3$8 VETERINARY PHYSIOLOGY 



importance. While it plays a certain part in digestion its 

 action is by no means indispensable, for it has been removed in 

 animals and in men without disturbance of the health. 



Vomiting 



Sometimes the stomach is emptied upwards through the 

 gullet instead of downwards through the pylorus. This act of 

 vomiting is generally a renex one, resulting from irritation of 

 the gastric mucous membrane and more rarely from stimulation 

 of other nerves. It is a reaction to nocuous stimuli. 



In vomiting, the glottis is closed, and, after a forced in- 

 spiratory effort by which air is drawn down into the gullet, 

 a forced and spasmodic expiration presses on the stomach, 

 while at the same time the cardiac sphincter is relaxed through 

 the action of the vagus, and the contents of the stomach are 

 sent upwards. They are at first prevented from passing into 

 the nares by the contraction of the muscles of the soft palate ; 

 but, as the act continues, these muscles are overcome, and the 

 vomited matter escapes through mouth and nose. The wall of 

 the stomach also seems to act, but its action is non-essential, 

 since vomiting may be produced in an animal hi which a 

 bladder has been inserted in place of the stomach. 



The centre which presides over the act is in the medulla 

 oblongata, and while it is usually reflexly called into action, 

 it may be stimulated directly by such drugs as apomorphine. 



B. Gastric Digestion in the Horse 



In the horse the process of gastric digestion differs from that 

 first described in the following particulars. 



In the first place, the horse has to eat a very large quantity 

 of food in proportion to the size of its stomach, and it is founc 

 that part of the food begins to pass very rapidly through the 

 stomach into the intestine. Colin found, when he killed a 

 horse which in two hours had eaten 2500 grms. of hay, that 

 the stomach contained only 1000 grms. But while this is the 

 case, a small residue of the meal remains for a very long time 

 in the stomach, and passes out only when the next me 

 is taken. 



The churning action of the stomach is less complete in the 



