149 



peptones and proteases. The digestion of the proteids is, of 

 course, due to the 



Gastric Juice. The gastric juice is a thin, colorless 

 liquid composed of about 99 per cent of water and about 

 i per cent of other substances. The latter are dissolved 

 in the water and include, besides several salts, three active 

 chemical agents hydrochloric acid, pepsin, and rennin. 

 Pepsin is the enzyme which acts upon proteids, but it is 

 able to act only in an acid medium a condition which is 

 supplied by the hydrochloric acid. Mixed with the hydro- 

 chloric acid it converts the proteids into peptones and 

 proteoses. 



Other Effects of the Gastric Juice. In addition to di- 

 gesting proteids, the gastric juice brings about several 

 minor effects, as follows: 



1. It checks, after a time, the digestion of the starch 

 which was begun in the mouth by the saliva. 1 This is due 

 to the presence of the hydrochloric acid, the ptyalin being 

 unable to act in an acid medium. 



2. While there is no appreciable action on the fat itself, 

 the proteid layers that inclose the fat particles are dissolved 

 away (Fig. 79), and the fat is set free. By this means the 

 fat is broken up and prepared for a special digestive action 

 in the small intestine. 



3. Dissolved albumin, like that in milk, is curded, or 

 coagulated, in the stomach. This action is due to the 

 rennin. The curded mass is then acted upon by the 

 pepsin and hydrochloric acid in the same manner as 

 the other proteids. 



1 The saliva may continue to act for a considerable time after the food enters the 

 stomach. " Careful examination of the contents of the fundus (large end of the 

 stomach) by Cannon and Day has shown that no inconsiderable amount of salivary 

 digestion occurs in the stomach." FISCHER, The Physiology of Alimentation. 



