DIGESTION 



249 



the stomach. The stomach is a muscular organ which 

 tapers toward the end away from the esophagus where 

 it is furnished with a muscular constriction, the pylorus. 

 The pylorus when contracted serves to retain food in 

 the stomach until it is digested. The stomach is lined 

 with a mucous membrane filled 

 with numerous gastric glands. 

 These glands secrete the gas- 

 tric juice, which is slightly 

 acid from the presence of a 

 small amount of hydrochloric 

 acid (about 0.2 per cent.). 

 They also produce a ferment 

 called pepsin which acts upon 

 the protein substances of our 

 food, converting them into a 

 soluble form called peptone. 

 Pepsin has no action upon fats 

 or carbohydrates, and it acts 

 upon proteins only in an acid 

 medium. The presence of 

 food in the stomach excites 

 the secretion of the gastric 

 juice, and the contractions of 

 the muscular walls of the 

 stomach which are set up by 

 the same cause produce a sort 

 of churning motion which mixes the gastric juice with the 

 food and indirectly aids in the process of digestion. When 

 the food has been acted on for a time in the stomach, the 

 pylorus relaxes and allows the more or less fluid mass to 

 escape into the small intestine. The latter is a long, coiled 

 tube with rather thin, muscular walls and an inner lining of 

 mucous membrane which contains numerous glands. It is 



PIG. 189. Diagram of stom- 

 ach and intestines. O, esoph- 

 agus; S, stomach; p, pylorus; i, 

 small intestine; co, colon or main 

 part of large intestine; R, rec- 

 tum or terminal division of large 

 intestine; A, appendix vermifor- 

 mis attached to the cecum, c. 

 The arrows indicate the direc- 

 tions taken by the food. 



