BIO 112 Lab 8

In the first part of this lab you will study the diversity of the remaining two classes of SubPhylum Vertebrate, the birds (Class Aves) and the mammals (Class Mammalia).

In the second part of this lab you will perform a virtual dissection (close examination of a dissected, plastic-embedded specimen) of a representative bird (Columba livia - the rock dove or common pigeon) and a detailed actual dissection of a mammal (Rattus norvegicus - the Norway rat).

PART I. DIVERSITY OF THE BIRDS AND MAMMALS

Materials

Representative live, preserved, skeletal, and/or fossil specimens of birds and mammals

Models for some taxonomic groups

Procedure

Specimens of each vertebrate taxon are on display. These specimens include skeletal remains, preserved animals, live animals, and some reproductive or behavioral artifacts (such as bird eggs and nests).

1. Work through these displays.

a. A dichotomous key which outlines the major distinguishing features of each vertebrate class is provided below.

b. Following the key is additional information about each class, as well as a guide to lower taxa (subclasses, infraclasses, superorders, orders) within each class. On the sample specimens, identify and examine all of the external structures written in bold print in this guide.

c. Additional information will be on display with the specimens.

d. Your textbook has additional information on these vertebrate classes. The Biology Department has several additional vertebrate biology textbooks.

2. Be able to describe the distinguishing features of each class of vertebrates. Within each class, be able to recognize the major subdivisions. Pay particular attention to terms and taxonomic groups in bold type.

3. Be able to use the distinguishing features to reliably classify these sample vertebrates and distinguish them from the groups that you examined in the previous animal labs.

Study Suggestions

1. Make detailed sketches and notes on specimens. This will help you to look at the specimens more closely, as well as to help you study later.

2. Plan to come view the specimens once or twice more before the lab test. Test yourself by attempting to identify the specimens as accurately as possible by their common names, as well as to classify them without first looking at their labels.

3. The words in bold print in the extended guide below are words you should know and/or structures you should be able to identify or describe.

A Dichotomous Key to the Vertebrate Classes

Note: This key is based, in some cases, on secondarily derived characteristics of adult animals. This key roughly follows actual phylogenetic relationships. Notice that this produces an "unbalanced" key with lots of exceptions (e.g. snakes are reptilian tetrapods but do not have four legs) and omissions (e.g. non-avian dinosaurs).

1. Organisms are without jaws CEPHALASPIDOMORPHI,

MYXINI [Week 7]

[also: lack paired appendages (fins)]

1. Organisms have jaws

2. Organisms have fins

3. Organisms have cartilaginous skeletons

CHONDRICHTHYES [Week 7]

[Also: most have no operculum covering gills slits; have 5-7 gills with

separate openings; have non-overlapping, placoid (bony) scales with

projecting points; have subterminal mouth; tail is either heterocercal

or whip-like; have no swim bladder but use oil instead for flotation]

3. Organisms have partially bony skeletons

OSTEICHTHYES [Week 7]
[Also: have an operculum over gill slits; have thin, overlapping dermal

scales; most use swim bladder for flotation]

2. Organisms are tetrapods ('four-footed') as adults or embryologically

3. Organisms have moist skin AMPHIBIA [Week 7]
[Also: terrestrial but remain tied to aquatic habitats; usually have

external fertilization; eggs have jelly-like membrane coverings;

development includes metamorphosis from aquatic larval form

to lung breathing adult (usually); are ectothermic

3. Organisms have dry skin

4. Organisms are ectothermic or heterothermic

REPTILIA [Week 7]

[Also: have scales; have amniotic egg with leathery shells;

have internal fertilization; have homodont dentition or

are adontoid]

4. Organisms are endothermic or homeothermic

5. Organisms have feathers AVES
[Also: have front limbs modified for flight (usually); have scales

on feet; have an amniotic egg with calcareous shell;

have beak and are devoid of teeth]

5. Organisms have fur (hair) MAMMALIA
[Also: have mammary glands; have heterodont dentition

(usually); have prehepatic diaphragms; have well-developed

forebrains]

CLASS AVES (birds)

The phylogeny of birds places them solidly within the reptiles, specifically as a surviving subset of the saurischian dinosaurs. Indeed, they are more closely related to the crocodilians than either is to any other extant group of vertebrates. The basic internal organ structure and function are very reptilian, including a diapsid skull, a posthepatic diaphragm and unidirectional air flow through the lungs, modification of the anterior part of the stomach into a muscular gizzard for grinding up ingested food, reproduction via amniotic eggs released from a cloaca, and epidermal scales (on parts of the bird body). Like crocodilians (and most dinosaurs), but unlike other reptiles, birds are largely endothermic and homeothermic, have a four-chambered heart with separate pulmonary and systemic ventricles, and show extended care of the young. For these reasons the Campbell text treats birds as a subset of the reptiles.

