104 THE FEEDING OF ANIMALS 



be very successfully produced in an artificially 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 dis- 

 solving lean meat, wheat gluten, and other proteins. The 

 active agent in causing this solution is pepsin, an unor- 

 ganized ferment or enzym (see Par. 128) which is present 

 in the gastric fluid of all animals. 



146. Changes in stomach digestion. This juice 

 changes proteins to peptones, bodies soluble and diffusible. 

 The change to peptones is not a single step, for the pro- 

 tein passes through successive stages as acid proteins and 

 proteoses before it reaches the peptone form. This is 

 largely what may be styled progressive hydrolysis. An- 

 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. Lipase, 

 an enzym that acts in the fats, is also possibly present 

 in the gastric juice of herbivorse. (See Par. 129.) 



147. Hydrochloric acid essential in stomach diges- 

 tion. The free hydrochloric acid in the gastric juice is 

 also actively concerned in protein digestion. It is found 

 that a solution of pepsin has a limited effect in the absence 

 of free acid, for when, during artificial digestion, the 

 supply of this acid is used up, it must be renewed or 

 digestion is checked. 



148. The stomachs of the horse and pig. These con- 

 sist of a single sac, so that digestion with these animals is 



