Lab 10: Tetrapod Vertebrates

1. Taxonomy for Lab 10

Subphylum Vertebrata: tetrapod vertebrates

  • Class Amphibia: amphibians (frogs, toads, salamanders, caecilians)
  • Class Mammalia: mammals
  • Nonavian reptiles: turtles and tortoises; crocodilians; lizards and snakes
  • Birds: ratites and carinates
2. Class Amphibia (caecilians, newts, salamanders, frogs, toads)

The Class Amphibia ("double life") contains over 4,000 species of animals that are somewhat transitional between fishes and reptiles. Amphibians make their first appearance in the fossil record almost 400 million years ago, and the surviving species represent a small fraction of the total number of amphibians that once existed on Earth. General characteristics of amphibians include a bony skeleton and usually four limbs. The skin of amphibians is moist and thin with no scales. Since it is too thin to provide much protection against dehydration or predators, they must employ other means of defense.

For example, amphibian skin contains many mucous glands that keep them moist and make them slippery, which helps them escape from predators. The skin of all amphibians also contains poison (serous) glands that produce toxins that range from mildly noxious to deadly poison (some are among the most potent toxins produced by any vertebrates). In keeping with their toxic nature, many amphibians are brightly marked with aposematic (warning) colors that advertise their toxicity. These colors are due to pigment-containing cells call chromatophores that come in several varieties.

Living amphibians include the: leg-less amphibians (caecilians), tailed-amphibians (salamanders and newts) and tail-less amphibians, or anurans, (frogs and toads).

Caecilians

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The caecilians include about 165 limbless amphibians that inhabit tropical forests of Central and South America, Africa and Southeast Asia. Their food consists mostly of earthworms and other small invertebrates. 

Salamanders

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Salamanders are tailed amphibians with short, stubby legs. Most species (like the Eastern red-backed salamander shown) have slender bodies, short noses, long tails and four toes on their front legs and five on their rear legs. Their moist skin restricts them to moist habitats in or near water, often in wetlands. Some salamanders are aquatic throughout life, some take to the water intermittently and some are entirely terrestrial as adults. Unique among vertebrates is the ability to regenerate lost limbs, as well as other body parts. All salamanders and newts hatch with gills, but these are usually lost in all but aquatic forms or those that fail to metamorphose. Lungs are the rule in adult forms, but some salamanders lack lungs or gills, using only their skin and mouth for gas exchange! 

Newts

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There is little actual distinction between newts and salamanders. In North America, newts tend be more aquatic with laterally compressed tails that aid in swimming. Another difference is that newts tend to be poisonous, and some of them are quite deadly if ingested. Consequently, they tend to be colorful, active during the day and quite bold (contrasted to most other species of salamanders, which are nocturnal). 

Frogs

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There are over 4,000 species of frogs and toads, ranging in size from 1 cm to over 30 cm! These amphibians lack tails and are therefore referred to as anurans. True frogs (like the wood frog shown in the image above) tend to have two bulging eyes, strong, long, webbed hind feet that are adapted for leaping and swimming, smooth or slimy skin. In general, frogs tend to prefer aquatic or moist environments and tend to lay their eggs in clusters. 

Toads

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True toads tend to have stubby bodies with short hind legs (for walking instead of hopping), warty and dry skin with paratoid (poison) glands behind their eyes. In general, toads prefer drier habitats than true frogs and tend to lay their eggs in long chains instead of clusters. The specimen shown above is the Colorado River toad.

3. Non-Avian Reptiles (turtles, tortoises, crocodilians, lizards, snakes

The reptiles make their appearance in the fossil record around 340 million years ago during the Paleozoic Era. By the beginning of the Mesozoic Era (about 250 million years ago) the ruling reptiles had radiated into just about every conceivable ecological niche ushering in the "Age of Reptiles". Certainly the most spectacular forms were the dinosaurs ("terrible lizards") that ruled the land for over 100 million years and then disappeared abruptly around 65 million years ago.

In general, modern reptiles show a number of developments that make them better suited to life on land than the amphibians. These include a waterproof skin, durable scales, a stronger skeleton, more efficient circulatory, respiratory and nervous systems and the ability to get rid of their nitrogenous wastes in a manner that requires the loss of very little water. In addition, unlike amphibians that are always tied to water for their reproduction, reptiles engage in internal fertilization and produce eggs that can be deposited on land (some species are even capable of giving birth to live young).

