Exercise 8 - Vertebrate Chordates - Mammalia

Fetal Pig:

Because of time limitations, we will emphasize the major features of the internal organs and circulation. The fetal pig offers a good example of a fairly generalized mammal, with structures very similar to that of higher primates. It is also large enough that dissection is not difficult and the circulatory system has been injected, with red and blue latex to facilitate identification of blood vessels.

Fetal pigs are obtained from commercial packing houses, injected and embalmed by biological supply houses from whom we purchase them. The venous system is injected (with blue latex) through the external jugular vein in the neck region, which accounts for the large gash in the neck on one side of your specimen. The arterial system has been injected with red latex. Wash your pig thoroughly and look over the external features. The umbilical vein contains 2 umbilical arteries (which should appear red), a single umbilical vein (blue), and a sac-like allantoic stalk which extends a short distance into the umbilical cord. Sex differences may at first be confusing. What may appear as a rudimentary penis at the base of the tail near the anus is actually a genital papilla in the female. In pigs, as in many other mammals (but not in humans), the penis is concealed in an abdominal sheath, the opening of which is a small hole just posterior to the umbilical cord. We will dissect out the male and female reproductive systems during the next lab.

we will be looking at internal organs only, not mussels, so we will not remove the skin from the pig's body. In dissecting your pig, remember that the goal is to expose as much as possible while cutting as little as possible . About 90% of dissection is simply tearing membranes and connective tissues. In particular, try not to cut any blood vessels! Only three need to be cut, and these will be specifically mentioned! Be patient in your dissection and do a good, clean job. The diagram on the blackboard will show the primary lines of cut to open the body and to expose subsurface structures.

We will first expose the structures in the throat area by dissection out a flap of skin covering the area. With a razor blade or scalpel, cut through the skin only starting at the upper end of the neck about an inch below the ear on one side. The skin has a layer of fat attached and is altogether about 1/8 to 1/4 in thick. Cut forward (anterioriorly) from this point along the ventral surface of the jaw until you are just anterior to the papilla which bears the whisker-like vibrissae, then cut straight across the chin region to the other jaw bone. Now extend the cut back (posteriorly) along the jaw back to the anterior end of the gash where the venous system was injected. At the posterior end of this gash, continue the cut posteriorly to just above the breast bone. to expose the throat region, take a corner of the skin and peel it away by using a pin or probe to tear the underlying membranes exposing the throat area. You will know you are deep enough, when you see some white glands and bands of muscle running down the center of the throat.

To open up the body region, it is best to start at the umbilical chord and cut forward. to begin, pull up gently on the umbilical chord and make a cut (using scissors) into the body cavity just anterior to the base of the chord. When you have a small hole that you can see into, insert the bottom edge of the scissors and cut forward. You will notice some resistance as you intersect the diaphragm, which separates the chest and abdominal cavities. Later, you will need to separate this muscle from the walls of the rib cage. In the meantime, continue cutting forward through the sternum, to meet the cut you made in the throat area.

To expose the body and chest cavities, you will need to cut dorsally along the bottom edges of the ribs on both sides (towards the pig's back) and separating the muscular diaphragm from the rib cage. do not remove the diaphragm, as you will need to be able to find it later.

Returning to the upper chest region, peel the skin away from the rest of the throat region and from over the chest. This will expose the transverse pectoral muscle which originates on the sternum and inserts on the humerus. In contracting, it adducts the arms inward toward the median line (hugging motion). You can feel the front end of the sternum. Running anteriorly you will see a band of muscle from the sternum to the neck region. Blot dry and locate the absolute center line, then separate the muscle along this line with a pin (not a razor) until you come to a small hard object embedded between the muscles up under the chin. This is the hyoid apparatus. What you are working on is the sternohyoid muscle which helps to position and move the larynx. The two halves should now be spread apart and you will find near the front end of the opening, a central hard object made of cartilage. This the larynx which lies just posterior to the hyoid bone. On either side of the larynx lies a large, soft, creamy-colored gland, the thymus gland. Carefully separate these and you will find two bands of small muscles running from the sternum to the back side of the larynx. Separate these carefully and you will locate a small reddish-brown gland about the size of a pea. This is the thyroid gland. Push it to one side and you will find a white tube, the trachea, running from the larynx. The trachea is supported internally by cartilaginous C- shaped rings and thus always exists as an open tube which connects the larynx to the lungs. Look carefully and you can see the rings with your naked eye.

