ANATOMY AND PHYSIOLOGY 7 



(X 2 or 3) the digestive system as thus seen, surrounded by an out- 

 line of the trunk and head, and show the position of the mouth cavity 

 and esophagus. 



Exercise 7. — The Digestive Tract in Cross Section. 



(d) The digestive organs must now be removed. Leave the heart, 

 lungs, large intestine, oviducts, testes, and related structures in place 

 and uninjured. Cut across the stomach at its anterior end, and sever 

 the small intestine near its union with the large intestine. Take out 

 the stomach, small intestine, spleen, pancreas, liver, and gall bladder 

 after cutting the mesentery that holds them in place. 



(e) With a sharp scalpel, a razor blade, or with scissors, cut cross 

 sections of the stomach toward its posterior end. Place these under 

 water and study the cut surfaces with a handlens. Identify the 

 mucous membrane, a conspicuous layer lining the cavity of the stom- 

 ach, the submucosa, a thin, somewhat brownish layer of varying 

 thickness, the muscular layer, conspicuous and of uniform thickness, 

 and the visceral peritoneum, which is very thin and visible only where 

 it is continuous with the mesentery. What are the functions of the 

 different layers? The mucous membrane and submucosa will be 

 folded if the stomach is not expanded with food. What are the func- 

 tions of such folds? Draw a section (X 5 to 8). 



Exercise 8. — Gastric Digestion. 



(f) The conditions under which digestion occurs in the living 

 animal can be duplicated to a considerable degree in experiments 

 with non-living material. Digestion of food will occur in test-tubes if 

 the proper chemicals are employed. The process is more rapid at the 

 temperature of the body but goes on quite actively at room tempera- 

 tures. 



(g) The way in which gastric digestion occurs can be found out 

 by means of a simple experiment. Label four test-tubes as follows: 

 (1) fibrin and distilled water; (2) fibrin, distilled water, and pepsin; 

 (3) fibrin, distilled water, and hydrochloric acid; (4) fibrin, distilled 

 water, pepsin, and hydrochloric acid. Place one small piece of fibrin, 

 a protein, in each tube and fill it half full of distilled water. Add one- 

 half pipette full of pepsin to tubes 2 and 4, and two drops of hydro- 

 chloric acid to tubes 3 and 4. Mix the contents of each tube thor- 

 oughly. Use care not to spill acid on the tables or on your clothing. 

 Notice the changes that occur in the fibrin during the laboratory 



