a histological cross-section of a seminiferous tubule. In spermatogenesis (a), the section outlines the progression of cell division and differentiation. It begins with Spermatogonium (2n) where cells go through mitosis to either primary spermatocyte (2n) or back up to spermatogonium. The cells from the Primary spermatocyte enter Meiosis I, dividing into two (1n). From the secondary spermatocyte (1n), these cells quickly enter Meiosis II. At Spermatid (1n), the four resulting haploid cells form a single primary spermatocyte. Finally, through spermiogenesis, round spermatids develop tails and condensed heads to become Spermatozoa (sperm). Part (b) is the cross-section of the seminiferous tubule leading to a microscopic view (micrograph) showing the physical organization of these cells within the testis. The micrograph shows the Lumen at the center with various parts labeled around the edges. Sertoli (sustentacular) cells are large cells that span the tubule wall. Leydig (interstitial) cells are located in the interstitial tissue outside the tubule. Spermatogonia are located at the very edge. Primary spermatocytes and early spermatids are found in the middle layers. The diagram also points out a lymphatic capillary, arteriole, and peritubular capillary.<\/p>\n

Figure 17.8: This diagram illustrates three types of sequential hermaphroditism in fish, where an individual changes its biological sex during its lifetime. Part A is Protogyny. An individual starts its life as a female (indicated by the \u2640 symbol and a smaller, yellow\/grey fish) and later transitions into a male (indicated by the \u2642 symbol and a larger, vibrantly colored blue and green fish). This is a one-way transition (single-headed arrow). Part B is Protandry. An individual starts its life as a male (\u2642) and later transitions into a female (\u2640). The illustration uses Clownfish as the example; in these species, the largest individual in a group typically becomes the female. This is also a one-way transition (single-headed arrow). Part C is the Bidirectional process. An individual has the capacity to change sex in either direction\u2014from female to male or male to female\u2014depending on social or environmental cues. The fish pictured (a goby) appears identical in both states. This is shown with a double-headed arrow, indicating the change is reversible or flexible.<\/p>\n

Figure 17.10: This image features an anatomical diagram and histological sections of the urogenital region of a lamprey. Part A (Left) is a three-dimensional cutaway illustration of the posterior body region. The digestive tract (gut) and the body cavity (coelom) are shown. The genital pore is labeled with a white arrow. The Wolffian Duct (WD) is shown as a separate tube above the gut. External features are labeled with the dorsal fin, cloacal labium, connective tissue, and the urogenital papilla. A 1cm scale bar is provided at the bottom left. Parts B & C are histological Sections on the right. These two black-and-white micrographs provide a microscopic view of the genital pore area. Image B shows the physical opening between the coelom and the urinary sinus. Image C is a higher-magnification view of the genital pore. It shows the cellular structure of the opening, including a label for a pycnotic nucleus (a small, dark, condensed nucleus, often indicating cell death or specialized epithelial turnover) at the margin of the pore.<\/p>\n

Figure 17.14: A black-and-white biological diagram compares two different arrangements of sperm production: the lobular type (left) and the tubular type (right). A central column of text with downward-pointing arrows tracks the maturation of germ cells from the top to the bottom: Spermatogonium, Spermatocyte, Spermatid, Sperm. The lobular type (left side) shows a structure where the sperm-producing cells are organized into distinct compartments along the wall. The lobular lumen is a central cavity where mature sperm collect. Spermatogonia are at the top and periphery, while mature sperm are released into the wide, lower sperm canal. Sertoli cells are identified as supportive cells within the lobule walls. The tubular type (right) shows a more streamlined, elongated structure where the maturation process happens in horizontal clusters within the tube. The Sertoli cell is labeled near a cluster of spermatids. Mature sperm are shown clustered together in a circular formation before moving downward toward the sperm canal. The bottom of the diagram shows both types converging into a common sperm canal at the base.<\/p>\n

Figure 17.17: An anatomical and histological diagram in black and white illustrates the reproductive system and sperm development. The macro anatomy (top left) shows a pair of organs labeled: testicle (an oval structure attached to the kidney), fat body (finger-like projections extending from the top of the testicle), and kidney (elongated, dark organ behind the testicle). The histology of the seminiferous tubule (center) shows a large cross-section surrounded by muscle cells. The epithelium is the inner lining of the tubule where sperm production occurs. The lumen is the central cavity of the tubule, filled with the tails of maturing sperm. The spermatogenesis (bottom and left) is an inset magnification and a series of three circular diagrams (left) show the stages of cell maturation: spermatogonia (the outermost layer of stem cells), spermatocytes (the next layer of cells undergoing division), spermatids (smaller cells moving toward the center), and spermatozoon (mature sperm cells with tails, ready for release into the lumen). The sperm variations (top right) show an arrow from the tubule pointing to six different morphologies of mature sperm, labeled a through f. These variations show different head shapes, ranging from straight and needle-like (a) to curved and hook-shaped (c, d, f) and even a spiral\/corkscrew shape (e).<\/p>\n