Now go back to the cut by the umbilical cord. Start the cut at the anterior end of the umbilical cord and cut around toward the posterior side of the cord but don't completely encircle the cord. Remember that in the male, the penis lies just underneath the skin posterior to the umbilical cord. If you cut completely around the cord, you will cut through the penis. Your finished cut should be "U" shaped with the open end of the U pointing posteriorly. At the end of the arms of the "U" continue to cut laterally and dorsally to open up the body cavity. Opening the peritoneum will disclose a lot of fluid, possibly brown in color. This is a cocktail of preservative, blood and other body fluids and should be washed out at the sink. Rinse the entire body cavity in running water.

Note that the umbilical cord has a large umbilical vein which runs from the cord into the liver. Cut the umbilical vein (this is the first blood vessel to cut!) half way between the liver and the point where the vein enters the umbilical cord. Now reflex the ventral abdominal wall back between the legs. By now the entire body cavity should be pretty well opened up. The big, brownish organ in front of the umbilical cord is liver, and it has three lobes. The gall bladder is behind and embedded in one of the lobes as a greenish round object (it may resemble a peeled grape). The bile duct will be a thin white tube running from the gall bladder down to the duodenum (the upper portion of the small intestine) The stomach is the large sac looping off to the pig's left (your right), and the spleen is a large, reddish flat object lying against the stomach on the pig's left side. By lifting up the stomach you can find the pancreas, a rather long, light colored, grainy textured object extending across the body behind (dorsal to) the stomach.. Carefully pick apart the mesenteries between the intestines and the larger, outside curvature of the stomach and also the peritoneum to better see the pancreas. The intestines form a convoluted mass and must be pushed off to one side (the pig's right). Notice that they are held snugly together, and to the body wall itself, by mesenteries which contain lots of blood vessels. You will have to use a pin and tear a lot of mesentery to get things cleared out. The small intestine tends to be pinkish while the large intestine (colon) is usually more greenish, there is no real difference in diameter at this stage; however, the small intestine has obvious blood vessels in the mesenteries between the loose loops of intestine while the large intestine is wrapped by mesenteries and hence appears as one mass. Part of the large intestine runs directly across the abdomen just below the stomach and may conceal or be attached to the pancreas. Again a pin will aid you in tearing the tissues away to show this, but this tissue is the peritoneum and not a mesentery. Try at all costs to avoid tearing into the intestine because if you do a lot of stuff (meconium) will spill out and have to be rinsed away (these by the way are not feces since the fetus has not been eating anything. These are mainly bile salts from the liver, desquamated intestinal cells and mucus). Follow the colon along and separate it from the body wall so it can be pushed to the left (your right) with the other intestines. Now see if you can find the junction of the large and small intestines. Here there is a short caecum which extends as a blind-ended sac about a centimeter long. The easiest way to locate the caecum is to push all of the pig's small intestines to your left, lifting them as you do so. Look for a small blin sac (appendix-like) where the small and large intestines meet. In humans, the vermiform appendix is attached to a shortened version of the caecum. Pigs lack an appendix. Now go back to the anterior side of the liver and find the diaphragm, the large dome-shaped muscle which divides the abdominal and thoracic cavities. You may remove as much of the diaphragm as is necessary to see the internal organs but leave a piece to identify when you take your quiz.. Note that a big vein (posterior vena cava) goes through the diaphragm, toward the heart, and also the esophagus, a soft, collapsible tube extending from the front end of the stomach to the mouth.

Return to the heart and remove the tough membrane (the pericardium) which surrounds it. It may have latex on it and perhaps have a little of the thymus gland on it too. Remove all of these structures, leaving the heart exposed. Do not remove the heart. On each side of the heart is a large soft lung. Remove the pig's left lung, cutting as close to the dorsal attachment and large vessels (these are the third vessels to cut!) as you can. Cut all of the left lung tissue and pleura clear down to the diaphragm and pull out the left lung. With your scalpel, make a cut through the center of the removed lung. Look at the lung with your dissecting microscope. Can you see different sized air tubes? Find the pulmonary artery and pulmonary vein. Which way does blood flow in these vessels (toward the lungs or away from the lungs)?__________________

Directly on the heart, near the top, are two dark, pleated lobes: the auricles (the chamber inside each auricle is called the atrium). Between the two auricles is a large whitish vessel angling off to the pig's left behind the left auricle. This is the pulmonary trunk. It arises from the right ventricle, crosses behind the left auricle and joins the large whitish dorsal aorta. Clear the remains of the pericardium away from the junction of the pulmonary trunk and the aorta and find this location. In the fetal pig this junction is large and obviously carries a lot of blood from the right ventricle directly into the aorta thus bypassing the fetal lungs. This fetal shortcut is called the ductus arteriosus. At birth, this shunt will close down and be retained in the adult as a ligament (ligamentum arteriosus). As this vessel constricts just after birth, all of the blood from the right ventricle then can't get through the ductus arteriosus and is forced into the pulmonary arteries and on to the lungs, the new site of oxygenation.

