SECRETION OF GASTRIC JUICE 129 



the protein of milk. It converts this substance into a clotted mass, 

 the curd. The pepsin carries on the digestion of curd more effi- 

 ciently in this form than it ^.Cardiac opening 

 could in the original form. 

 Food is kept in constant 

 motion in the stomach. 

 By means of the action of 

 the stomach muscles, food 

 is churned back and forth 



- , , ., . The diagram on the left shows the shape of the 



and Up and down Until it stomach when empty; and the one on the right shows 

 i i , 1/2 the approximate shape after a hearty meal. 



is reduced to much finer 



particles than were formed in the mouth. The partially digested 

 food is in liquid form and is called chyme. The reduction of the 

 food into these very fine particles is invaluable in increasing the 

 amount of food surface to be exposed to the action of digestive 

 juices in the small intestines. The cardiac sphincter prevents the 

 return of food to the gullet during the churning process. At 

 intervals the pyloric sphincter opens and some of the chyme is 

 forced into the small intestine by a wave of contraction. 



The stomach acts as a reservoir, holding the food and feeding it 

 at regular intervals to the small intestine. The time required for 

 gastric digestion of a meal depends upon the quantity and kind of 

 food eaten. An average meal requires about five hours for gastric 

 digestion. Solid particles tend either to keep the pyloric valve 

 closed, or to force it to relax, because of fatigue, before the food 

 has reached a semi-fluid condition. It is largely the acidity of the 

 chyme that causes the relaxation of this valve, but in the small in- 

 testine the acid has just the opposite effect. When the acidulous 

 chyme passes into the intestine, it causes the sphincter to contract. 

 The pylorus then remains closed until the acid has been neutral- 

 ized by the alkalinity of the intestinal juice. Since few enzymes 

 are produced in the stomach and their digestive action is incom- 

 plete, the digestion of the nutrients continues in the small intestine. 



