DIGESTION 



317 



lated by a local nervous mechanism which is reilexly brought 

 into play by the escape of the acid gastric contents into the 

 duodenum. This leads to an immediate closure of the 

 pylorus, which does not again open till the contents of the 

 duodenum have been neutralised by the alkaline secretions 

 which are poured into it. 



The rate of passage from the stomach of various kinds of food 

 has been studied by feeding cats with equal amounts of different 

 kinds of food mixed with bismuth, and then, by X-rays, 

 getting the outline of the contents of the small intestine at 



Prepyloric Sphincter. 



Pyloric Sphincter. 



Fig. 153. — Stomach of a Dog fed successively with three different foods 

 to show the absence of mixing at the cardiac end. 



to pass on 



different periods. Carbohydrates were found 

 most rapidly and fats most slowly (fig. 15 2). 



In man after a moderate meal the stomach is usually 

 emptied in about four hours. 



2. Nervous Mechanism of Gastric Movements — 



(a) Intrinsic. — Even after the section of all the gastric 

 nerves, the movements of the stomach may be observed to go 

 on regularly. They are therefore due to a mechanism in the 

 wall of the organ, and, in all probability, judging from the 

 analogy of the small intestine (p. 833), they are controlled 

 by the plexus of neurons in the muscular coat. 



(h) Extrinsic. — (i.) The vagus maintains the tone of the 

 muscular coat and augments the movements of the pylorus. 

 It also seems to act upon the cardiac sphincter to relax it. 

 (ii.) The sympathetic fibres decrease the tone and the 

 movements, but seem to maintain the contraction of the 

 pyloric sphincter. Their terminations are stimulated by 

 adrenalin. 