In terms of their classification, since reptiles do not constitute a monophyletic group they are no longer considered as a valid taxon that is separate from birds. Recall that a monophyletic group must contains all known descendants of a single common ancestor, but the reptiles are considered paraphyletic because the group does not contain all of the descendants of a common reptile ancestor – the common ancestor of reptiles is also the ancestor of birds!

Consequently, we will refer to the animals traditionally included in the Class Reptilia as non-avian reptiles. Examples include: turtles, lizards, snakes and crocodilians.

Turtles

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Turtles and tortoises are among the most often seen and easily recognized non-avian reptiles. All species are encased in a pair of bony shells, a dorsal carapace and a ventral plastron. The shells (which are covered with a horny layer of keratin) are formed from sections of dermal bones that are fused to each other and to the ribs and thoracic vertebrae. Although all turtles (such as the painted turtle shown in the image above) are oviparous, burying their eggs in the soil, in some species, the temperature at which the eggs are incubated determines the sex of the offspring (cooler temperatures produce males). 

Lizards

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The more than 3,000 species of lizards belong to the most successful group of non-avian reptiles. Part of this success is due to their highly mobile jaws that can be used to seize and swallow large prey. Although many forms are found in the tropics, some species (such as the eastern fence lizard shown above) range well into temperate regions as well as the driest deserts of the world. Most lizards feed on insects and other arthropods, but the larger ones (such as iguanas) feed on plant material, and monitor lizards that can reach a length of up to 3 m and weigh up to 250 kg can even feed on pigs, deer and other mammals. 

Snakes 

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Like lizards, the more than 2,500 species of snakes owe much of their success to their highly mobile jaws that can be used to seize and swallow large prey. In spite of the fact that rarely do they pose any real threat to people in developed countries, snakes are often unjustly persecuted and deserve to be protected not only for the beneficial role they play in the control of rodent populations but also for the contribution they make to the Earth's biodiversity. It is against the law in many countries (including most of the United States) to kill nonvenomous species and sometimes even venomous ones like the timber rattlesnake shown above, which is protected in Wisconsin!

Crocodilians

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Modern crocodilians (a small remnant of a once large and successful group) are (along with birds) the only living members of the archosaurian line of reptiles that gave rise to both groups. All crocodilians (like the American alligator shown above) have well-reinforced skulls with massive jaw musculature that provides a wide gape and rapid, powerful closure over prey. Their many sharp teeth, which are set in sockets (thecodont), are designed for gasping but not for chewing. A muscular gizzard (like that of birds) finishes the job of grinding up food with the help of swallowed stones and other items. 

4. Birds (ratites and carinates)

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Birds (which are divided into two non-taxonomic, functional groups: the ratites, or flightless birds, and the carinates, flying forms with keeled sternums) make their appearance in the fossil record about 150 million years ago, most likely having evolved from small, bipedal dinosaurs. With over 9,000 species (more than any other vertebrate group except for the ray-finned fishes), they are found in just about every habitat on earth. In terms of their phylogeny, birds and crocodilians form a monophyletic sister group and are therefore placed in the clade Archosauria, a group that also contains the extinct dinosaurs and pterosaurs (flying reptiles). 

5. Class Mammalia (mammals)

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The Class Mammalia contains over 4,000 species, ranging in size from tiny bats and shrews to whales weighing over 100 tons! After the disappearance of the dinosaurs around 65 million years ago, mammals underwent a tremendous adaptive radiation filling practically all of the vacated ecological niches.  

6. Frog heart model (ventral view)

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The image above shows a ventral view of a frog heart. The amphibian heart contains three principal chambers, a thick-walled ventricle (1) and the thin-walled right atrium (2) and left atrium (3). The conus arteriosus (4) arises from the ventricle and divides to form the truncus arteriosus (5) on each side of the heart. Each truncus arteriosus divides into three major vessels, the pulmocutaneous arch (6) ,which goes to the lungs and skin, the systemic arch (7), which goes to the body, and the carotid arch (8), which go to the head region. 

7. Frog heart model (dorsal view)

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On the dorsal side of the heart observe the thin walled sinus venosus (4) that is formed by the convergence of the anterior vena cavae and posterior vena cava. Pulmonary veins (5) bringing oxygenated blood from the lungs join to enter the left atrium (3). Also seen on the model is the thick-walled ventricle (1), thin-walled right atrium (2), pulmocutaneous arches (6), systemic arches (7) and carotid arches (8). 