Figure 17.21: An anatomical line drawing titled “TURTLE” displays the reproductive and excretory systems of a male turtle through a longitudinal section and three cross-sections. The longitudinal section at the top (A) shows a side view of the posterior anatomy. The excretory organs include the gut, ureter, bladder, and a cloacal pouch or accessory bladder. The reproductive structures highlight the internal phallus including the bulb of corpus cavernosum, the glans, the corpus fibrosum, and the retractor penis muscle. Cross-section Markers are vertical lines labeled B, C, and D that indicate where the corresponding cross-sectional views are taken. Three circular diagrams (Cross-sections B, C, D) show the internal arrangement of tissues at different points along the body. View B is located furthest back, showing the gut at the top, the urogenital sinus in the center, and the ureter and vas deferens nearby. At the bottom are the coelomic canal and the bulb of corpus cavernosum. View C shows the cloacal pouch extending outward like wings. Below it are the corpus cavernosum, corpus fibrosum, and retractor penis muscle. View D is located near the glans, showing an oval section containing a central seminal groove supported by the corpus fibrosum and the retractor penis muscle.<\/p>\n

Figure 17.36: Comprehensive scientific illustration showing the reproductive anatomy in frogs, arranged as a circular sequence of six numbered diagrams (I\u2013VI) surrounding a central large diagram of the ovary. The center is a cross-sectional view of the ovary showing the germinal epithelium on the outer surface, with multiple follicles at various stages of development visible within the ovarian cortex, and blood vessels throughout. Stage I (upper right) is a primordial follicle showing a single follicular cell, nucleus, and Balbiani body. Stage II is an early follicle with developing cytoplasm and initial yolk around a vitelline cover. Stage III is a growing follicle showing increased yolk accumulation, cytoplasm, blood vessels, and surrounding cubic cells. Stage IV is a more advanced follicle with a cavity, theca cell layer, vitello (yolk), and granulosa cells. Stage V is a large pre-ovulatory follicle with a prominent cavity, theca cells, granulosa cells, blood vessels, and an oogonium inset. Stage VI is a nearly mature oocyte showing the oocyte itself, polar body, zona pellucida, vitelline envelope, and surrounding granulosa cells. The upper left is an anatomical diagram of the female reproductive system showing fat body (tentacle-like structures), ovary, oviduct, cloaca, kidney, and granulosa cells with a vitelline cover.<\/p>\n

Figure 17.40: Schematic diagram illustrating the reproductive system of an ovary-bearing chicken. At the top, a large white\/gray oval structure represents the ovary, with multiple blue spheres of varying sizes clustered on its surface representing oocytes or follicles at different stages of maturation. The largest blue sphere represents a mature follicle, surrounded by smaller ones, and a cluster of very small blue spheres at the top going into the left ovary (A). Below the ovary, three successive stages are depicted descending vertically from the infundibulum (B): first, a gray crescent-shaped structure with a blue sphere (magnum); second, a gray oval with a central blue sphere in the isthmus (D); and third, a plain gray oval with no blue sphere in the uterus (E). The tube tapers down into the vagina (F) and out the cloaca (G). The large intestine (H) and rudiment of right oviduct (I) are shown to the right of the vagina.<\/p>\n

Figure 17.41: Three-panel photograph showing variation in avian vagina morphology, with scale bars in each panel. Anatomical structures are identified by white arrowheads and letter labels. Panel A (upper left) is a dorsal view of the vagina from a pheasant, showing the uterus (U), vagina (V), and cloaca (Cl) labeled, with white arrowheads pointing to junctions between segments. Yellow follicles or eggs are visible at the upper left, and the paired uteri extend upward. Panel B (lower left) is a closer view of the vaginal and cloacal region, labeling the uterus (U), vagina (V), and cloaca (Cl). Asterisks (* and **) mark specific anatomical features of interest and white arrowheads indicate junctions. Panel C (right) is a fully dissected and unfolded reproductive tract laid out on a black background, showing the complete length from uterus (U) at the top to cloaca (Cl) at the bottom. White arrowheads at multiple points indicate regional boundaries or structural features, and a bracket labeled S.S. indicates a specific segment of the oviduct. A scale bar is visible at the right.<\/p>\n

Figure 17.44: Comparative anatomical illustration showing the diversity of female reproductive tract organization across mammals, divided into two main groups. On the left, Marsupials are represented by a single diagram labeled “double uterus” and “double vagina,” showing two separate uteri (yellow) and two separate vaginal canals (pink\/red) that do not fuse, with the urinary bladder (blue) visible below. On the right, Placentates (placental mammals) are represented by three diagrams showing progressive fusion of the reproductive tract from left to right: double uterus (two separate uteri with a single vagina), bicornuate uterus (two uterine horns partially fused at the lower end, with a single vagina), and single uterus (fully fused uterus with a single uterine body). The rightmost diagram labels all structures: fallopian tubes at the top (yellow), connecting to the uterus (red), vagina (pink), urinary bladder (blue), and sinus urogenitalis. A bracket below the three placental diagrams indicates they all share a single vagina, contrasting with the marsupial double vagina.<\/p>\n

