38 PHYSIOLOGY OP FARM ANIMALS [CH. 



of a clog so operated upon may he followed by gastrie seerelioii. 

 These experiments show that gastric secretion may result from 

 the exciting action of an internal secretion normally produced 

 in the mucous membrane of the pyloric portion of the stomach 

 and passed into the blood in which it is carried to all parts of the 

 body including the whole secretory area of the stomach where it 

 evokes the activity of the gastric glands. The chemical excitant 

 having the specific action described has been designated gastrin 

 or gastric secretin, and it belongs to a class of internal secretions 

 which Starling has called by the name of hormones. Such bodies 

 are internal secretions having a specific stimulating action upon 

 organs or tissues other than those which manufacture them. 



There can be no doubt that the general principles underlying 

 the mechanisms of digestion just described apply as much to the 

 horse and other animals as to the dog. A further knowledge of 

 the digestive processes in the horse has been obtained by means 

 of feeding experiments. 



It has been noted that under normal conditions in the horse 

 the stomach is hardly ever empty. Even many hours after 

 feeding, ^ome food is still present, and it is not until a horse has 

 been starved for twenty-four hours that its stomach is found to 

 be really devoid of food. At the time of feeding, however, food 

 usually passes out of the stomach into the small intestine with 

 considerable rapidity, and when the stomach has become two- 

 thirds full the rate of ingress of food and the rate of egress are 

 equal. But as soon as the feed is finished the passage of the 

 food into the intestine at once slows down. 



The condition of the digesting stomach varies somewhat 

 according to the nature and quantity of the food, and the order 

 in which it is supplied. Hay after mixing with about four times 

 its weight of saliva and after mastication in the mouth passes 

 into the stomach, and if it be empt}^ the hay comes to lie in the 

 jiyloric region. As the stomach gradually fills it gets acted upon 

 by the gastric juice, and some of it, in a partially digested state, 

 commences to pass out into the intestine. At the end of the 

 feed the material left in the stomach is found to be comminuted 

 and to resemble green and yellow faeces, the colour being due to 

 the acid of the gastric juice. The entire stomach is at this time 

 generally acid throughout, and if any alkalinity can be detected 

 it is caused by the swallowed saliva. 