8. Frog heart model (internal view)

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In this internal view of the model, the ventral portion of the heart has been removed to reveal the arrangement of the three chambers. Note the spiral valve (1) within the conus arteriosus. This valve directs oxygenated and deoxygenated blood into appropriate channels. By blocking and unblocking the common entrance to the left and right pulmocutaneous arches (6), blood low in oxygen is shunted directly to the lungs and skin while oxygenated blood is directed to the carotid (8) arches and systemic arches (7). Note that the direction of flow of oxygenated and deoxygenated blood through the heart is indicated with red and blue arrows respectively. Other structures labeled on the model include the ventricle (4), right atrium (2), left atrium (3) and the right and left truncus arteriosus (5). 

9. Frog brain model (dorsal view)

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This image shows some of the major structures visible on the dorsal surface of the frog brain. The vertebrate brain is divided into three main regions, some of which are further subdivided. Anterior most is the forebrain, which is divided into the telencephalon and diencephalon. The mid brain, or mesencephalon, develops without further subdivision. The hindbrain differentiates into the metencephalon and myelencephalon. The anterior-most telencephalon bears two olfactory lobes (1) and two cerebral hemispheres (2). The olfactory lobes terminate in the olfactory nerves, which carry impulses from the nasal cavities to the brain.

The mesencephalon, immediately posterior to the diencephalon, bears two large optic lobes (3) that serve to integrate nerve impulses from the eyes. Note that the optic lobes of the frog are large, which reflects the importance of sight to these visual predators. Posterior to the mesencephalon is the metencephalon, which is represented by a narrow, transverse portion of the brain called the cerebellum (4); the cerebellum is involved in motor coordination in the frog.

The most posterior portion of the brain is the myelencephalon, consisting of the medulla oblongata (5) that tapers gradually into the spinal cord. A depression between the two sides of the medulla oblongata called the choroid plexus (6) is partially responsible for the secretion of the lymph-like cerebrospinal fluid that fills spaces called ventricles in the brain and the central canal of the spinal cord. 

10. Frog brain model (ventral view)

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This image shows some of the major structures visible on the ventral surface of the frog brain. Note the previously mentioned olfactory lobes (2) and associated olfactory nerves (1). On the ventral surface of the diencephalon, the two optic nerves (4) cross to form the optic chiasma and from there extend to the optic tracts (5) that carry impulses to the optic lobes on the dorsal surface of the brain.

Note: The olfactory and optic nerves are but two of the 10 pairs of cranial nerves possessed by all amphibians.

Posterior to the optic chiasma is a ventral outgrowth of the diencephalon called the pituitary gland, or hypophysis (6). This endocrine gland (which actually consists of two major subdivisions with different embryonic origins) regulates many body functions including in amphibians, changes in skin color. Anterior to the optic chiasma are the large cerebral hemispheres (3) of the telencephalon. Once again, the medulla oblongata (7) of the myelencephalon can be seen, along with the cranial nerves (shown in yellow on the model) that arise from this most posterior portion of the brain. 

11. Leopard frog skeleton (anterior-dorsal view)

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The above image show a dorsal view of the anterior part of a leopard frog skeleton. The skeleton of the frog consists chiefly of bony and cartilaginous elements. The functions of a skeleton include providing support for the body, protection of delicate internal organs and attachment surfaces for muscles. In vertebrates, the axial skeleton consists of the skull, vertebral column, sternum (breast bone) and ribs (which are not present in amphibians).

The vertebral column of frogs is made up of 10 vertebrae, the first of which is called the atlas (7), which articulates with the base of the skull. The atlas is the only cervical vertebra in the frog. The next seven vertebrae are abdominal vertebrae, followed by the sacral vertebra whose strong transverse processes form the sacrum (8) that join with the ileum (9). The last vertebra is a long and highly modified bone called the urostyle (10).

Note: Most vertebrates have a tail supported by caudal vertebrate, but frogs and toads are atypical in that they lack any tail and are therefore called anurans ("tail less amphibians").

Observe the fact that the forelimbs of the frog consist of two stout bones - a proximal humerus (1) and a distal radioulna (2). The hand is composed of a number of carpals (3), metacarpals (4) and distal elements called phalanges (5).

The forelimbs are supported by a number of bones that make up the pectoral girdle. These bones include the suprascapulas (6) and the scapulas, clavicles, coracoids and sternum, which are best seen in the ventral view of the frog on the next page. 

