^r'TJfff DIGESTION 



213 





the proteid part of the milk. It is especially abundant 

 in childhood. The other ferment, called pepsin, softens the 

 proteid part of food and reduces it to peptone, in which 

 form it is soluble in water. Pepsin, however, can act only 

 when the hydrochloric acid has accumulated to an 

 amount sufficient to neutralize the alkaline condition caused 

 by the saliva. This usually requires about thirty or forty 

 minutes. The hydro- 

 chloric acid, by its 

 presence in sufficient 

 quantity, not only en- 

 ables the pepsin to act, 

 but also prevents fer- 

 mentation of the food 

 and kills all germs that 

 may enter the stomach. 



379. The saliva con- 

 tinues its work even in 

 the stomach until neu- 

 tralized by the acid. 

 The pepsin then begins 

 to act. The outside of 

 the food particles is 

 acted upon first, and 

 this digested part is 



then rubbed off by the peristaltic movements, and the 

 next layer is acted upon. Its action is confined to the 

 proteids. In fat meat the albuminous walls of the cells 

 are eaten away and the fat is set free but not digested. 

 Starch may also be set free from albuminous envelopes. 



380. The food is thus reduced to a semi-fluid condition 

 and is called chyme. A slight absorption may take place 

 in the stomach, of a portion of the proteids digested there. 

 Some of the sugar resulting from the salivary digestion of 

 starch by the saliva, may also be absorbed. Nearly all the 



FIG. 169. A Section through the Walls of the 

 Stomach. Magnified only 15 diameters. 



i, surface of the mucous membrane, showing the 

 openings of the gastric glands; 2, mucous mem- 

 brane, composed almost entirely of glands; 3, sub- 

 mucous or areolar tissue; 4, transverse muscular 

 fibers; 5, longitudinal muscular fibers; 6, perito- 

 neal coat. 



