Vertebrate Anatomy - Mammals - White Rat.

Part One.




Secure your specimen ventral side up, with the legs spread laterally with thread or pins (thread under the pan works better, usually). Refer to one of your diagram handouts for the dissection pattern. Beginning at the opened skin of the throat, and cutting very shallowly (scissors work better than scalpels for this), make a medial ventral incision the length of the body, then lateral incisions at the collar bones, the posterior margin of the rib cage, and at the hips, cutting deep enough to reach the body cavities but not so deep as to damage the organs inside. Free the resulting flaps of tissue, gently disconnecting it from underlying tissue with a blunt probe or careful cutting. You can pin the flaps aside or completely remove them.

Digestive System.

Look in the abdominal cavity. The abdominal organs may still be covered with a membrane, the peritoneum, but this usually comes off with the overlying layers. If necessary, wash the organs under the faucet and drain the body cavity. Another membrane, the mesentery, surrounds and supports most of the digestive system and its related circulation, and in human males is a primary storage site for fat, causing "beer bellies" in some men. The primary fat storage site for most human females is usually in the sides and backs of the hips.

1. Why would the mesentery be used for primary fat storage in men but not women?

BONUS: Why might the use of the hips for such storage in women be a deceptive secondary sexual characteristic?

Much of the anterior abdomen is occupied, especially on the right side, by the liver. Inspect it. The liver does many more things than making bile. It collects almost all of the blood circulated through the intestines and processes many of the chemicals picked up there (such as alcohol, which your liver detoxifies). Waste products from protein metabolism are processed into the less-toxic form of urea, which will be removed in the kidneys. Old red blood cells are broken down, with important things like the iron recycled; the liver is also a major staging site for white blood cells of the immune system. Temporary storage of sugar occurs in the liver: in response to insulin hormone from the pancreas, sugar in the blood is absorbed and stored as the simple starch glycogen; another pancreas hormone, glucagon, causes conversion of glycogen back to sugar and its release into the blood as needed. That's a partial list of liver functions. Tucked into a recess underneath in most vertebrates (but not, apparently, rats) is the gall bladder, where a secretion called bile, also produced in the liver, is stored. Bile is a salty fluid (if too concentrated, crystals can form - gallstones) used in the small intestine to emulsify fat, physically making tiny, digestible blobs out of big, separated-out globs of fatty materials (dish detergents do the same thing to grease). There are several ducts in the region - a duct connects the liver to the small intestine, with a branch to the gall bladder and a branch to the pancreas.

2. Why would the liver be the largest internal organ?

3. Count the lobes (sections) of the liver. Don't count the long, narrow spleen, which is lateral to the stomach on the left side. How many lobes are there?

4. The liver does not divide jobs up among the lobes (if it did, each lobe would be a separate organ). Why, then, divide the mass of the liver up into sections? (The organ underneath the liver on the left side might supply a clue...)

5. Having your gall bladder removed does not have a serious effect upon your digestive abilities, even though fat emulsification is very important. Why not?

Remove the liver. Take the spleen off the stomach. The spleen is not a digestive organ, but rather a major storage site for oxygen-carrying red blood cells, and immune-system white blood cells, as well as a processor. In areas of the world with many disease organisms, the stress on the body's immune system can cause both spleen and liver to expand, giving a "pot belly" appearance that is sometimes wrongly attributed to malnutrition. The spleen is covered by a thin membrane, ruptures easily and is difficult to repair, but you can have it removed and live.

6. People whose spleens have been removed are often told to avoid traveling to Third-World countries. Why?

Find the stomach. This organ has an incoming tube, the esophagus, anterior and an outgoing tube, the small intestine, posterior and medial. Remove the stomach with about 1 cm. of those tubes attached. The stomach serves two major functions: it churns up the balls of food sent down the esophagus from the mouth (we chew the food up but stick it together into a ball - a bolus - to swallow it), reseparating the pieces, and it begins the chemical breakdown of the food with strong acids and with a powerful protein-digesting enzyme. The stomach (and much of the intestines) is lined with a mucus layer to protect the linings from the digestive secretions; in places where the mucus thins, self-digestion can happen, creating ulcers.

7. Cut open the stomach, but don't cut near the two tubes. Describe the lining.

8. Fold the stomach back and look at the structures on the inside where the tubes connect. What sort of structures are they?

