GASTRIC DIGESTION. 75 



nin is more easily destroyed by heating its solution than is 

 pepsin. The methods of obtaining it are similar to those 

 employed with pepsin. 



The gastric juice acts only upon the nitrogenous con- 

 stituents of the food. The albuminous substances are first 

 changed into acid albumin by the free acid; then this is 

 decomposed by the pepsin and hydrochloric acid, forming 

 proteose, which passes into peptone. Since the process is 

 a continuous one, all these products may be found in the 

 stomach at the same time, though in the first stages the 

 acid albumin is in excess and at the last the peptones. 

 The connective tissues are digested by the gastric juice, 

 though the proteolytic ferment of the pancreas does not 

 dissolve them. The membranes which surround the fat- 

 cells are also dissolved, setting the fat free. Thus the food 

 is changed into chyme: a pulpy mass which can be readily 

 penetrated by the intestinal fluids. In the first stages of 

 digestion the saliva may continue to act on the starchy 

 materials, and during the first fifteen to twenty minutes 

 there is a formation of lactic acid, which disappears after 

 this time. Milk is coagulated, partly by the acid, partly 

 by the rennin. The absorption of the peptones commences 

 in the stomach, but the digestion is not completed here, the 

 chyme passing through the pylorus into the intestine. 



179. PREPARATION OF PEPSIN. Separate the mu- 

 cous membrane of a pig's stomach from the muscular tis- 

 sue. After rinsing it with water chop it finely. Make a 

 dilute solution of hydrochloric acid by the addition of 1 

 cubic centimeter of concentrated acid 1 to 150 cubic centi- 

 meters of water. This will contain about 0.2 per cent, 

 of the pure acid. Extract the pepsin from the chopped 



1 The U. S. P. acid containing about 32-per-cent. HC1. 