However, birds have enough unique, shared, derived characteristics to warrant placing them in their own taxonomic Class Aves. These include:

1) calcified egg shells

2) modification of the forelimbs into wings

3) highly modified skulls with enlarged craniums and toothless and

keratinized beaks

4) enlarged brains with very well-developed forebrains

5) fusion of the pelvic bone into a synsacrum arch

6) fusion of the clavicles into a springy furculum (wishbone)

7) fusion of sternal bones and modification into a prominent carina (keel) for

attaching flight muscles

8) and elaboration of the respiratory systems into multiple passages, air sacs,

and hollow spaces within the long bones.

Most of these can be regarded as adaptations (or preadaptations) for flight, although some birds have secondarily lost this capacity and some of the features. The most distinguishing features of birds are feathers, keratinized elaborations of basic reptilian scales, which originally evolved to serve insulating and perhaps display functions.

Compare the skeleton and skull of the modern pigeon Columba livia to those of the modern crocodilian Alligator mississippiensis, as well as to the models and casts of the flying reptile pterosaur Pterodactylus, the saurischian dinosaur Velociraptor, and the "ancestral bird" Achaeopteryx. What are the similarities and what are the differences? Note the very modern arrangement and structure of the flight feathers of Archeopteryx. What shared skeletal structures of the bird are distinct from Archeopteryx?

The extant members of Class Aves are generally divided into two Subclasses - Paleognathidae (the ratitie birds) and Neognathidae (the carinate birds). Avian systematics below this level are still "under revision", with considerable disagreement on both group memberships and phylogenic relationships.

Subclass Paleognathidae - ratite birds

The ratites are (mostly) flightless birds with a heavy synsacrum, a flattened carina (keel), reduced clavicles, and reduced wings. They comprise perhaps five extant Orders and include the ostriches, emus, rheas, cassowaries, tinamous, and kiwis.

Examine the emu (Class Cassuariformes) skeleton on display. Note the skeletal differences from the "standard" carinate bird design.

Subclass Neognathidae - carinate birds

The carinates are (mostly) flying birds, and by far the more diverse and speciose extant group. Infraclass Galloanseres is a group of somewhat more primitive carinate birds which ncludes the fowl (e.g. chickens and pheasants - Order Galliformes) and waterfowl (e.g. ducks, gees, swans - Order Anseres). Infraclass Neoaves includes the rest of the carinate birds, divided into 15-20 orders. By far the most diverse and speciose is Order Passerformes, the perching birds or songbirds.

Observe the diversity of the carinate bird skeletons and skulls on display. How do these differ from the emu skeleton? The tiny finch skeleton is the only member of Order Passeriformes. Observe also the other avian artifacts, including feathers, eggs, and nests.

CLASS MAMMALIA (mammals)

The two most obvious distinguishing features of mammals are hair and mammary glands. Hairs are keratinized structures derived from scales, and provide an insulating coat for most mammals. Mammary glands are modified sweat glands which provide nourishment for infant offspring.

Additional shared, derived structures are:

1) In terrestrial mammals the limbs are positioned directly under the trunk and the

vertebral column is braced to hold both the trunk and the head up. This allows

very rapid and efficient locomotion, either by running or hopping.

2) Mammals have akinetic skulls, comparatively large craniums, and large, elaborate

brains.

3) Most mammals have heterodont teeth, that is, several kinds of teeth which serve

specialized functions. Most ammals feed by biting and chewing. Chewing is

uniquely mammalian, and is made possible by lateral mobility of the jaw joints,

as well as cheeks and lips which keep food in the mouth.

4) Mammals have an extended soft plalate which allows a tight oral seal for suckling.