    What purposes would these structures serve? (Each serves a slightly different purpose, so this is a two-part answer!)

Most of what's left in the abdominal cavity is the intestines, small and large. The intestines are held together by the mesentery membrane, which also supports the pancreas. As mentioned before, the pancreas controls blood sugar levels with hormone commands mostly to the liver, but it also makes many digestive chemicals. Enzymes, specifically targeted at particular molecules, are made in the pancreas, as is sodium bicarbonate (the same chemical in baking soda), which neutralizes the acids in the "soup" that comes out of the stomach. Most large molecules are broken down to absorbable size as they travel along the small intestine; an exception is cellulose, "fiber," which is undigestible to most mammals except those few with bacterial "buddies" to do the job for them. Separate out the small intestine, clipping gently through the mesentery holding it together, until you have separated out a long single tube (you're going to measure them for question 10). At a certain, clearly recognizable point, the small intestine joins the large intestine, or colon, at the cecum. In the colon, water is drawn out of the food, and a few vitamins and minerals are processed by specialized bacteria that live there. The cecum is a sac for fermentation of fibrous plant materials, also processed by symbiotic bacteria.

9. What clearly marks where the small intestine joins the large intestine (a similar set-up can be found in all mammals)?

10. How long is the small intestine?

        How long is the large intestine?


Urogenital System.

With the intestines removed, the cavity within the peritoneal membrane is pretty much empty; however, beyond the peritoneal membrane dorsally are the two kidneys. Medial on the dorsal body wall are the two main posterior blood vessels: the abdominal (or dorsal) aorta, a artery, and the inferior vena cava, a vein; both send branches, renal vessels, to the kidneys. Expose the kidneys and the tubes connected to them, which will include ureters, which connect to the bladder posteriorly. On the kidneys' anterior edge, or slightly separate from them but often difficult to see are the adrenal glands, which produce a wide variety of hormones (including adrenaline). In the kidneys, blood passes through physical filters that remove all molecules below a certain size (proteins are too big to be lost); then, in a series of tubes, small molecules the body needs back (sugar, water, some salts, etc...) are reabsorbed by the blood and larger molecules the body does not need are pumped out - the resulting, waste-filled fluid is urine, which leaves through the ureters and is stored in the urinary bladder. From there, urine leaves the body through a single urethra.

11. Remove one kidney and cut it longitudinally to get a dorsal and ventral half.  Draw one side here, labeling the thin outside capsule; the underlying cortex, where the filters are; the inner medulla, full of collecting tubules;  and possibly the central cavity, where urine collects, if it's visible.




Make sure to find another specimen of the opposite sex to yours - you'll need it for comparison purposes.

In females, the ovaries, which produce egg cells and female hormones, are small and somewhat peanut-shaped, located just posterior to the kidneys, inside the peritoneal membrane. These organs produce egg cells (ova) in the middle of blister-like follicles, and female hormones in the follicle lining cells. To find an intact ovary, you may need to look at the side opposite to where you removed the kidney. The oviducts lead from the ovaries to the medial, Y-shaped uterus, which connects by way of the vagina to the urogenital opening.

In males, testes, which produce sperm and male hormones, start out up inside the body cavity during fetal development, then migrate out through two canals into the scrotum, passing through two slits in the abdominal wall. Attached to each testis is an epididymis, which stores sperm and leads to the sperm duct. The duct, and some semen glands, attach to the urethra, which carries sperm out of the body through the penis.

12. Describe the following for the male and female:

Location. Glands. Associated Tubes.




13. Remove a gonad from your specimen and dissect it. Describe or draw its internal structure.

Circle: M or F:

Part Two. Thoracic Cavity.

The organs of the chest, or thoracic cavity, are also covered by various membranes: the lungs on each side are encased in pleural membranes, while the heart, located in the middle, is covered by the pericardium. These lubricated surfaces allow the two sets of organs to move vigorously without wearing each other away. The posterior wall of the cavity is the diaphragm (it may have been damaged during dissection). When its muscle contracts, the dome-shape flattens and moves downward, increasing the space of the chest cavity; the lungs will inflate with air to fill that extra space. Ventral and somewhat anterior to the heart is the thymus gland. In this organ, some white blood cells from the bone marrow mature, becoming T Cells (it is one type of T Cell that is attacked by the AIDS virus). Activity in the thymus gland begins during the first few months of life, peaks around puberty, then drops off steadily; our increased susceptibility to disease in old age is probably due at least partly to the shrinkage of our thymus glands.

