FOOD AND DIGESTION 



103 



the kind of food eaten, the way it has been cooked, and 

 the thoroughness with which it has been chewed. The 

 gastric juice is chiefly water, and contains two ferments 

 called pepsin and rennin y and a small quantity of hydro- 

 chloric acid. Rennin acts upon the curd of milk, and is 

 abundant only during infancy. Hydrochloric acid kills 

 germs that may enter the stomach, and changes the food 

 which has been made alkaline by the saliva into an acid 

 condition (Exp. i). This enables ft\t pepsin to act upon the 

 proteid part of the food, for pepsin will not act while the 

 food is alkaline. Gastric juice digests lean meat, which is 

 a proteid food, by first dissolving the connective tissue that 

 holds the fibers in place, and they fall apart ; it then acts 

 upon the fibers separately and makes them soluble. Like 

 human fatty tissue (Fig. 14), fat meat consists of cells 

 filled with fat and held together by threads of connective 

 tissue. The cell walls and the threads, both being proteid, 

 are soon dissolved by the gastric juice, and the free fat is 

 melted into oil, but still undigested. 

 The food is reduced in the stomach to 

 a creamy, half-fluid mass called chyme. 

 Where the stomach opens into the 

 small intestine, there is a folding in or 

 narrowing of the tube so as to form a 

 kind of valve called the pylorus. After 

 the food has been changed to chyme, 

 this fold relaxes every minute or two, 

 and allows some of the chyme to 

 escape into the intestine. 



The small intestine is about one inch 

 in diameter and twenty feet long, with FIG. 96. A PORTION 

 many coils and turns in its course (Fig. OF SMALL INT S- 



' % TINE cut open to show 



90). Its mucous lining is wrinkled into the folds in its lining. 