Figure 17.48: Color diagram showing a developing fetus in the uterus and a detailed inset of the placental interface. The left side shows a fetus in a curled position within the amniotic sac, filled with amniotic fluid. Labels identify the surrounding layers and structures: placenta (the thick organ lining the uterine wall), yolk sac (a small yellow pouch outside the main embryo), amnion and chorion (the inner and outer membranes surrounding the fetus), umbilical cord (connecting the fetus to the placenta), and uterus (the muscular organ containing the pregnancy). An enlarged inset on the right shows the exchange point between maternal and fetal blood systems. Chorionic villi are tree-like branching structures containing fetal blood vessels (blue umbilical arteries and red umbilical veins). These extend into the intervillous space. Maternal Blood Supply are uterine arteries and veins that pump blood into the intervillous space, where it bathes the chorionic villi. The umbilical vein (carrying oxygenated blood to the fetus) and umbilical artery (carrying deoxygenated blood away) are shown traveling through the umbilical cord.<\/p>\n

Figure 17.50: This medical diagram provides a comprehensive anatomical view of the female reproductive system, including a central illustration of the organs and two histological (microscopic) insets. The central image shows a frontal view of the uterus, ovaries, and associated structures. The uterine tube (oviduct) consists of three parts: the Infundibulum (with finger-like Fimbriae near the ovary), the Ampulla (middle section), and the Isthmus (the narrow section connecting to the uterus). Labeled parts of the uterus include the Fundus (top rounded portion), the Uterine isthmus (lower narrow portion), and the Cervix (the opening to the vagina). Uterine Wall Layers are labeled in an inset to the right: endometrium (the inner lining), myometrium (the thick middle muscular layer), perimetrium (the outer serous layer.) Ovaries and ligaments show the ovaries held in place by the suspensory ligament, ovarian ligament, and the broad ligament. The vasculature displays the network of the ovarian artery and vein, uterine artery and vein, and vaginal artery. The vagina is the canal leading from the cervix to the exterior of the body. The ovarian section (Left) is a microscopic view of the Ovarian cortex, showing the Tunica albuginea (outer capsule) and the Edge of a follicle. The uterine section (Right) is a microscopic view detailing the transition between the three layers of the uterine wall (Endometrium, Myometrium, and Perimetrium).<\/p>\n

Figure 17.51: This medical illustration displays the external and internal anatomy of the human female genitalia, referred to as the vulva. The left panel shows the surface anatomy as viewed from the front. Labels include: Prepuce (the hood of skin covering the clitoris), Glans clitoris (the external, visible portion of the clitoris), labia minora (the smaller, inner folds of skin), labia majora (the larger, outer folds of skin), urethral opening (the small opening through which urine is excreted), vaginal opening (the entrance to the vaginal canal), and anus (labeled at the bottom of the frame, separate from the vulva). The right panel shows the deeper, internal structures that sit beneath the skin: Corpus cavernosum (the internal erectile tissue of the clitoris that extends backward, bulb of vestibule (two masses of erectile tissue situated on either side of the vaginal opening), Bartholin\u2019s glands (small glands located near the vaginal opening), opening of right Bartholin\u2019s gland (a specific label pointing to the duct where the gland’s fluid enters the vestibule).<\/p>\n

Figure 17.55: Black and white anatomical illustration comparing two pelvic skeletons side by side, viewed from the front, representing the female pelvis (left) and male pelvis (right). Both illustrations have a red oval drawn on them to highlight the pelvic region for comparison. Four structures are labeled with lines pointing to both pelves: Hip bone (large lateral bones forming the sides of the pelvis), Sacrum (the triangular bone at the posterior center), Pelvic brim (the curved ridge outlining the pelvic inlet, traced by the red oval), and Subpubic angle (the angle formed beneath the pubic symphysis at the bottom, indicated by bracket lines spanning both pelves). The female pelvis (left) has a noticeably wider, rounder pelvic inlet and a broader subpubic angle compared to the narrower, more heart-shaped inlet and more acute subpubic angle of the male pelvis (left), illustrating the key anatomical differences between the two.<\/p>\n

Figure 18.9: Four-part diagram showing embryonic brain development from primary to secondary brain vesicles, with lateral (side) profile views bookending two labeled cross-sectional illustrations. The far left is a lateral view of a three- to four-week embryo showing the curved, early brain shape. The second illustration (a) is a labeled illustration of the three primary brain vesicles in a three- to four-week embryo, showing three color-coded segments from top to bottom: Prosencephalon (Forebrain) in light gray, Mesencephalon (Midbrain) in gold\/yellow, and Rhombencephalon (Hindbrain) in magenta\/pink, with a brown spinal cord below. The third illustration (b) of the five secondary brain vesicles in a five-week embryo, showing how each primary vesicle subdivides, with arrows indicating the developmental progression. The Prosencephalon becomes the Telencephalon (developing into the Cerebrum) and Diencephalon (developing into the eye cup, Thalamus, hypothalamus, and epithalamus). The Mesencephalon becomes the Mesencephalon (Midbrain). The Rhombencephalon becomes the Metencephalon (developing into the Pons and Cerebellum) and Myelencephalon (developing into the Medulla oblongata). The far right is a lateral view of the 5-week embryo brain showing its more developed, curved shape with distinct regions visible.<\/p>\n