12. Leopard frog skeleton (anterior-ventral view)

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The above image show a ventral view of the anterior part of a leopard frog skeleton.

Once again, note that the forelimbs of the frog consist of two stout bones - a proximal humerus (1) and a distal radioulna (2). The hand is composed of a number of carpals (3), metacarpals (4) and distal elements called phalanges (5).

The forelimbs are supported by a number of bones that make up the pectoral girdle. These bones include the suprascapulas (seen in the dorsal view shown on the previous page), the scapulas (6), clavicles (7), coracoids (8) and sternum (9), which is composed of several separate bones and pieces of cartilage that are not easily distinguished as separate elements on the image above. 

13. Bull frog skeleton (lateral view)

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This image shows the skeleton of a bullfrog in a normal sitting position. Note the massive size of the fused radioulna (5) of the forelimbs and tibiofibula (13) of the hindlimbs, which reflects the ability of these bones to withstand the stresses imposed by the long jumps and subsequent landings of these heavy-bodied frogs.

Other labeled bones include the: suprascapula (1), scapula (2), posterior part of the sternum (3), humerus (4), carpals (6), metacarpals (7), phalanges (8), sacrum (9), ilium (10), ischium (11), femur (12), two tarsals — the astragalus (14) and calcaneus (15) and metatarsals (16). 

14. Bull frog skeleton (posterior view)

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This image shows a posterior view of the skeleton of a bullfrog. Aspects of the vertebral column and pelvic girdle that are visible include the first cervical vertebra that is called the atlas (1), the sacral vertebra whose transverse processes form the sacrum (2), the ilium (3), the ischium (4) and the urostyle (5).

As shown in the image, each hind limb is composed of a stout femur (6) and tibiofibula (7), two tarsal bones called the calcaneus (8) and the astragalus (9), the metatarsals (10) and distal bony elements called phalanges (11).

The suprascapula (12) and scapula (13) of the pectoral girdle are visible as well as the humerus (14) of each the forelimb. 

15. Dorsal muscles of the frog thigh

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This image shows several of the major muscles on the dorsal surface of the frog thigh. The gluteus muscle (1), which originates on the ilium and inserts on the femur, rotates the thigh. The piriformis (2) is a small muscle located near the opening of the cloaca. The muscle originates on the urostyle, inserts on the femur and functions to extend and rotate the thigh. As the name indicates, the triceps femoris (3) is divided into three parts that originate and insert on different skeletal elements. All function, however, to flex the thigh and extend the shank. The semimembranosus (4), which originates on the ischium and pubis and inserts on the tibiofibula, extends the thigh and flexes the shank. The biceps femoris (5) found between the triceps femoris and semimembranosus originates on the ilium and inserts on the tibiofibula and femur. The biceps femoris extends and adducts the thigh and flexes the shank. Also shown on the image are the two major muscles of dorsal surface of the frog shank, the gastrocnemius (6) and peroneus (7). 

16. Ventral muscles of the frog thigh 1

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This image shows the major muscles of the ventral surface of the frog thigh. The sartorius (1) is a long, strap-shaped muscle that covers the anterior surface of the thigh. It originates on the pubis, inserts on the tibiofibula and acts to flex the thigh and shank. The sartorius (2) of the right leg is shown with its distal end cut and deflected to make it more visible. The adductor longus (3), which originates on the pubis and inserts on the femur, is a thin, strap-shaped muscle beneath the sartorius. Note: This muscle of the right leg has also been cut at its distal end and deflected to make it more visible. As the name implies, the adductor longus functions to adduct the thigh. The adductor magnus (4), which also adducts the thigh, is a large muscle seen as a triangle near the groin when the sartorius is in place. The muscle originates on the ischium and pubis and inserts on the femur. The semitendinosus (5) is a deep muscle with two heads that lies under and between the gracilis major and adductor magnus. The muscle originates on the ischium, inserts on the tibiofibula. The semitendinosus extends and adducts the thigh and flexes the knee. The gracilis major (6) is a large muscle that partly covers adductor magnus. It originates on the pubis, inserts on the tibiofibula and acts to extend the thigh and flex the shank. 

