DIGESTION 323 



sugar, salt, and a few other substances are dissolved. 

 Chemically, saliva acts upon starch, changing it into a 

 form of sugar. Only a small part of the starch which 

 we eat has time to be changed during the short time 

 the food is in the mouth. The food is pushed back 

 from the mouth by the tongue. It passes down the 

 esophagus and enters the stomach. 



Digestion in stomach. The walls of the stomach are 

 composed of several layers of muscles. By the action 

 of these muscles the food is churned about and thus 

 thoroughly mixed with the gastric juice. The stomach 

 may be closed by means of the ring of muscles at the 

 lower end. At intervals it is opened to allow the food 

 which is suitably prepared to pass on. 



Gastric juice is a thin colorless liquid consisting 

 mainly of water (99.4%). In it are also small amounts 

 of hydrochloric acid and two enzymes, pepsin and 

 rennin. The pepsin and hydrochloric acid act upon 

 proteins. As a result of this action, protein is ready 

 for diffusion. Eennin acts only upon the protein of 

 milk. This it curdles, after which the protein is acted 

 upon by the pepsin. Rennin is frequently used in the 

 manufacture of cheese. In the stomach fats are melted 

 and oils are emulsified; that is, they are broken up into 

 very small drops and then scattered through the fluid. 



The food remains in the stomach about three hours. 

 The gastric juice starts to flow when food enters the 

 stomach. The flow may be further stimulated by the 

 flavor of food. It has been estimated that about three 