5) Mammals are endothermic and most are fully homeothermic.

6) Mammals have true four-chambered hearts with separate systemic and pulmonary

circulatory loops.

7) Most mammals (with the exception of the monotremes) bear live young and

provide extended parental care.

The three major clades of mammals are the monotremes (Subclass Protheria - platypuses and spiny anteaters), marsupials (Subclass Metatheria - pouched animals such as kangaroos and opossums) and the placental mammals (Subclass Eutheria - all other mammals).

Subclass Protheria: Order Monotremata - the egg-laying mammals

Monotremata is the only surviving order of the protherians and retain several reptilian traits. The word "monotreme" refers to the common urogenital opening or cloaca. Monotremes lay leathery eggs, are incompletely homeothermic, have small and fairly reptilian brains, and have scaled appendages. Like the other mammals they nourish the young with milk from mammary glands, but they lack nipples. There are only five surviving species of monotreme, four species of echidna ("spiny anteater") and one species of platypus. All of the monotremes are native to Australia.

Examine the platypus skeleton and skull on display and compare it to both the skeletons of both lizards and placental mammals. Can you identify both mammalian and reptilian features?

Subclass Theria: Infraclass Metatheria - the marsupials

In marsupials the young are "born" at an embryonic level of development, migrate across the abdomen of the mother, enter a marsupium or pouch, and attach to a mammary nipple. Subsequent development takes place in this pouch, until the young become independent. This is actually a very efficient system, which allows a female marsupial to simultaneously have three litters of offspring - one "on foot", one in the pouch, and one in the uterus.

Fossil evidence indicates that the marsupials originated in the southern supercontinent of Gondwana and spread through what is now Africa and Antarctica to what is now greater Australia. They are remain native to South America (Superorder Ameridelphia - two orders) and Australia (Superorder Australidelphia - five orders).

Examine the skeleton of the Virginia opossum, a member of the largest Ameridelphian Order Didelphimorphia. This is the only extant marsupial species which has crossed the geologically recent Central American land bridge and become established in North America. Find the characteristic marsupial bones of the pelvis. Note the chevron bones of the tail, which apparently function to prevent compression of the caudal artery when the animal is suspended from its prehensile tail. Note also that the first digits of the hind feet are partially opposable.

Now examine and compare the skeleton of the Tammar wallaby. This small Australian kangaroo is a member of the largest Australidelphian Order Diprotodontia. Note the characteristic prepelvic marsupial bones and the long tail which stores fat and serves as a counterweight enabling complete saltation - locomotion by two-legged hopping using exclusively the large hind legs. Note also the curious pattern of the medial toes on the hind legs, in which a single fleshy toe contains two small skeletal digits (syndactyly).

Subclass Theria: Infraclass Eutheria - the placentals
Placental mammals are the most speciose, widely distributed, and varied subclass. They are characterized by the development of a placenta, which nourishes the developing fetus within the uterus and allows live birth of relatively developed offspring. We have multiple skull and/or skeletal examples of all four of the great eutherian biogeographic clades, which diverged as the supercontinent of Panagea split 170 mya.

Magnaorder Afrotheria - six orders

This is thought to be the oldest clade and originated in the southern continent (Gondwana). As the name suggests, most modern members are native to Africa. The best known afrotherians are the relatively large elephants (Order Proboscidia), manatees and dugongs (Order Sirenia), and aardvarks (Order Tubulidentata).

Examine the skulls of the sengis (elephant shrews) and the rock hyrax on display. These are members of two of the other three afrotherian orders of comparatively smaller mammals.

Magnaorder Xenarthra - two orders

This is also a southern continental clade, with all of its member groups originating in South America. The clade name refers to the several peculiar skeletal stuctures unique to this clade.

Examine and compare the skeletons of the nine-banded armadillo (Order Cingulata) and the Hoffman's two-toed sloth (Order Pilosa). Notice both the peculiar double articulation of the hip bones to the pelvis and the unique dentition of each.

Magnaorder Euarchontoglires - five orders

Euarchotoglires includes clade Glires (rodents and rabbits) and clade Euarchonta (primates and relatives). Even though many of its members are now tropical and/or native to southern continents, this clade originated in the northern part of Pangea (Laurasia). Orders Rodentia and Primates are two of the most successful, widespread, and speciose of the mammalian clades.

