78 THE HOESE IN HEALTH AND DISEASE 



horse. Schattke found an acid reaction in all parts of the horse's 

 stomach ninety minutes after feeding; previous to that time the 

 esophageal portion gave an alkaline or neutral reaction. 



Pepsin acts principally on the highly organized protein mole- 

 cules, converting them into peptones. This conversion is a very 

 complex process and is accompanied with hydration. Pepsin is 

 active only on acidified protein. The presence of the latter stimu- 

 lates the glands of the stomach to activity. 



Rennin has the power to curdle milk. It acts upon the casein- 

 ogen of milk and splits it into a proteose-like body and soluble 

 casein. The latter combines with calcium to form insoluble 

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

 horse's stomach. Probably the foal's stomach secretes much larger 

 quantities. 



All experiments recorded in the literature show that when the 

 stomach ingesta are ready to be passed into the intestine they con- 

 tain a surprisingly constant amount of water. If dry food is 

 eaten, the stomach secretes water to make up the deficiency. On 

 the other hand, if too much water is present, it is passed on to the 

 intestines before gastric digestion is completed. This goes to show 

 that the secretions of hydrochloric acid, pepsin, and water by the 

 gastric glands are independent of each other. 



The epoch-making results obtained by Pawlow on dogs, as a 

 result of establishing a gastric fistula for obtaining gastic juice and 

 observing its secretion, have led to an understanding of many ob- 

 scure phases of digestion, and have provided a basis on which 

 physiologists have been enabled greatly to strengthen our knowl- 

 edge of dietetics. He demonstrated that the mere sight or smell of 

 food not only causes "the mouth to water," but also stimulates 

 the glands in the walls of the stomach to start secreting gastric 

 juice. When the food enters the mouth both secretions become 

 more active. The gastric secretion keeps up until the entire mass 

 of food in the stomach has undergone gastric digestion. 



The Mechanical Action of the Stomach. — The old idea of a 

 churning action in the stomach has given way before recent obser- 

 vations to better founded knowledge. The stomach of the horse 

 is never empty, so gastric digestion is a continuous process in this 

 animal. Sisson has observed that the pylorus is usually open in 

 subjects killed for dissection, and that part of an ordinary feed of 

 hay and oats passes directly into the intestine during ingestion. 