Figure 18.10: Diagram showing four illustrations (A\u2013D) of spinal cord anatomy, depicted in gold\/yellow against a white background, representing different developmental stages or species comparisons. Illustration A (red-eared slider) is a long, narrow spinal cord with short, evenly spaced nerve roots branching symmetrically along its entire length, tapering to a fine point at the bottom. Illustration B (harbor seal) is a wider spinal cord with longer, more spread nerve roots that angle downward, also tapering to a point at the bottom. Illustration C (human) shows a spinal cord with very long, sweeping nerve roots that extend far downward and outward, creating a dramatic elongated fringe effect, with the roots converging toward the bottom. Illustration D (common toad) is a shorter, broader spinal cord shown from the top with thick, prominent nerve roots curving outward and downward from the base, showing fewer but larger root bundles.<\/p>\n

Figure 18.11: Diagram showing five simplified cross-sectional illustrations (A\u2013E) of spinal cord anatomy, depicted in purple and pink\/lavender against a white background. Each illustration shows the outer white matter (dark purple) surrounding the inner gray matter (light pink\/lavender) in varying shapes across different species. Image A (common toad) is a rounded cross-section with a large, wide pink interior region and a small central canal visible, with minimal gray matter differentiation. Image B (red-eared slider) is a similar rounded shape with a more defined butterfly\/H-shaped gray matter region beginning to emerge. Image C (Hermann’s tortoise) is a cross-section with a prominent, well-defined butterfly\/H-shaped gray matter pattern with clear dorsal and ventral horns, and relatively less white matter. Image D (tegu) is a cross-section with a broad, squat H-shaped gray matter region with wide ventral horns, surrounded by proportionally less white matter. Image E (reticulated python) is a large, rounded cross-section with a tall, narrow butterfly-shaped gray matter region and a substantial amount of surrounding white matter.<\/p>\n

Figure 18.15: Detailed anatomical illustration of an adult lamprey brain shown from a lateral view, with the brain dissected and labeled to show its key structures. From front (left) to back (right), the labeled regions include: the large olfactory bulb (shown in purple\/violet at the far left, prominent in sharks due to their acute sense of smell), the olfactory nerve connecting it to the brain, the medial pallium (hippocampus) (orange\/tan region), the lateral pallium (cerebral hemisphere) (tan, lower front region), the optic nerve (projecting downward), the adenohypophysis (blue, on the underside), the infundibulum (stalk connecting to the pituitary), the habenula (small dark blue structure on top), the pineal gland (projecting upward from the top), the midbrain (large rounded dorsal structure in the center-right), the oculomotor nerve (projecting from the underside toward the right), and the spinal cord (tapering off to the far right).<\/p>\n

Figure 18.21: This image displays four sets of real anatomical specimens of bird brains, labeled a, b, c, and d. Each set includes a ventral view (top row) and a dorsal view (bottom row). The specimens are labeled with several neuroanatomical abbreviations. OB (Olfactory Bulb) is located at the very front (anterior) of the brain. In specimen b, it is labeled as ‘OB’, suggesting a unique or primitive form in that species. ON (Optic Nerve) is visible in the ventral views as white, cross-like structures (the optic chiasm) where the eyes connect to the brain. OT (Optic Tectum) is highlighted with dashed circles on the sides of the brain. Wulst is a prominent, rounded elevation on the dorsal surface of the forebrain (telencephalon). V (Vallecula) is indicated by a dashed line; this is the furrow or groove that separates the Wulst from the rest of the telencephalon. Specimen \u201ca\u201d features elongated olfactory bulbs and a distinct Wulst. Specimen \u201cb\u201d shows a much broader, more heart-shaped forebrain with very large olfactory structures. Specimen \u201cc\u201d shows a paler specimen with a highly pronounced, rounded Wulst. Specimen \u201cd\u201d displays a more compact brain structure with clearly defined optic tecta.<\/p>\n

Figure 18.24: Color-coded anatomical illustration of the human brain in lateral (side) view, showing the four main cerebral lobes and key surface landmarks, each region distinguished by a distinct color. The frontal lobe is shown in pink\/salmon and occupies the anterior (front) portion of the brain. The parietal lobe is shown in purple\/mauve and sits posterior to the frontal lobe. The occipital lobe is shown in green and is located at the rear of the brain. The temporal lobe is shown in dark blue\/slate and runs along the lower lateral surface. Labeled sulci (grooves) and gyri (ridges) include the central sulcus (separating the frontal and parietal lobes), the precentral gyrus (just anterior to the central sulcus), the postcentral gyrus (just posterior to the central sulcus, associated with somatosensory function), the lateral sulcus (separating the temporal lobe from the frontal and parietal lobes), and the parieto-occipital sulcus (marking the boundary between the parietal and occipital lobes). The brainstem and cerebellum are partially visible at the base of the illustration.<\/p>\n

