THE DIGESTIVE SYSTEM 95 



constant amount of water. If dry food is eaten, more water is 

 secreted to make up the deficiency. On the contrary, if too 

 much water is present, it is passed on to the intestines before 

 gastric digestion is completed. This indicates that the secretions 

 of acid, pepsin, and water by the stomach glands are independ- 

 ent of each other. 



Pepsin acts principally on the highly organized protein mole- 

 cules, converting them into peptones. This conversion is a 

 very complex process and is accompanied by hydration. Pep- 

 sin is active only on acidified protein. The presence of the 

 latter stimulates the glands of the stomach to activity. 



Rennin has the power of curdling milk. It acts upon the 

 caseinogen of milk and splits it into a proteose-like body and 

 soluble casein. The latter combines with calcium to form insol- 

 uble casein. Hammarsten says that some rennin is found in the 

 adult horse's stomach. Probably the foal's stomach secretes 

 much larger quantities. 



The flow of gastric juice commences before food reaches the 

 stomach, in fact the mere sight or smell of appetizing food starts 

 action in the glands of the stomach. When food enters the 

 mouth and the nerves of taste are stimulated, the secretion be- 

 comes even more pronounced. It keeps up until the entire 

 mass of food in the stomach has undergone gastric digestion — a 

 period of about three hours in the horse. 



When horses are fed different kinds of foods in succession, it 

 has been found, by killing them at varying intervals after feeding, 

 that the first food taken passes into the most ventral part of the 

 stomach. Subsequently, ingested food is arranged in layers, 

 provided its consistency is sufficient to hold it from forming a 

 mechanical mixture with previously ingested materials (Fig. 29). 



There is some difference of opinion in regard to the effect of 

 watering a horse after feeding. It is evident that when a horse 

 which has eaten an ordinary feed drinks any great quantity of 

 water, both cannot be accommodated in the stomach. Scheun- 

 ert and Schattke 1 conclude, from a series of experiments which 

 they carried out: (1) That the drinking of water in any amount 

 desired by the animal exerted no deleterious effect on digestion. 

 (2) That the increase in the water content of the ingesta was in- 



1 Cited by Sisson. 



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