DIGESTION 319 



and more rarely from stimulation of other nerves. It is a 

 reaction to nocuous stimuli. In some animals, as the dog, 

 it may be voluntarily induced. 



Usually vomiting is preceded by a free secretion of saliva. 

 The glottis is then closed, and, after a forced inspiratory 

 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, and the con- 

 tents of the stomach are shot upwards. They are prevented 

 from passing into the nares by the contraction of the muscles 

 of the soft palate. The wall of the stomach also acts, the 

 pyloric end being firmly contracted and the cardiac end being 

 also in a state of tonus. But its action is non-essential, since 

 vomiting may be produced in an animal in 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. 



IV. INTESTINAL DIGESTION. 



After being subjected to gastric digestion the food is 

 generally reduced to a semi-fluid grey pultaceous condition 

 of strongly acid reaction known as chyme, and, in this 

 condition, it enters the duodenum. 



Here it meets three different secretions : — 



A. Pancreatic secretion. 



B. Bile. 



C Intestinal secretion. 



A. Pancreatic Secretion. 



The secretion of the pancreas may be procured by making 

 either a temporary or a permanent fistula. In the former 

 case, the duct is exposed, and a cannula fastened in it ; in 

 the latter, the duct is made to open on the surface of the 

 abdomen, a small piece of the intestinal wall, with the 

 raucous membrane round the opening of the duct, being 