Figure 19.2: Diagram comparing the parasympathetic (left) and sympathetic (right) divisions of the autonomic nervous system and their opposing effects on various organs. The central illustration shows a sagittal view of the brain and spinal cord, with the vagus nerve, medulla oblongata, ganglion, solar plexus, and chain of sympathetic ganglia labeled. Lines extend from the central nervous system to illustrated organs on each side, with text describing each division’s effect. Parasympathetic effects (left side): the ganglion and medulla oblongata, stimulates flow of saliva. The vagus nerve slows heartbeat, constricts bronchi, stimulates peristalsis and secretion, and stimulates release of bile. The end of the spinal cord has a leader line to contracts bladder. Sympathetic effects (right side): The chain of sympathetic ganglia dilates pupil and inhibits flow of saliva. Multiple leader lines mark from the spine to the various effects: accelerates heartbeat, dilates bronchi, inhibits peristalsis and secretion, triggers conversion of glycogen to glucose, stimulates secretion of adrenaline and noradrenaline, and inhibits bladder contraction.<\/p>\n

Figure 19.7: Three-panel anatomical diagram illustrating the evolution of the recurrent laryngeal nerve from various species. Panel A (upper left, shark) is a simplified frontal view of the thoracic cavity showing the spine, ribs, and a red structure representing the heart or major vessels. Numbered labels (1, 2, 3, 9) point to various structures, with yellow structures representing nerve pathways running alongside red vascular structures. Panel B (upper right, mouse) is a more detailed frontal view of the heart and great vessels. Red structures represent the heart and arteries, yellow structures represent nerves, and a purple structure indicates a vessel or nerve loop (likely the recurrent laryngeal nerve loop around the aorta). Numbered labels 1 through 9 identify specific anatomical structures Panel C (lower, giraffe) is a lateral view of an elongated neck and thorax, used to illustrate the impractically long path of the recurrent laryngeal nerve. Yellow nerve pathways (labeled 1 and 4) descend from the brain region (label 10, shown in yellow) down the neck, loop around cardiovascular structures at the base (labels 3, 5, 6, 9), and return upward. Label 11 points to an additional structure along the pathway.<\/p>\n

Figure 20.1: Scientific infographic illustrating the electromagnetic spectrum against a dramatic landscape background with mountains, sky, and a satellite visible in the upper right. A wave diagram runs horizontally across the middle of the image, showing waves becoming progressively shorter and more frequent from left to right. A vertical beam of white light splitting into a rainbow (visible light spectrum) is depicted at the center. The spectrum is divided into labeled regions from left to right: Radio Waves (longest wavelength, ~10\u00b2 meters, shown with AM radio and FM radio tower icons), Microwaves (wavelength ~1mm, shown with cell phone\/Wi-Fi and microwave oven icons), Infrared (wavelength ~thickness of paper, shown with a human figure representing body heat and a remote control icon), Visible Light (the narrow band humans can see, shown with an eye icon), Ultraviolet (shown with a sunscreen\/skin icon), X-rays (shown with a medical X-ray image icon), and Gamma Waves (shortest wavelength, ~size of atomic nuclei, shown with a nuclear reactor icon). Above the wave diagram, atmospheric opacity windows are labeled, indicating which wavelengths penetrate Earth’s atmosphere, including the Radio Window and Optical Window. Frequency values in Hertz are shown along the top and wavelength values in meters along the bottom.<\/p>\n

Figure 20.2: Two-panel anatomical illustration of the primate eye and retina. Panel A (Primate eye) shows a cross-sectional line diagram of the whole eye with a yellow arrow labeled “LIGHT” indicating the direction of incoming light through the cornea. Labeled structures include: sclera (outer white coat), choroid (vascular layer), posterior chamber, iris, ciliary body, fovea (point of sharpest vision), vitreous body (gel-filled interior), cornea (transparent front surface), anterior chamber, suspensory ligament, lens, optic nerve, and retina. A red arrow points from the retina to Panel B. Panel B (Retina) is a detailed color illustration of the retinal cell layers shown in cross-section, depicting the cellular organization from the vitreous side (left) to the outer pigment epithelium (right). Labeled cell types and structures from left to right include: ganglion cells (innermost layer, with visible nuclei and axons), bipolar cells (intermediate neurons), cone cells (color photoreceptors), rods (dim-light photoreceptors), connecting stalks, nuclei, discs (membranous photopigment-containing structures), mitochondria, Golgi apparatus, melanin granules, and pigment epithelium (outermost layer).<\/p>\n

