126 A Manual of Veterinary Physiology. 



dition it produces no action on food. It is essential to its 

 function that it should be associated with an acid, which the 

 majority of observers regard as hydrochloric. In this con- 

 dition it readily acts on proteids, converting them into 

 peptones. This function is spoken of as peptonizing. 



Pepsin may be destroyed by heat. It loses its activity 

 at 134° to 136° F., though in the dried state it may be 

 raised to 212° F. without being destroj^ed. It best mani- 

 fests its activity at a temperature of 104° F. 



Pepsin is described as a ferment, inasmuch as, besides 

 splitting up the proteid substances of food, one of its lead- 

 ing characteristics is that a very small amount of it is 

 capable of performing a considerable amount of work. It 

 is probable that so long as it is acting in an acid medium it 

 is not appreciably used up. 



The Rennin, or milk-curdling ferment, is also formed in 

 the same cells as the pepsin ; it is destroyed at a lower 

 temperature than pepsin. By means of magnesium car- 

 bonate the two ferments may be separated. 



The action of rennin is to clot the casein of milk. This 

 process is used in the manufacture of cheese, an infusion of 

 the mucous membrane of the stomach being sufficient to 

 set up this action in the milk. The ferment acts without 

 the presence of an acid, but it is essential in order that 

 the rennin may clot casein that calcium phosphate should 

 be present. 



In the fourth stomach of the calf and sheep this ferment 

 may be readily found, and it has been said to exist in the 

 stomach of the horse. 



A third or lactic acid ferment has been described as 

 existing in the gastric juice, which has the power of con- 

 verting milk-sugar into lactic acid. Dilute caustic soda, 

 which will destroy both pepsin and rennin, has no effect on 

 this ferment. 



Mucin. — The remarkable amount of mucin in the stomach 

 of the horse is secreted by epithelial cells and mucous 

 glands, as previously mentioned. How much mucin is 

 formed, independent of the gastric juice, Ave have no means 