Examine and compare the specimens on display of all five Euarchontogliran orders, including the:

rodents (Order Rodentia),

rabbits (Order Lagomorpha),

tree shrews (Order Scandentia),

culagos or flying lemurs (Order Dermoptera), and

prosimians, monkeys, apes (Order Primates).

What are the distinguishing features of each of these five orders? To which order do you belong?

Magnaorder Laurasiatheria - six orders

Laurasiatheria is a close sister clade to Euarchontoglires and the two fall together in the larger clade Boreotheria (the "northern animals"). Orders Cetartiodactyla and Carnivora are the two most successful and diverse of the Laurasiatherian orders.

Examine and compare the specimens of all six Laurasiatherian orders, including the:

core insectivores - shrews, hedgehogs, and moles (Order Eulipotyphla),

even-toed ungulates - cattle, deer, antelopes, pigs, cameloids, giraffes,

toothed whales, baleen whales (Order Cetartiodactyla),

odd-toed ungulates - horses, tapirs, rhinoceruses (Order Perissodactyla),

bats (Order Chiroptera),

pangolins (Order Pholidota), and

carnivores - dogs, bears, weasels, skunks, raccoons, seals, sea lions, walruses, cats,

civets, mongooses (Order Carnivora).

What are the distinguishing features of each of these six orders? Which have the most distinct or unique members?

PART II. VERTEBRATE ANATOMY

Materials

Mounted and preserved specimens: pigeon, rats, cat nervous system, sheep heart,

sheep brains
Guides to anatomy and dissection

Dissecting trays and instruments

Procedure

A prepared "plastomount" of a dissected pigeon Columba livia will be used for studying basic bird internal anatomy. Your mammalian dissection will focus on the Norway rat Rattus norvegicus, supplemented by examinations of the sheep heart, pig uterus, sheep brain, and cat spinal cord.

1. For the plastomount pigeon and the cat nervous system fluid mount work through the guide below. Try to identify all of the external and internal structures in bold. For each structure, know the basic function and the organ system to which it belongs.

2. For the rat and sheep brain dissections, conduct a thorough examination of the external structures. Then follow the dissection guide below to expose and identify internal structures. Be sure to complete each numbered step in sequence. Again, try to identify all of the structures in bold. For each structure, know the basic function and the organ system to which it belongs.

3. In the pigeon and rat focus on the major structures/specializations associated with each of the following organ systems:

digestive respiratory circulatory excretory reproductive

integumentary neural/sensory skeletomuscular

Study Suggestions

1. Make detailed sketches and notes on specimens. This will help you to look at the specimens more closely, as well as to help you study later.

2. Plan to come in and work with the specimens once or twice more before the lab test. Test yourself by attempting to identify the specimens as accurately as possible by their common names, as well as to classify them without first looking at their labels.

3. The words in bold print in the extended guide below are words you should know and/or structures you should be able to identify or describe.

4. There are many useful on-line video guides for dissection and structure identification for most of these animals.

AVIAN ANATOMY

1. On the head of the plastomount pigeon find the maxillary (upper) and mandibular beak surrounding the mouth, the crown of the head, the eyes, and the ear coverts. Working down the body find the nape of the neck, throat, breast, belly, back, rump (pygidium) and cloaca.

2. On the pigeon skeleton locate these three uniquely avian structures: the furculum (wishbone - fused clavicles), carina or keel (modified sternum), and synsacrum (fused hip and sacral spine bones). How is each of these an adaptation for flight?

3. On the plastomount pigeon trace the respiratory system, from the external nares (nostrils), through the trachea (#3), to the parabronchi (lungs - unlabeled to either side caudal to the crop). Trace the digestive system from the mouth, through the esophagus (#4), to the crop (#5), gizzard (#10), small intestine (#14,15), caecum and rectum (#16), and cloaca (#18). Find the two abdominal extrinsic digestive glands, the liver (#9) and pancreas (#13). What is the function of each of these organs in its respective system?

4. Find the heart (#8) and the symmetrical aortic arches (#6) of the circulatory system and the kidneys (#11) of the excretory system. What is the function of each of these organs in its respective system?

