DIGESTION. 



83 



cess. On the other hand, pepsin alone is unable to perform any dis- 

 solution or digestion of the foods with which it comes into contact. 

 But, if to it a 0.2-per-cent. solution of hydrochloric acid is added, 

 proteolysis proceeds quickly and energetically. The powers of the 

 gastric juice cannot be attributed to the presence, then, of its acid or 

 pepsin alone, but to a combination which may be termed pepsin-acid. 

 Thus gastric digestion is an acid digestion, and demands a knowledge 



Fig. 19. Dogs to whom a Fictitious Meal is Given. They have a 

 fistula in the oesophagus and a fistula in the stomach. After a photo- 

 graph taken in the laboratory of Pawlow. (GLEY.) 



of chemistry, for it is in many respects a chemical act. The result 

 of the action of gastric juice on food is essentially the same whether 

 the act takes place within the body or outside of it. Life has nothing 

 to do with it, for it is a chemical action on the proteids of the food. 

 In the stomach, then, the main process of digestion is the conversion 

 of the proteids, through intermediate stages, into peptones, for pro- 



