DIGESTIVE SYSTEM IN MAN 



83 



Parotid 

 gland 



118. The stomach. Whatever fermentation has been started 

 by the saUva in the mouth continues in the mass of food until 

 this reaches the stomach. Here, however, it stops the moment 

 the acid, or sour, stomach juice comes in contact with the 

 saliva. It seems that the saHva ferment cannot act in the 

 presence of acid. 



The stom.ach juice contains a special ferment known as 

 pepsin. Pepsin, in the presence of acid, acts upon the proteins 

 in the food, changing them 

 into soluble compounds of 

 similar composition, known 

 as peptones. Peptones dif- 

 fer from proteins chiefly in 

 this one fact, that the former 

 are soluble in water and are 

 capable of diffusing through 

 membranes, while the pro- 

 teins are generally not 

 capable of such diffusion. 

 In the stomach the swal- 

 lowed substances are thor- 

 oughly mixed with the 

 gastric, or stomach, juice by 

 the action of the muscles 

 in the stomach wall. The 



stomach wall contains layers of muscle cells running in differ- 

 ent directions, as well as gland cells in which are produced the 

 particular substances found in the gastric juice (see F'ig. 30). 

 As the changing of proteins to peptones goes on, the mix- 

 ture in the stomach becomes more and more liquid and more 

 and more acid. From time to time a quantity of the liquid in 

 the stomach is squirted out into the beginning of the intestine 

 by the opening of the connection (Fig. 28, e) and the contrac- 

 tion of the stomach at the same time. After a while most of 

 the contents of the stomach has been changed to a mixture 



Blood 



vessels ^^, ,__^ 



Sublingual 



Svhmaxillary 



The salivary glands 



There are three sets of glands which produce 

 saliva : the parotid, in the chee]<, just in front 

 of the ears ; the submaxillary, under the angles 

 of the jaw ; and the sublingual, under the tongue 



Fig. 29. 



