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 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, and a small quantity of hydrochloric 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. 1). This enables the 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.

— cut open to show the folds in its lining.

The small intestine is about one inch in diameter and twenty feet long, with many coils and turns in its course (Fig. 90). Its mucous lining is wrinkled into