MAMMALIAN ANATOMY

Skeletal System

1. Closely examine one of the rat skeletons. The axial skeleton consists of the skull, vertebral columns, ribs, and sternum. On the rat skull identify the cranio-facial and maxillary (upper jaw) regions and the mandible. The vertebral column has cervical (neck), thoracic (chest), lumbosacral (lower back), and caudal (tail) regions. Identify the paired ribs and the segmented sternum.

2. Working from proximal to distal on each anterior appendage distinguish the scapula, humerus, radius and ulna, carpals, metacarpals, and phalanges. On each posterior appendage distinguish the innominate (hip) bones, femur, tibiofibula, tarsals, metatarsals, and phalanges.

Initial Rat Dissection

First a few notes on dissecting procedures. Put on a pair of gloves and keep them on. Find your scalpel. Put it somewhere safely out of reach and don't use it for rat dissection unless specifically instructed to do so. Whenever possible, use "blunt dissection" to separate tissues by gently tearing along natural lines. Your gloved fingers and your blunt probe are the best tools for this. Make the minimal required incisions using scissors only. The figures at the end of this lab will help you with the dissection and identification of structures.

1. To begin the dissection, place the rat ventral side up in the dissecting pan. Use sturdy string to attach each forelimb to a corner post of the dissecting pan, pulling the strings tight enough to spread out the legs.

2. Lift a small flap of skin along the midline in the lower abdominal region. Insert one blade of your scissors and cut the skin anteriorly up to the chin. Make sure that you lift the skin as you cut and keep the scissors blades tilted slightly up to avoid cutting the underlying muscle layers. Now make similar ventral skin incisions from the anterior end of the incision laterally to the base of the ear, from the mid-thoracic region laterally to each elbow, from the lower abdominal region laterally to each knee.

3. Use your fingers to separate the skin flaps from the underlying muscle by tearing through the web-like, connective tissue hypodermis and fascia. Pin these skin flaps out. DO NOT completely detach these skin flaps from the body.

Body Cavities, Respiratory, Digestive, and Excretory Systems

1. Head and Neck

a. Locate the external nares leading in from the nasal vestibule to the nasal cavity on each side of the nasal septum. If you wish, you may use your scalpel to very CAREFULLY make a midline incision into the roof of your rat's mouth to expose these structures and the nasopharynx behind them.

b. Identify the large parotid salivary glands on each side of the head between the external ear and the angle of the jaw.

c. Using large scissors, cut through the angle of each side of the jaw. Open the mouth and look inside into the oral cavity. In back of the oral cavity lies the oropharynx, a region shared between the digestive and respiratory systems. Identify the glottis (the opening of the larynx) and the epiglottis, a small elastic flap of tissue.

d. In the upper neck region identify the paired submaxillary and sublingual salivary glands at the angle of the jaw. The hyoid bone lies between the submaxillary glands and provides an anchoring point for medial neck muscles and the base of the tongue. Use your scissors, fingers, and blunt probe separate these medial neck muscles along the midline below the hyoid to expose the larynx and trachea. You can recognize the trachea by its "corrugated" appearance. Pull the trachea to one side to see the smooth esophagus lying behind (dorsal to) it. Be VERY CAREFUL not to cut through the blood vessels which pass over the lower trachea as they exit the heart.

2. Thoracic Cavity

a. To open the thoracic cavity, insert the one tip of your scissors under the tip of the sternum or breastbone and cut anteriorly up to the base of the neck. Again, try to keep your scissors angled upwards to avoid damaging underlying organs. This is especially important in the upper chest, where you will be cutting over the great vessels of the heart.

b. Find the dome-shaped diaphragm, which separates the thoracic and abdominal cavities. How does contraction of this muscle increase the size of the thoracic cavity during respiratory inspiration? How do the other chest muscles cooperate in respiration?

c. Carefully cut laterally in each direction around the margin of the diaphragm to separate it from the lower ribs. Now spread the ribs by breaking them with your fingers along each side of the animal. Pin the rib cage open. If you cannot get a satisfactory exposure of the thoracic cavity it is OK to cut through the ribs at each lateral margin and remove them entirely. However, DO NOT DISPOSE OF THE RIB SECTIONS.

d. The midline region of the thoracic cavity is called the mediastinum and is dominated by the heart. The heart lies within a compartment of the coelom called the pericardial cavity. You will examine the heart further in the next section. For now, try to avoid cutting or tearing the great vessels carrying blood to and from the heart.