Go back to the surface of the heart and find the two ventricles, the thick walled muscles which make up the bulk of the heart tissue. Look between the right and left ventricles for a slight groove. In this groove you may be able to see the coronary artery. There is another one on the back side of the heart. Find the right auricle and trace the large blue vessel which runs into it (the anterior vena cava which you previously found). This large vessel drains all of the blood from the head and arms back into the right auricle. The posterior vena cava also joins the anterior vena cava (AVC) just before entering the right atrium. The posterior vena cava drains all of the blood from the posterior half of the pig back toward the heart. Trace the anterior vena cava forward until it branches into two veins, the right and left brachiocephalic veins. Trace the brachiocephalic vein anteriorly until it branches into the internal jugular vein and the external jugular vein. These two veins drain blood from the head into the brachiocephalic vein. Also joining the brachiocephalic vein is a vein coming in from the arm region, the subclavian vein.

In order to see the dorsally located arteries in the neck region, we have to get the ventrally located veins out of the way. Push the heart to the pig's left and note that the AVC runs down behind the right atrium and joins the posterior vena cava coming up from the abdomen. The two merge and discharge blood into the right atrium. Now get a pin in behind the AVC just where it joins the heart and cut through it (the AVC). That's the second vessel you've cut. Take up the free, cut end and work it anteriorly as you lift it up so that the AVC is freed from the tissues behind it. You should be able to lay it up to one side. Push the thymus gland aside and cut one of the jugular veins (that's three vessels!) to aid in getting the anterior veins out of the way.

Find the large artery coming forward from behind the right auricle. This is the aorta, and will require a bit of cleanup to get the thymus, pericardium and assorted mesenteries away. The main aortic arch curves off to the pig's left and makes a large loop before coming back to the center next to the spinal column. It extends back along the spine between the kidneys. Note that the aorta is whitish in color; it is so thick that the red latex doesn't show through. Why would the aorta be so thick and the posterior vena cava be so thin walled?

Notice that on top of the aortic arch there is a branch that goes straight forward about a half-inch; this is the brachiocephalic artery. It supplies all of the blood to the arms and head. Notice that it divides into two carotid arteries, one of which runs along either side of the trachea. Clear away any thymus gland or connective tissue to trace the brachiocephalic and carotid arteries up toward the base of the neck. Go back to the brachiocephalic artery and notice that just before the division for the carotid arteries that the large right subclavian artery projects off toward the right shoulder. Where is the left subclavian artery? Not where you would expect it to be! Come back to the aortic arch and trace it about 5-6 mm to the pig's left. There you will find the left subclavian artery projecting directly off toward to the left shoulder. Hence, the right subclavian artery comes off the brachiocephalic artery while the left subclavian artery comes directly off the aorta. The arterial system is thus asymmetrical. Flop the veins back in place over the arterial system. Are the veins of the head and neck region symmetrical or asymmetrical?

Follow the aorta on posteriorly past the ductus arteriosus. Just where it goes between the kidneys, it runs parallel with the posterior vena cava, both being about the same diameter. The renal arteries are obvious; follow one into a kidney (taking care not to damage the whitish coiled ureter which also comes out of the kidney and lies nearby). Just a few millimeters anterior to the renal arteries, you will find the single mesenteric artery which carries blood from the aorta to the intestines.. Now follow the aorta on posteriorly past the kidneys and you will find that it branches into four smaller arteries. The outermost two are the iliac arteries going into the legs, and the innermost two are the umbilical arteries which extend up into the umbilical cord and supply blood to the fetal side of the placenta.

In working toward the rear end of the coelomic cavity, be careful not to damage either the ureters or, especially in the female, the small reproductive organs. We shall examine these next lab period. Take your time and use a dissecting needle or pin (not your scalpel or a razor blade) in tracing the blood vessels. Notice that the kidneys, aorta, posterior vena cava, etc. are covered by peritoneum and hence are not inside the coelom (they are said to be retroperitoneal). You must pick the peritoneum away to get at these structures. Where the peritoneum leaves the body wall (as the inner lining) and turns down to enclose the intestines it is then called mesentery and exists as two very thin membranes lying close together. In the coils of the small intestine you can see the numerous very small arteries supported by this mesentery.

If you have time to do so, try taking out a few millimeters of small intestine about 5 cms beyond the pyloric sphincter and splitting it open. Examine the lining with a dissecting microscope to see the villi. Do you recall from the cross-sectional slide of the intestine that you saw a few weeks ago what type of epithelial cells line the villi of the small intestine?

When you have located all of the organs of the respiratory, circulatory and digestive systems listed in today's exercise, take a few minutes to review their location, and then locate the organs on the human model.

For all internal organs found today you should know (1) the organ-system to which it belongs and (2) its primary function or functions.