1. Count the lobes of the lungs, comparing right lung to left. Which side seems to have a clearly extra lobe, and where is it?

BONUS. What feature of internal anatomy leads to the development of that extra lobe?

Remove the tissue around the heart and carefully take out the heart itself with the main attached vessels. A mammal's (birds, too) heart contains four chambers, two each on the right and left sides: 2 each of the upper chamber, the atrium, and 2 ventricles. The right atrium receives blood from the vena cavas, the main veins bringing blood in from the body, where cells have removed oxygen and added carbon dioxide, among other things. The blood is nutrient-rich, however, since much of it has come in from the digestive system, by way of the liver. Blood is pushed from the right atrium down into the right ventricle and from there out to the lungs for carbon dioxide removal and oxygen charging; it returns to the left atrium, is pushed into the left ventricle, the pumped out the aorta to the body circulation. Note the network of blood vessels across the surface of the heart - these branch off the aorta to feed the heart muscle  (the blood flowing through the chambers does not feed the heart). If these small vessels clog up, the parts of the heart they would ordinarily be feeding seize up, causing a heart attack.

2. The heart has huge nutrient and oxygen requirements. Explain how the circulation system is organized to make sure that the heart gets all that it needs.



3. Ectothermic / poikilothermic animals, such as frogs or snakes, have a single ventricle that receives both right- and left-side blood. What important purpose is served by dividing that chamber into right and left ventricles, as endothermic / homeothermic mammals and birds do? Don't be too vague.

4. Cut through the heart, producing a front and back half. Draw and label, indicating what chambers are visible.


5. The left ventricle is thicker and stronger than the right ventricle. Why?

Behind the heart is a stiff tube, whitish with rings around it, which splits into tubes that run to the lobes of the lungs. The main tube is the trachea, and the branches are bronchi. The rings around the tube are cartilage, and reinforce the tubes in a way similar to the way that vacuum cleaner hoses are reinforced.

6. Explain why these tubes require the reinforcing rings of cartilage - what are they reinforced against?

7. Dorsal to the trachea is the esophagus, the tube that brings food from the throat to the stomach. How is its appearance different from the trachea's?

Working up the trachea to the larynx (voice box) in the throat area, the thyroid gland should be a small structure on the surface . This organ produces hormones that control metabolic rates, the general level of chemical activity in the cells. On each side, somewhat anteriorly, of the larynx are salivary glands (there are others up by the tongue and cheeks). Saliva from these glands sticks chewed-up food into a slick, swallowable ball. Saliva also contains an amylase, an enzyme that can break down starches to sugars.

10. Why doesn�t saliva contain enzymes to break down other food molecules, such as proteins or lipids?




Part Three. The Head.

The top, somewhat rounded part of the head is the cranium; it protects the brain. Carefully remove the skin and underlying tissue from this area, exposing the skull. You need to remove the top of the skull - the best way is to use a scissor tip to get just inside, then snip along the seam where the skull bone knit together. Be careful not to dig into the brain beneath. Don't try to take the entire bone off at once - cut it into small pieces as you go. The part of the brain that you're exposing is the cerebrum, the main information processor. You may see some thin covering membranes: these meninges surround the brain and spinal cord.

The larger anterior, upper brain, the cerebrum, is the area that processes "higher" thinking, including complex memory analysis and decision-making. The cerebellum, posterior and slightly underneath, coordinates muscles and controls balance. Posterior still is the medulla, which runs such "automatic" functions as heartbeat, breathing, and digestion. Very carefully remove the brain, noting that major cranial nerves are connected underneath in addition to the spinal cord that runs out the posterior of the skull and down runs down a canal in the vertebral column.

1. Describe the appearance of the cerebrum.

2. Describe the appearance of the cerebellum.

3. If blood flow is cut off to a part of the brain, a stroke can be the result. If permanent damage occurs, all manner of functions can be compromised, from basic control to coordination, memory, or even specific abilities like language or place recognition. Stroke victims with unusual impairments afterward have contributed to our mapping of the brain's functional layout - we now know where many processes are done in the brain. How would this be done?





First Written 1990;  Web Version 2001;  Last Update 2014,  M. McDarby




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