Figure 20.3: Comparative anatomical illustration showing cross-sectional line diagrams of eyes from six vertebrate groups, arranged in two rows of three, each labeled below. All diagrams show the lens as a prominent central oval structure. Panel A (Lamprey) labels include spectacle, cornea, iris, choroid, retina, protractor lentis muscle, and corneal muscle. Panel B (Shark) is a simpler diagram showing the lens, cornea, choroid, and retina with minimal additional structures. Panel C (Teleost) labels include conjunctiva, iris, suspensory ligament, sclera, choroid, retina, optic nerve, and retractor lentis muscle, with a notably large round lens. Panel D (Amphibian) labels include cornea, protractor lentis muscle, suspensory ligament, optic nerve, sclera, retina, choroid, nictitating membrane, and lower eye lid. Panel E (Lizard) labels include ciliary muscle, papillary cone, optic nerve, iris, cornea, lens, vitreous body, fovea, sclera, retina, choroid, and sclearal ossicle (a ring of small bones in the sclera). Panel F (Bird) labels include ciliary body and muscle, suspensory ligament, optic nerve, iris, cornea, pecten (a pleated vascular structure unique to birds projecting into the vitreous), lens, vitreous body, fovea, sclera, retina, and choroid, and sclearal ossicle.<\/p>\n

Figure 20.7: Multi-panel anatomical illustration showing the organization of taste receptors on the human tongue, progressing from gross anatomy to cellular detail. Center left is a dorsal view of the human tongue showing the distribution of different papilla types across the surface, with small boxes indicating the regions magnified in surrounding panels. Top center and right are a circumvallate (vallate) papilla is shown in cross-section, with taste buds labeled along its walls, and a histological photomicrograph (upper right) showing the actual tissue appearance of the circumvallate papilla in pink-stained section, revealing the taste bud-lined trench. The bottom left shows three additional papilla types shown in isolation: fungiform papilla (broad, mushroom-shaped), filiform papilla (narrow, pointed, hair-like), and foliate papilla (leaf-like folds with taste buds labeled along the sides). The bottom right is a detailed cross-sectional diagram of a single taste bud, labeling: taste hairs (microvilli projecting into the taste pore), taste pore (opening at the surface), gustatory cells (the primary sensory receptor cells), basal cells (stem cells at the base), and transitional cells (intermediate cell type).<\/p>\n

Figure 20.10: Detailed anatomical illustration showing a cross-sectional view of the human ear, divided into three labeled regions along the bottom: External ear, Middle ear, and Inner ear. The external ear structures labeled are the auricle (the visible outer ear cartilage, shown in brown) and the ear canal (the passage leading inward to the eardrum). The middle ear structures labeled are the tympanic membrane (eardrum, separating the external and middle ear), the three ossicles \u2014 malleus, incus, and stapes (attached to the oval window) \u2014 and the tympanic cavity (the air-filled middle ear space). The inner ear structures labeled are the vestibule, vestibular nerve, cochlear nerve (both shown in yellow as they merge into the vestibulocochlear nerve), round window, cochlea (the snail-shaped hearing organ), and Eustachian tube (connecting the middle ear to the nasopharynx for pressure equalization). The surrounding temporal bone is rendered in beige\/tan with a spongy texture, and the middle ear cavity is shown in red.<\/p>\n

Figure 20.11: Anatomical illustration showing a cross-sectional view of the cochlea, with a small inset diagram of the whole cochlear spiral in the upper left indicating the region being magnified. The main image shows a single turn of the cochlea cut in cross-section, revealing its internal compartments and structures. Labeled structures include: bony cochlear wall (the outer osseous shell), scala vestibuli (the upper fluid-filled chamber), cochlear duct (the middle triangular compartment containing endolymph), tectorial membrane (a gelatinous membrane overlying the hair cells), basilar membrane (the floor of the cochlear duct supporting the organ of Corti), scala tympani (the lower fluid-filled chamber), organ of Corti (the sensory epithelium containing hair cells, labeled on the right with a leader line pointing to the structure sitting on the basilar membrane), spiral ganglion (tan dots on the yellow cochlear branches that are the collection of sensory neuron cell bodies at the base), and cochlear branch of N VIII (the cochlear portion of the vestibulocochlear nerve, shown in yellow exiting to the right).<\/p>\n

Figure 21.4: A detailed flow chart illustrating the Negative Feedback Loop of hormone regulation, specifically focusing on the release of glucocorticoids. The process is shown in a continuous cycle of four main steps starting from a state of homeostasis. Stage 1, Imbalance: A cross-section of a blood vessel shows a low concentration of blue dots representing glucocorticoids. The hypothalamus perceives low blood concentrations of glucocorticoids via sensors in the blood vessels. Stage 2, Hormone Release: An illustration of the brain’s hypothalamus and pituitary gland. The Hypothalamus releases Corticotropin-releasing hormone (CRH). A dashed line shows CRH traveling to start a hormone cascade that triggers the adrenal glands to release glucocorticoid into blood. Stage 3, Correction: An illustration of an adrenal gland (sitting atop a kidney). Blood concentration of glucocorticoids increases. A dotted line runs from the end of this stage (glucocorticoid release) to show another cross-section of a blood vessel with increasing density of blue dots (glucocorticoid levels in the blood increase). Stage 4, Negative Feedback: The hypothalamus illustration returns, but with a red “X” over the “CRH release” label. As glucocorticoid levels reach a normal concentration, the hypothalamus perceives this “Correction” and stops releasing CRH. This negative feedback returns the system to a state of Homeostasis.<\/p>\n