17. Ventral muscles of the frog thigh 2

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This image shows a close-up view of some of the major muscles of the ventral surface of the frog thigh seen on the previous page. Note again the sartorius (1) muscle that covers the anterior surface of the thigh, the adductor magnus (2) and gracilis major (3). Not seen on the previous page is the gracilis minor (4), a thin, strap-shaped muscle that covers the posterior margin of the thigh. This muscle has the same origin, insertion and action as the larger gracilis major, that is, it originates on the pubis, inserts on the tibiofibula and acts to extend the thigh and flex the shank. 

18. Dorsal muscles of the frog shank

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This image shows two of the major muscles found on the frog shank. The large gastrocnemius (1), the calf muscle that is located on the medial surface of the shank, originates on the femur, inserts on the Achilles tendon and flexes the shank and foot. The peroneus (2), which is located lateral to the gastrocnemius, also originates on the femur but inserts on the distal end of the tibiofibula. The peroneus extends the shank and foot.  

19. Ventral muscles of the frog shank

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This image shows a ventral view of some of the other major muscles of the frog shank. Once again note the large gastrocnemius (1) that covers much of the posterior surface of the shank. Between the gastrocnemius and the tibiofibula (2) is the tibialis posticus (3), a small muscle tightly attached to the posterior surface of the tibiofibula. This muscle originates on the tibiofibula, inserts on the tarsal bones and flexes the foot. The extensor cruris (4) is a short muscle found tightly attached to the upper two thirds of the tibiofibula. It originates on the femur, inserts on the tibiofibula and extends the shank. The tibialis anticus longus (5) is a small muscle with two bellies that lies anterior to the tibiofibula. It originates on the femur and inserts on the tarsal bones. The tibialis anticus longus lifts the foot and flexes the ankle. 

20. Frog oral cavity (ventral view)

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In this image of a preserved frog, the lower jaw and tongue have been removed to reveal the details of the upper surface of the oral cavity. Frogs are predators that capture prey (usually insects) with sticky tongues that are attached at the front, an arrangement that allows them to be everted to some distance. Prey are held by a two rows of tiny, sharp maxillary teeth (1) located on each side of the upper jaw as well as a pair of more centrally located, larger vomerine teeth (2).

Frogs breath by taking in air through a pair of external nares that enter the oral cavity through openings called internal nares (3). Note the openings to the eustachian tubes (4) that communicate with the middle ear cavity. These structures allow vertebrates to equalize the pressure on both sides of the eardrum. 

21. Frog internal anatomy 1

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The above image shows a ventral view of a dissected preserved frog with the abdominal skin and muscles removed. Note that the largest and most conspicuous organ is the liver (1), which is divided into three lobes. Located between the right and left lobes of the liver is the gall bladder (2), which stores bile (a digestive juice) that is produced by the liver. When needed for digestion, the gall bladder secretes a small amount that aids in the breakdown of food, specifically fats. Structures belonging to the digestive system that can be seen include the stomach (3), small intestine (4) and large intestine (5). Other labeled structures include the bright orange or yellow fat bodies (6) that provide enough energy for a frog or toad to go without food during hibernation or estivation (burrowing to escape summer heat and arid conditions) for over a year, the heart (7) and deflated urinary bladder (8).

Note: The urinary bladder of anurans is a membranous structure that is only inflated when full of urine. Because it is so thin and highly vascular, frogs and toads can actually reabsorb water from the bladder during times of drought, using it as a reservoir in such situations. 

22. Frog internal anatomy 2

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The above image shows another ventral view of a dissected frog with the liver and gall bladder removed to reveal the underlying organs. Structures belonging to the digestive system that can be seen include the stomach (1), small intestine (2) and large intestine (3). Other labeled structures that can be seen include the right and left lungs (4), ventricle (5), left atrium (6) and right atrium (7) of the heart and the spleen (8), which is part of the circulatory system. 

23. Frog internal anatomy 3

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The above image shows a ventral view of the dorsal wall of a preserved male frog. Structures of the urogenital system that can be seen include the left kidney (1), attached to which at its anterior end is an oval, cream colored testis (2). The (deflated) urinary bladder (3) can also be seen covering a portion of the posterior end of the left kidney. Although this is a male frog, males of many species of frogs retain vestigial oviducts (4) that serve no actual purpose. In females, of course, eggs are produced and retained within the oviducts until they are released during mating. Other labeled structures include fat bodies (5), the stomach (6), the median lobe of the liver (7) and the left lung (8). 