e. The lungs lie on either side of a central region of the thorax called the mediastinum. Each lung is enclosed within its own compartment of the coelom (true body cavity), called the pleural cavity. The pleural cavities are "virtual spaces" which are fluid-filled and occupy very little volume in the living animal. Why is it essential that air be kept out of these spaces in a live animal; i.e. what happens to the lung if air is allowed to enter this space?

f. Follow the trachea down behind the heart to the point where it divides into two primary bronchi, one of which goes to each lung. Try to follow each bronchus into its lung, noticing that it divides into several secondary bronchi as it does so. Each secondary bronchus services one lobe of the lung. How many lung lobes does each rat lung have?

g. Follow the esophagus down behind the heart to teh point at which it passed through the diaphragm.

h. Insert one of the plastic pipettes into the glottis of your rat and attempt to inflate the lungs.

3. Abdominal Cavity

a. Using scissors, make a midline incision through all of the muscle layers of the abdomen, starting from the level of the diaphragm at the tip of the sternum and proceeding down to the lower extent of your original skin incision. Again, use forceps to lift the muscles as you cut and keep your scissors blades angled up to avoid cutting the underlying viscera. Cut laterally through the abdominal musculature on both sides, following the lowest rib and the edge of the diaphragm. Make a second set of cuts though the muscles at the posterior region of the abdomen. Pin out these muscle flaps. Again, DO NOT completely detach these muscle flaps from the body.

b. You should now be looking into the abdominal peritoneal cavity, the abdominal compartment of the coelom. This is also a "virtual space", meaning that in the living animal it is collapsed into a very small volume, filled with a slippery, serous fluid. The shiny surface of the inner body walls is the parietal peritoneum and most internal organs of the abdomen are similarly covered with visceral peritoneum.

c. The stomach is a J-shaped bag in the upper-left quadrant of the abdomen. At the anterior end it is joined to the esophagus, which has passed though the diaphragm. Find this esophageal or cardiac junction. At the lower end of the stomach is the pyloric sphincter, a muscular valve which controls the passage of partially digested food (chyme) from the stomach to the small intestine or enteron. Loops of the small intestine are suspended from the posterior body wall by doubled layers of peritoneum called mesenteries. Note that the blood supply (and nerves) to the small intestine travel through the mesenteries.

d. Two large digestive glands develop as complex outpocketings of the duodenum and continue to empty their secretory products into the duodenum. The liver is a large, brownish, lobed organ in the upper-right quadrant of the abdomen. What are the digestive functions of the liver? How many lobes can you count in the rat liver? The pancreas is a more diffuse, spongy, yellowish organ extending across the abdomen below and behind the stomach. What are the roles of pancreatic secretions in digestion? Try to find the place where the common bile duct and pancreatic duct connect these two secretory organs into the first region of the small intestine, the duodenum.

e. Locate the elongated spleen opposite the liver in the upper left quadrant of the abdomen. The spleen is a major organ of the immune system.

f. Make a short longitudinal cut into the small intestine, fold open the cut and look for the circular infolded rings called plicae circularis, as well as the smaller finger-like villi projecting into the intestinal lumen. What is the function of these infoldings?

g. Use your fingers to �walk� the length of the small intestine to the ileocaecal junction where it joins with the large intestine or colon. The large bag-shaped structure at this location is the caecum. What is its function in the rat? Cut open the caecum and examine its contents.

h. Continue to trace the colon from the ileo-caecal junction to the rectum. What are the main functions of the colon and rectum?

Circulatory System

In the tetrapods, blood generally flows in a double-circuit circulation. In such a system blood flows from the heart to the lungs (and/or skin), then back to the heart, then to the tissues, then back to the heart again. The first loop which oxygenates blood is called the pulmonary or pulmocutaneous circuit, while the second loop which carries this oxygenated blood to the tissues is called the systemic circuit.

