350 VETERINARY PHYSIOLOGY 



The condition of the stomach varies greatly in fasting and 

 after feeding. 



Gastric digestion in the dog and pig will first be considered. 



A. Gastric Digestion in the Dog and Pig 

 Stomach during Fasting 



The organ is collapsed, and the mucous membrane is thrown 

 into large ridges. It is pale in colour because the blood vessels 

 are not dilated. Movements are not marked and the secretion 

 is scanty, only a little mucus being formed on the surface of 

 the lining membrane. 



Stomach after Feeding 



When food is taken the blood vessels dilate, a secretion is 

 poured out, and movements of the organ become more marked. 



1. Vascular Changes. The arterioles dilate, and the mucous 

 membrane becomes bright red in colour. This is a reflex vaso- 

 dilator effect, impulses passing up the vagus to a vaso-dilator 

 centre in the medulla, and coming down the vagus from that 

 centre. Section of the vagi prevents its onset. 



2. Secretion. There is a free flow of gastric juice from all 

 the glands in the mucous membrane. 



(a) Characters of Gastric Secretion. The gastric juice is a 

 clear watery fluid, which is markedly acid from the presence of 

 free hydrochloric acid. In the dog the free acid may amount 

 to 0'2 per cent., but in man it is less abundant, and when the 

 gastric juice is mixed with food the acid rapidly combines 

 with alkalies and with proteins, and is no longer free. In 

 addition to the HC1, small quantities of organic salts are 

 present. Traces of proteins may also be demonstrated, and 

 with these two enzymes are associated one a proteolytic or 

 protein-digesting enzyme, pepsin, the other a milk-curdling 

 enzyme, rennin. 



(b) Course of Gastric Digestion (1) Amylolytie Period. The 

 action of the gastric juice does not at once become manifest. 

 For a short time after the food is swallowed, in the pig the 

 ptyalin of the saliva goes on acting, and the various micro- 

 organisms swallowed with the food grow and multiply, and thus 