24. Frog circulatory system 1

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This image shows a ventral view of the heart of a preserved bullfrog. The heart of all amphibians contains three chambers – a muscular ventricle and two thin-walled atria. Although there is some mixing of oxygenated and de-oxygenated blood, this problem is reduced considerably due to the presence of a spiral valve that shunts oxygenated blood to the body and poorly oxygenated blood to the lungs and skin. For a more detailed view of the major structures and vessels of the heart, see the images of the frog heart model.

On the image above observe the large ventricle (1), the left atrium (2) and right atrium (3). Note that the conus arteriosus (4) coming off the ventricle divides into two great vessels, the left truncus arteriosus (5) and right truncus arteriosus (6). Each truncus arteriosus give rise in turn to three vessels called the aortic arches, which consist of the common carotid artery (which goes to the head region), the pulmocutaneous artery (which goes to the lungs and skin) and the systemic arch (which goes to the rest of the body). Also note that two of the three lobes of the liver (7), which is the largest organ in the body cavity, are shown on the image. 

25. Frog circulatory system 2

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The image shows a ventral view of some of the major arteries of a preserved bullfrog. Arteries and veins of dissection specimens are often injected with colored latex to make it easier to visualize the extent of the circulatory system. By convention, arteries are injected with red latex and veins with blue latex. Arteries are blood vessels that conduct blood away from the heart, while veins conduct blood toward the heart. Because of this fact, arteries must withstand much greater blood pressure and are therefore thicker (and easier to find).

Labeled arteries include the two systemic arches (1) that come from the heart. These blood vessels join to form the large dorsal aorta (2), which continues posteriorly posteriorly until it splits into the two common iliac arteries that supply blood to the legs. At the point where the systemic arches join, the dorsal aorta gives off a short celiacomesenteric artery (3) that divides into the celiac artery (4) that goes to the stomach (5), pancreas and liver and the mesenteric artery (6) that goes to the small intestine, large intestine and spleen. Also seen on the image is the large posterior vena cava (7), which receives blood from the liver, kidneys and gonads. 

26. Frog circulatory system 3

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The image above shows another ventral view of some of the major arteries of a preserved bullfrog. Note again the two systemic arches (1) that come from the heart. These blood vessels join to form the large dorsal aorta (2), which continues posteriorly posteriorly until it splits into the two common iliac arteries (3) that supply blood to the legs. As mentioned on the previous slide, at the point where the systemic arches join, the dorsal aorta gives off a short celiacomesenteric artery (4) that divides into the celiac artery (5) that goes to the stomach, pancreas and liver and the mesenteric artery (6) that goes to the small and large intestine and spleen (7).  

27. Frog circulatory system 4

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Although veins normally carry blood directly from capillaries to the heart, in amphibians, this plan is interrupted by capillary beds in two portal systems – the hepatic and renal portal systems. The hepatic portal system consist of two main vessels, the ventral abdominal vein (1) in the ventral body wall (often broken during dissection), which collects blood from the pelvis, and the hepatic portal vein, which receives blood from the stomach (gastric vein), intestine (mesenteric vein) and spleen (splenic vein). The renal portal system conducts blood from the hind legs directly to the kidneys via the renal portal veins. Other labeled structures on the image include the heart (2) and three lobes of the liver (3). 

28. Bird skeleton

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  1. Synsacrum
  2. Pygostyle
  3. Uncinate processes
  4. Keeled sternum
  5. Furcula 
  6. Quadrate bone

Many aspects of the bird skeleton reflect the special demands of flight. For example, note the extensive fusion of vertebral elements. The posterior thoracic, all of the lumbar and sacral and the anterior caudal vertebrae have been fused into a single structure called the synsacrum (1). The posterior caudal vertebrate are fused into the pygostyle (2), which supports the tail. Observe the small, forward-pointing projections called uncinate processes (3) that connect one rib with the preceding one. These structures serve to provide further support and rigidity for the airframe.

Note that the clavicles are fused together to form the furcula (4), which is often called the "wishbone", and that the large sternum has a well-developed keel (5), which provides a large surface area for the attachment of the major flight muscles.

Finally, observe the articulation of the avian jaw (which is identical to that of the non-avian reptiles). Although not visible as a separate element, the articular bone of the lower jaw articulates with the quadrate bone (6) on the bird skull. In mammals, the dentary bone (mandible), the only bone in the lower jaw, articulates with the squamosal bone (a portion of the temporal bone in humans). No longer needed with this new jaw articulation, the articular and quadrate bones migrate into the middle ear to become the malleus and incus respectively, two of the three mammalian ear ossicles that conduct sound from the tympanum to the inner ear. 

