Digestion The Stomach 113 



tificially prepared liquid. If the mucous lining of a 

 pig's stomach, after carefully cleaning without washing 

 with water, is warmed for some hours in a very dilute 

 solution of hydrochloric acid, an extract is obtained 

 which has the power of dissolving lean meat, wheat 

 gluten and other proteid substances. The active agent 

 in causing this solution is pepsin, an unorganized fer- 

 ment or enzym which is present in the gastric fluid of 

 all animals. It changes albuminoids to peptones, bod- 

 ies so soluble and diffusible that they pass readily into 

 certain small vessels which are distributed in the walls 

 of the alimentary canal and thus become available as 

 nutrients. The other ferment present in the gastric 

 juice is the one which gives to rennet its value as a 

 means -of coagulating the casein of milk in cheese- 

 making, and is called rennin. The action of this latter 

 body is especially prominent in the stomach of the calf 

 when fed exclusively on milk, and it is the calf's active 

 stomach, the fourth in. the mature animal, which is the 

 source of commercial rennet. 



The free hydrochloric acid in the gastric juice is also 

 actively concerned in proteid digestion. It is found 

 that a solution of pepsin has a limited effect in the 

 absence of free acid, for when, during artificial diges- 

 tion, the supply of this acid is used up it must be 

 renewed or digestion is checked. 



The stomach of the horse and pig consists of a 

 single sac, so that digestion with these animals is a 

 much simpler matter mechanically than with ruminants. 

 Chemically, the results are essentially similar, i. e., the 

 protein is in part changed to peptones. The food, after 