1. Sheep Heart

Examine the freeze-dried sheep heart. Using the figures and the model of the human heart as guides, trace the flow of blood through the chambers with their associated valves and the major vessels entering and exiting the heart: Deoxygenated blood enters the right atrium via the precava and postcava, then passes through the tricuspid valve, to the right ventricle, through the pulmonary semilunar valve to the pulmonary arteries, and on to the lungs. Oxygenated blood reenters the heart at the left atrium via pulmonary veins, passes through the bicuspid valve into the left ventricle, is pumped through the aortic semilunar valve and into the aorta, and the systemic circuit. This systemic circuit distributes blood to the body.

2. Rat Circulatory System

a. In the thoracic cavity find the pericardium, a membranous sac, enclosing the heart. As with the other body cavity regions, the pericardium has parietal and visceral layers. Try to locate the four chambers of the rat heart.

b. In your rat the systemic arteries are filled with red latex, however their thick walls may obscure this color. Using the accompanying figures as a guide, identify the following arterial structures:

The aortic arch lies on top of the heart and carries blood out of the left ventricle. Its major branches are two medial carotid arteries, which supply blood to the head and two lateral subclavian arteries, which supply blood to the forelimbs. The pulmonary trunk exits the right ventricle and passes behind the aortic arch, where it branches into right and left pulmonary arteries, which carry blood to the lungs.

The thoracic aorta turns posteriorly and runs dorsal to heart and through the diaphragm. Lift the heart to locate this vessel. As it enters the abdominal cavity it becomes the abdominal aorta, which runs medially along the dorsal wall of the abdomen.

The first major branch of the abdominal aorta is the coeliac artery, which carries blood to the liver, spleen, stomach and pancreas. The superior mesenteric artery carries blood to the small intestine via the mesenteries. The paired renal arteries carry blood to the kidneys.

In the posterior abdomen the aorta branches into three main arteries: the inferior mesenteric which carries blood to the large intestine and connects in a loop with the superior mesenteric artery, and the two common iliac arteries which carry blood to the hindlimbs.

c. Systemic veins are filled with blue latex and should appear blue in your rat. Using the accompanying figures as a guide, identify the following venous structures:

In the upper thorax blood returns from the head in the jugular veins, while blood returns from the forelimbs in the subclavian veins. On each side of the back of the heart these join to form the right and left precava (or right and left superior vena cava) which empty into the right atrium of the heart. The thoracic postcava (or inferior vena cava) carries venous return from the lower part of the body, passes through the diaphragm, and also empties into the right atrium. You may have to lift the right lung to find this vessel. The pulmonary veins carrying blood from the lungs to the left atrium are very short and you will probably not be able to identify them.

In the posterior abdomen the paired common iliac veins return blood from the hindlimbs and join to form the abdominal postcava. The other major abdominal tributaries of the postcava are the paired renal veins, which return blood from the kidneys, and the hepatic vein, which drains the liver. The hepatic vein is located on the back or dorsal aspect of the liver and is generally too short to see.

d. The hepatic portal system is a special set of veins, which drain the intestines, spleen, pancreas, and stomach into the liver. These should be filled with yellow latex in your rat. Locate branches of the mesenteric veins in the mesenteries and follow them to the large hepatic portal vein, which enters the liver. Can you find the tributary veins?

Excretory and Reproductive Systems

1. Excretory System

a. Locate the paired bean-shaped kidneys of the excretory (renal or urinary) system. The kidneys lie against the lateral posterior wall in the upper abdominal cavity on each side of the abdomen. Each kidney in nestled in fat and actually lies behind and outside of the peritoneal cavity.

b. From each kidney trace the threadlike ureter down to the midline urinary bladder, lying over (ventral to) the rectum.

c. The urinary bladder exits the body via the urethra. Trace the course of the urethra as it exits the urinary bladder and passes through the body wall. In the female the urethra is very short and opens into the peripheral part of the vagina. In the male the urethra travels through the penis and exits at the glans.

