DIGESTION. 95 



and ruptures the cellulose walls so as to permit the introduction of gastric 

 juice and the solution of its nutritive principles. 



The principal action of the gastric juice, however, is the transformation 

 of the different protein principles of the food into peptones, the intermediate 

 stages of which are due to the influence of the acid and pepsin respectively. 

 As soon as any one of the proteins is penetrated by the acid it is converted into 

 acid-protein, a fact which indicates that the first step in gastric digestion is the 

 acidification of the proteins. This having been accomplished, the pepsin 

 becomes operative and in a varying length of time transforms the acid-protein 

 into a new form of protein termed peptone. In this transformation it is 

 possible to isolate intermediate bodies by the addition of ammonium and 

 magnesium sulphates, to which the term proteases has been given. Because 

 of the order in which they are obtained they have been divided into primary 

 and secondary. This supposed change is represented by the following 

 scheme: 



Protein 



I 



Acid-protein 



I 



Proteose (primary) 



I 



Proteose (secondary) 



I 



Peptone 



Peptones. Peptones are the final products of the digestion of protein 

 bodies in the stomach and differ from the bodies from which they are derived 

 in the following particulars: 



1. They are diffusible i. e., capable of passing readily through animal 

 membranes. 



2. They are soluble in water and in saline solution. 



3. They are non-coagulable by heat and nitric or acetic acids. They can 

 be readily precipitated, however, by tannic acid, by bile acids, and by 

 mercuric chlorid. 



The duration of gastric digestion will depend largely upon the quantity 

 and quality of the food. The digestion of the average meal occupies from 

 three to five hours. 



Movements of the Stomach. During the period of gastric digestion the 

 walls of the stomach become the seat of a series of movements, somewhat 



