Exercise XIV 



VERTEBRATE ANATOMY 71 



an organ that you have removed, a single-edged 

 razor blade ordinarily does better than the 

 scalpel. 



Open the abdomen about midway down from 

 the rib cage. Extend a longitudinal slit forward 

 to the ribs (be careful not to go beyond) and back 

 to the genital openings. Make lateral slits down 

 the side of the body wall parallel to the rib cage. 

 Observe the packing of the abdominal contents; 

 poke about, noting connections and relative 

 positions. Don't be afraid to lift the organs 

 away from one another. Watch the slow peristal- 

 tic movements of the intestine. Pull the liver 

 down and observe the lung through the trans- 

 parent part of the diaphragm. 



Cut through one side of the diaphragm and the 

 rib cage on that side, staying close to the midline. 

 Note the collapse of the lung. (Why does it 

 collapse?) Pull open the rib cage and observe 

 the heartbeat. Describe the motions. Do all the 

 parts beat simultaneously? Collapse the lung 

 on the other side and remove the ribs and ster- 

 num, over the heart. This will allow you to 

 inspect the heart more closely. Try to trace the 

 major vessels to and from the heart (the vena cava 

 and aorta). Can you distinguish arteries and 

 veins? It might help to remove the white thymus 

 gland, which is found anterior to the heart and 

 may obscure its atria (auricles). (You may 

 keep your animal's heart beating longer and ob- 

 serve the expansion of the lungs by inserting a 

 dropper pipet attached to a rubber tube into the 

 trachea and breathing for the rat. The instruc- 

 tor will demonstrate this for you.) 



By this time, anoxia is probably overcoming 

 the heart and other tissues. The muscles may 

 twitch, but these are "automatic" responses; the 

 animals feels nothing. After you have observed 

 the heart, cut a slit into the end of the ventricle 

 with your scalpel to bleed the animal. Take it 

 over to the sink, and wash away the blood. Now 

 start dissecting the abdominal contents. 



First, examine the liver. The veins draining the 

 intestines pass through the liver on their way 

 to the heart. (Of what importance do you think 

 this might be in the light of liver function?) 

 Trace these veins as best you can to and from the 



liver. Remove the liver carefully, freeing it as far 

 as possible from its attachments to surrounding 

 tissues. Examine the excised liver, noting its 

 consistency, lobes, etc. (Make such a cursory 

 examination of every organ you remove.) 



The stomach is found just below the liver on 

 the animal's left. Find the esophagus, following 

 it to where it penetrates the diaphragm. (We will 

 dissect it completely later.) Examine the in- 

 testinal tract. Pull it out, noting the mesentery 

 that attaches the intestine to the body wall (all 

 the organs in the body cavity are surrounded and 

 supported by mesenteries). Note the fanlike 

 arrangement of blood vessels in the mesentery. 

 Note the large caecum of the rat. (Do we have a 

 caecum?) Below the caecum, the digestive tract 

 is called the colon ; above, the intestine. Next find 

 the pancreas, buried in the mesenteries just below 

 the stomach. Starting at either end, free and re- 

 move the intestinal tract in one long string. How 

 long is the intestine? Comparing your height 

 and the length of the rat, how long would you 

 estimate your own intestinal tract to be? 



Look next at the spleen, located below and to 

 the right of the stomach. Why is the speen so 

 dark in color? What is known of its function? 



Study the kidneys. Try to trace the ureters to 

 the bladder. Find the adrenal glands in the fat just 

 above the kidney. These are round, brownish 

 bodies, easy to miss. Note the prominent blood 

 vessels leading from the kidney to enter or leave 

 the heart through the vena cava and the aorta. 

 Remove the kidneys and adrenal glands. (Be 

 sure to slice open these organs and look at their 

 cross sections.) 



Now move to the thorax. If a model of the 

 human thorax is available in your laboratory, 

 study it before going on with your dissection. 

 Look at the lungs, heart, esophagus, and major 

 vessels that run forward from the heart and then 

 back along the ridge of the spinal column. Fol- 

 low the aorta and vena cava backward as far 

 as you can. Try to reach the place where they 

 fork before entering the legs. Remove the heart; 

 identify the ventricles and atria. Slice it to look 

 at the internal structure. (We will study the 

 mammalian heart in some detail later in the 