Figure 21.6: A flow chart diagram mapping the hormones produced by the pituitary gland and their various target organs in the body. The pituitary is split into two distinct lobes (posterior on the left and anterior on the right). Posterior Pituitary (Left Lobe) releases two primary hormones that act on specific tissues: Antidiuretic hormone and oxytocin that targets an unidentified organ but indicated by an outward arrow). Anterior Pituitary (Right Lobe) produces a wide range of hormones that regulate other endocrine glands and body processes. Prolactin targets the mammary glands and ovaries (to produce estrogens and progesterone). LH (Luteinizing Hormone) targets the ovaries (to produce estrogens and progesterone) and the testes (to produce androgens). FSH (Follicle-Stimulating Hormone) also targets the ovaries (to produce estrogens, progesterone) and testes (to produce androgens). TSH (Thyroid-Stimulating Hormone) targets the Thyroid gland to produce thyroid hormones (T3, T4). MSH (Melanocyte-Stimulating Hormone) targets pigment cells in the skin. ACTH (Adrenocorticotropic Hormone) targets the adrenal gland to produce glucocorticoids. GH (Growth Hormone) targets bones to stimulate growth.<\/p>\n

Figure 21.9: A complex scientific diagram illustrating the comparative anatomy and evolution of the pituitary gland across different vertebrate groups. The central element is a phylogenetic wheel, with anatomical cross-sections for specific species radiating outward. The diagram uses a color-coded legend to identify specific regions. grey: brain, brown: Pars nervosa, green: Pars intermedia, pink\/blue\/purple: Pars distalis (divided into Rostral, Medial\/Proximal, and Ventralis), black: Pars tuberalis, orange: median eminence, red lines: vascular system\/blood flow. Comparative Species (Clockwise from top) begins with A (Lamprey\/Hagfish) shows a primitive, elongated structure with distinct blue, pink, brown and green segments, and simple downward blood flow. B (Shark\/Elasmobranch) shows a long structure at the bottom with distinct orange, light blue, pink, brown, and green segments. It features a highly specialized ventralis region (dark blue) extending downward and a complex vascular network. C (Teleost Fish) displays a compact gland where the brain tissue (grey) interdigitates deeply with the pars distalis. D (Lungfish) shows an elongated pars distalis with a prominent orange median eminence. E (Frog\/Amphibian) is a more globular structure where the pars distalis (pink) is situated below the brain, with blood vessels entering through the median eminence. F (Turtle\/Reptile) displays a large, layered pars distalis with clear compartmentalization between the blue and pink regions. G (Mouse\/Mammal) features the classic mammalian structure with a well-defined pars nervosa (brown) and a large, unified pars distalis (pink) fed by a portal system through the median eminence.<\/p>\n

Figure 21.15: This diagram illustrates the homeostatic regulation of blood calcium levels (Ca^{2+}), showing the opposing roles of the thyroid and parathyroid glands. The system is depicted as a seesaw balanced on a fulcrum to represent equilibrium. High Blood Calcium (Upper Loop) shows when calcium levels rise above the set point (homeostasis). An imbalance occurs where blood Ca^{2+} is too high, which causes the thyroid gland to release hormones (calcitonin, though not explicitly labeled) to lower calcium. This stimulates bone calcium deposit, where excess calcium is stored in the bone tissue. The result is that blood Ca^{2+} levels decrease (indicated by the downward blue arrow) to return to homeostasis. Low Blood Calcium (Lower Loop) shows when calcium levels fall below the set point. An imbalance occurs where blood Ca^{2+} is too low, thus the Parathyroid Gland (shown as four small nodes) releases Parathyroid Hormone (PTH). PTH stimulates osteoclasts (blue multi-nucleated cells) to break down bone matrix. The result is that calcium is released from the bone into the blood, causing blood Ca^{2+} levels to rise (indicated by the upward blue arrow) back toward homeostasis.<\/p>\n

Figure 21.19: A detailed black-and-white anatomical diagram of the adrenal glands, showing their location in the body and their internal cellular structure. Gross Anatomy (Top) have two adrenal glands that are shown sitting directly on top of each kidney. The diagram includes the major blood vessels connecting to the kidneys. Gland Cross-Section (Bottom Left) is a close-up view of a single adrenal gland sliced open to show two main functional areas: Adrenal Medulla (the innermost core of the gland), and Adrenal Cortex (the thick outer layer surrounding the medulla). Microscopic View of the Cortex (Bottom Right) is a highly magnified rectangular inset that shows the three distinct layers (zones) of the adrenal cortex, each responsible for producing different hormones: Zona glomerulosa (the outermost layer, characterized by rounded clusters of cells), Zona fasciculata (the middle and thickest layer, consisting of cells arranged in long, straight columns), and Zona reticularis (the innermost layer of the cortex, featuring a net-like arrangement of cells adjacent to the medulla).<\/p>\n

