74 THE GASTRIC JUICE. 



divided precipitates of other substances, as is the case with 

 many of the ferments. 



The pepsin thus obtained is probably not the pure 

 substance. It is a white or yellowish-white, amorphous 

 powder or scale when dried. It is hygroscopic in the air 

 and has a slightly saline or acidulous taste, with no offen- 

 sive odor. It is soluble in about 100 parts of water, but 

 more easily on the addition of hydrochloric acid. Because 

 of its reactions pepsin is generally regarded as a protein. 

 Its molecular composition is unknown. 



Pepsin is inactive in neutral or alkaline liquids, but 

 in slightly acid fluids it dissolves coagulated albuminous 

 compounds, with the formation of proteoses and peptones. 

 It acts most rapidly with hydrochloric acid, though others 

 may be used instead. The best strength of acid for the 

 purpose varies with the kind of material to be digested from 

 0.1 per cent, to 0.3 per cent, of hydrochloric acid. Pepsin 

 from warm-blooded animals digests best at 38. It is 

 destroyed by heating the solution. Its action is hindered 

 by the presence of the products of digestion, but if these 

 are removed as fast as they are formed it will change to 

 the soluble form many thousand times its weight of albu- 

 minous material. 



The second ferment of the gastric juice, rennin, is 

 always present in human gastric juice under normal condi- 

 tions. It exists in the mucous membrane in the form of a 

 zymogen, which is sometimes inactive until it is set free 

 by an acid. Hence if it is extracted by water it may not 

 give its characteristic reaction. This is especially true in 

 the case of birds. or fish. This characteristic reaction is the 

 coagulation of milk or casein in a neutral or faintly-alka- 

 line solution in which calcium salts are present. It does 

 not give the albumin reactions when in a pure state. Ben- 



