DIGESTION AND RESORPTION 83 



specific gravity of 1.005 to I - I an d containing a maximum of 

 about 2.5 per cent of solids. The combined action of the pepsin 

 and hydrochloric acid of the gastric juice splits the proteins 

 of the feed into derived proteins, especially proteoses and 

 peptones, and to some extent into polypeptids. 1 The hydro- 

 chloric acid of the gastric juice has also an important anti- 

 septic action and likewise serves to dissolve more or less of 

 the ash of the feed. 



In addition to its digestive action on proteins, the gastric 

 juice contains an enzym which brings about the coagulation 

 of the caseinogen of milk the rennet ferment, or chymosin. 

 According to some investigators, chymosin is identical with 

 pepsin, but the weight of opinion seems to be in favor of its 

 independent existence. 



119. The passage of feed from the stomach. The lower 

 or posterior end of the stomach is closed by a sphincter muscle 

 called the pylorus, which prevents the ingested feed from pass- 

 ing into the next division of the alimentary canal, the duodenum, 

 or being forced into it by the contractions of the stomach. 

 When in the course of gastric digestion, however; the difference 

 between the acid reaction of the stomach contents and the al- 

 kaline reaction which normally prevails in the duodenum 

 reaches a certain level, the pylorus relaxes and allows the per- 

 istaltic contraction of the stomach to press a portion of its acid 

 contents into the duodenum. The partial neutralization of 

 the duodenal contents which results causes the pylorus to close 

 again until the alkaline reaction is restored, when the cycle may 

 be repeated. 



The mechanism of this process has been especially studied by 

 Cannon in carnivora, but it may be presumed that what is true of these 

 animals is also substantially true of herbivora, although experimental 

 proof of this is lacking. 



While both protein and carbohydrates undergo considerable 

 digestion in the stomach, it is evident that one important 

 function which the stomach performs is that of a receptacle 

 which prevents too rapid passage of the feed into the duodenum 

 and at the same time initiates chemical changes and prepares 



1 By prolonged peptic digestion in vitro amino acids may also be produced but 

 it is not believed that this occurs in natural digestion. 