Figure 21.22: This anatomical illustration provides a detailed comparative overview of the male and female reproductive gonads (Testis and Ovary) and their microscopic structures. The left side of the diagram focuses on the male reproductive system. The macro view is a cross-section of the testis that reveals internal compartments containing coiled seminiferous tubules, where sperm production occurs. The microscopic view (Cross section of seminiferous tubule) is circular detail showing the cellular arrangement within a tubule includes spermatogonium with stem cells located at the outer edge of the tubule, Sertoli cells are the large, supportive cells (highlighted in green) and Leydig cells are small clusters of blue cells located in the interstitial space between the tubules. The right side of the diagram illustrates the female reproductive system. The macro view, the ovary is shown containing follicles at various stages of the menstrual cycle including a developing follicle (small circular structures growing within the ovary), mature follicle (a large, fluid-filled follicle ready for ovulation), and Corpus luteum (shown in stages of forming and regressing after the egg has been released). The microscopic view (Cross section of follicle) is a circular detail shows a mature follicle containing oocyte (the central egg cell), granulosa cells (a thick layer of cells, highlighted in purple, surrounding the oocyte), and Theca cells (the outermost layer of cells, highlighted in yellow, that work with granulosa cells to produce estrogen.<\/p>\n

Figure 21.24: This anatomical diagram illustrates the Hypothalamic-Pituitary-Gonadal (HPG) axis, the primary hormonal system responsible for regulating reproductive functions in both males and females. The image is structured into three main components. Part 1 is the Hormonal Flowchart (Left) that shows a vertical pathway describing the sequence of hormone signaling. The hypothalamus is the control center that secretes GnRH (Gonadotropin-releasing hormone). Anterior pituitary is stimulated by GnRH to release LH (Luteinizing hormone) and FSH (Follicle-stimulating hormone). Gonad is the target organs (testis or ovary) that receive LH and FSH. Sex steroids are the final products are the sex steroids. Part 2 is the Negative Feedback Loop showing a large, curved arrow pointing from the sex steroids back up to the Anterior pituitary and the Hypothalamus. It is labeled with negative signs (-). Part 3 shows Anatomical Illustrations (Right) in detailed black-and-white line drawings representing the physical organs involved. The top is a cross-section of the brain showing the hypothalamus and pituitary gland. Bottom Left is a cross-section of a testis, showing internal seminiferous tubules. Bottom Right is a cross-section of an ovary, showing follicles at various stages of development and a corpus luteum.<\/p>\n

Figure 21.25: This infographic illustrates the menstrual cycle by mapping hormonal fluctuations against the physical changes in the ovary over a 28-day period. Section 1 are the Ovarian Stages (Top Row). The diagram tracks the development of the follicle and the egg across three main phases: Follicular Phase (Days 1\u201313) shows the growth from a small developing follicle to a large, pink mature follicle. Ovulation (Day 14) is a large, irregular white structure represents the ruptured follicle releasing the egg (pink dot). This is labeled as the fertile period. Luteal Phase (Days 15\u201328) shows the transformation of the ruptured follicle into an early corpus luteum (star-shaped), which eventually becomes a smaller regressing corpus luteum as the cycle ends. Section 2 shows Hormone Levels (Middle Section). Colored line graphs represent the concentrations of four key hormones. LH (red) shows a massive spike right before ovulation. FSH (orange) shows a moderate rise during the follicular phase and a small peak alongside LH. Estrogen (green) peaks twice\u2014once just before ovulation to trigger the LH surge, and again during the mid-luteal phase. Progesterone (blue) remains low until after ovulation, where it rises significantly during the luteal phase to support the uterine lining. Section 3 shows the timeline (Bottom Row). The horizontal axis is labeled Days and includes markers for Day 1, 7, 14, 21, and 28 to provide a chronological scale for the biological events.<\/p>\n","protected":false},"author":3,"menu_order":2,"template":"","meta":{"pb_show_title":"","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"back-matter-type":[],"contributor":[],"license":[],"_links":{"self":[{"href":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/wp-json\/pressbooks\/v2\/back-matter\/1170"}],"collection":[{"href":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/wp-json\/pressbooks\/v2\/back-matter"}],"about":[{"href":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/wp-json\/wp\/v2\/types\/back-matter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/wp-json\/wp\/v2\/users\/3"}],"version-history":[{"count":6,"href":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/wp-json\/pressbooks\/v2\/back-matter\/1170\/revisions"}],"predecessor-version":[{"id":1180,"href":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/wp-json\/pressbooks\/v2\/back-matter\/1170\/revisions\/1180"}],"metadata":[{"href":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/wp-json\/pressbooks\/v2\/back-matter\/1170\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/wp-json\/wp\/v2\/media?parent=1170"}],"wp:term":[{"taxonomy":"back-matter-type","embeddable":true,"href":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/wp-json\/pressbooks\/v2\/back-matter-type?post=1170"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/wp-json\/wp\/v2\/contributor?post=1170"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.palni.org\/comparativevertebrateandhumananatomy\/wp-json\/wp\/v2\/license?post=1170"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}