2. Female Reproductive System

a. On a female rat locate the opening of the vagina, just ventral to the anus (which, in turn is just ventral to the tail). Carefully extend the midline abdominal incisions through the skin and muscle to the vaginal opening. The large uterus is y-shaped, with a body, which lies dorsal to the urinary bladder and two large uterine horns, which extend anteriorly, almost to the level of the kidneys. At the tip of each uterine horn you should be able to trace the small, coiled oviduct (Fallopian tube), to the small, globular ovary. The entire female reproductive tract on each side is suspended in a fold of the peritoneum called the mesometrium.

b. If you are having difficulty identifying the relatively tiny reproductive structures of the female rat, try observing the display specimen of an isolated pregnant female pig reproductive tract. Like most mammals (including rats), pigs are multiparous, meaning they gestate several offspring at a time. The two large uterine horns make up most of this specimen. Trace the two uterine horns to where they meet in a small uterine body. Below this is the much thicker-walled and more muscular vagina. Now trace each uterine horn out to its distal end. Along the way you will encounter several thickened regions. Some of these have been opened to reveal the miniature pig embryos within. At the distal end of each uterine horn is the much smaller, tightly-coiled oviduct, or Fallopian tube. Trace the oviduct to its distal end and locate the lumpy ovary, covered by the membranous mesovarium, an infolding of the abdominal peritoneum.

3. Male Reproductive System

a. This is a much more difficult dissection. On a male rat start by carefully extending your midline abdominal incision posteriorly, along one side of the penis, down the central seam (raphe) of the scrotum, to the opening of the anus. Cut through the scrotal sac on one side to expose the testis. The confusingly-named tunica vaginalis is a remnant of the peritoneum which surrounds the testis. The epididymis is an extensively coiled tube on the dorsal surface of the testis.

b. The vas deferens leaves the scrotum as part of the spermatic cord, and enters the abdominal cavity by way of the inguinal canal. Carefully cut through the inguinal canal on one side, following the course of the spermatic cord. In the abdominal cavity the vas deferens passes through the large prostate gland ventral to the urinary bladder, where it empties into the urethra. Find the large seminal vesicles and coagulating glands, which also empty into the prostatic urethra. What is the function of each of these structures in the production of sperm and semen?

Find the pubic symphysis which forms the ventral midline bridge of the bony pelvic girdle. The urethra runs posteriorly dorsal to the pubic symphysis, then turns ventrally and anteriorly, where it enters the base of the penis. Carefully cut through the pubic symphysis, then trace the urethra through this path.

Nervous System

1. Sheep Brain

Obtain a sheep brain and place in a dissecting pan. Using a kitchen knife carefully cut it in half down the midline (midsagittal section). We will work as a class to locate the following brain regions and structures:

a. Meninges -- the triple layer of tough connective tissue which surrounds the brain and covers the spinal cord. (These may have already been removed from your sheep brain.)

b. Cerebrum (forebrain) -- the largest and most developed portion of the mammalian brain. It serves as the site of coordination of sensory stimuli, control of learned behavior, and muscular activity. Identify the frontal, parietal, temporal, and occipital lobes of the cerebrum.

c. Hypothalamus (forebrain) -- the "head ganglion of the autonomic nervous system". This ventral midline region is a visceral motor control center. The stalk of the pituitary gland extends from its ventral surface just caudal to the optic chiasm.

d. Thalamus (forebrain) -- a dorsal relay site for sensory nerve fibers found at the base of the forebrain.

e. Superior and Inferior Colliculi (midbrain) � these are dorsal relay centers for the visual and auditory systems, respectively

f. Cerebellum (hindbrain) -- the center of equilibrium, posture, and movement. It is a relatively large dorsal structure found below and posterior to the occipital lobe of the cerebral cortex.

g. Pons (hindbrain) -- located ventrally between the medulla oblongata and the midbrain. The pons is made up of thick bundles of fibers that carry impulses from one side of the cerebellum to the other.

h. Medulla Oblongata (hindbrain) -- the conical posterior continuation of the brain stem. It controls numerous involuntary activities such as heart rate, respiration, vasomotor tone, and swallowing.

2. Cat Nervous System

Note: this is an extremely delicate prep - please do NOT handle it or pick it up.

a. Locate the cerebral hemispheres, cerebellum, and medulla oblongata at the rostral end of the nervous system. Within which region of the brain is each of these located?

b. Observe the spinal cord and paired spinal nerves, composed of both sensory and motor axons. Notice the small dorsal root ganglion at the base of each spinal nerve. These contain the cell bodies of all of the sensory neurons carrying information into the spinal cord. Note particularly the vagus nerve (10th cranial nerve), which exits the skull and travels caudally to the larynx, heart, and digestive organs. What are its functions? Find the cauda equina at the extreme caudal end of the spinal cord.