DIGESTION AND RESORPTION 93 



as more than a trace of free acid accumulates, or to speak more 

 exactly, as soon as the concentration of the hydrogen ions ex- 

 ceeds a certain small limit, the action of the ferment is not only 

 inhibited, but the ptyalin is digested by the pepsin. 



This, however, is far from happening immediately upon the 

 entry of the feed into the stomach. The secretion of the gastric 

 juice requires a certain length of time. Moreover, the contents 

 of the stomach are semi-solid rather than liquid and while the 

 muscular contractions of the stomach serve to mix the feed to 

 some extent, this effect is less than is often assumed. Frozen 

 sections of animals to which variously colored feeds have 

 been given show the contents of the stomach to be distinctly 

 stratified some time after the ingestion of feed. Furthermore, 

 the gastric juice is secreted only in the pyloric portion of 

 the stomach (116). Time is required, therefore, for the 

 acid to penetrate and acidify the whole mass and conse- 

 quently the action of the ptyalin may continue for a con- 

 siderable period. 



Extensive investigations, especially by Ellenberger and 

 Hofmeister, upon gastric digestion in the horse and hog have 

 demonstrated that in these animals the action of the saliva in 

 the stomach upon the starch of the feed plays an important 

 part in digestion. In the horse (116), the left end of the stomach 

 is really a dilation of the esophagus. In the hog, while nearly 

 the entire surface of the stomach is lined with mucous membrane, 

 the left-hand end contains no peptic glands. When the stomach 

 is filled with feed, therefore, it is evident that the action of the 

 hydrochloric acid will begin along the walls of the fundus of 

 the stomach and only gradually spread to the rest of the con- 

 tents. It is true that lactic fermentation usually sets in during 

 this period, aiding to acidify the stomach contents but, as 

 already stated, ptyalin is less sensitive to organic than to 

 inorganic acids. It has been found that the solution of starch 

 may continue to a greater or less extent for as much as four or 

 five hours both in the horse and hog. In ruminants, the con- 

 ditions are even more favorable for salivary action, since the 

 feed remains in contact with the saliva in the rumen for a con- 

 siderable time, the contents of this stomach being maintained 

 slightly alkaline by the large amount of saliva secreted by these 

 animals (115). It may be assumed, therefore, in spite of the 