Synsacrum and pygostyle

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The above image shows how the rigid airframe of the bird skeleton is formed by extensive fusion of the vertebral elements. As mentioned in the overview of the bird skeleton, the posterior thoracic, all of the lumbar and sacral and the anterior caudal vertebrae have all been fused into a single structure called the synsacrum (1). The posterior caudal vertebrate are fused into the pygostyle (2), which supports the tail. 

Uncinate processes of the bird rib cage

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The above image shows how the uncinate processes (3) of a bird connect one rib with the preceding one, providing further support and rigidity for the airframe. 

Keeled sternum and furcula

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Note on the image above that unlike those of most tetrapods, the sternum (4) of bird is not flat but rather extends anteriorly as a large keel that provides a large surface area for the attachment of the two major flight muscles (the pectoralis major and supracoracoideus). Also observe how the clavicles of the bird have become fused into a structure called the furcula (5), which is often referred to as the "wish bone". 

Quadrate bone of the bird jaw

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Finally, observe the articulation of the avian jaw (which is identical to that of the non-avian reptiles). Although not visible as a separate element, the articular of the lower jaw articulates with the quadrate bone (6) on the bird skull. In mammals, the dentary bone (mandible), the only bone in the lower jaw, articulates with the squamosal bone (a portion of the temporal bone in humans). No longer needed with this new jaw articulation, the articular and quadrate bones migrate into the middle ear to become the malleus and incus respectively, two of the three mammalian ear ossicles that conduct sound from the tympanum to the inner ear. 

29. Cat skeleton

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The above image highlights the various regions of the vertebral column of the cat. Note that there are seven cervical (neck) vertebrae (1), which is the same number found in most mammals. The first two cervical vertebrae (C1 & C2) are called the atlas and axis respectively. There are 13 thoracic vertebrae (2), which in addition to their function of axial support, bear the ribs. Next come seven lumbar vertebrae (3), followed by three sacral vertebrae (4) that are fused into the sacrum and a variable number (21-23) caudal vertebrae (5) that support the tail. For more detailed views of the skull and cervical region, the pectoral girdle, font limbs and sternum, the pelvic girdle and the hind limbs, see below.

Skull/Cervical Region

Lab-10 42

This image shows the region of the cat skeleton containing the skull and the first six (of seven) cervical vertebrae. The first cervical vertebra (1), which is called the atlas, articulates with two projections of the skull called occipital condyles and permits up and down motion of the head. The second cervical vertebra (2), which is called the axis, permits rotational movement of the skull.

Pectoral Girdle/Sternum

Lab-10 43

This image shows the region of the cat skeleton containing the pectoral girdle and its associated appendages as well as the sternum. The front limbs are composed of three main bones ─ the humerus (1), radius (2) and ulna (3). These appendages are supported by the pectoral girdle, which consists a left and right scapula (4) in cats. Notice that the clavicles of the cat are free-floating and embedded in muscle tissue, that is, they do not articulate with any other bones (as they do in humans). Note: On the prepared skeleton these bones are supported by two pieces of wire attached to the anterior most bone of the sternum and that on the image above, only the left clavicle (5) is labeled

The sternum (which serves as for the attachment of the cartilaginous portions of nine pairs of ribs) is composed of eight segments called sternebrae, the first of which is a laterally compressed structure called the manubrium (6), followed by six sternebrae (7) that form the body of the sternum and a caudal piece called the xiphoid process (8). 

Pelvic Girdle

Lab-10 44

This image shows the region of the cat skeleton containing the pelvic girdle and its attached appendages*, the femurs (1). The pelvic girdle consists of three pairs of bones, the ilium (2), the ischium (3) and the pubis (4) that are fused together in a rigid structure. Also seen on the image are four of the last seven lumbar vertebrae (5), the three sacral vertebrae (6) that are fused to form a sacrum to which the pelvic girdle is attached and two of the caudal vertebrae (7).

*Note: To see the other major bones of the cat's hind limbs, click here. 

Hind Limbs 

Lab-10 45

 

This image shows the three major bones of the cat's hind limbs, the femurs (1), the tibias (2) and the fibulas (3) as well as the largest of the seven tarsal bones, the calcaneus (4), or heel bone. Also seen on the image is the patella (5), or "knee cap", of the left leg as well as the ischium (6) and pubis (7) of the pelvic girdle and a number of caudal vertebrae (8).