THE PYLORIC SPHINCTER 131 



of contraction. The effect of a succession of waves of this 

 sort upon the gastric contents was shown by Cannon, 

 who administered to an animal small capsules containing 

 a large quantity of bismuth in a meal containing a small 

 amount of bismuth. The capsules thus appeared by the 

 X-rays as dark shadows in a faint shadow. At each wave 

 the capsules were conveyed a short distance, until they 

 slipped back through the advancing ring. They thus 

 arrived by a to and fro movement at the pyloric vestibule. 

 Finally, a wave carried them up to the pylorus, from 

 which they were returned in the back-wash to the point 

 from which they first started. 



It is thus evident that gastric peristalsis has the effect 

 of mixing very thoroughly the food and the gastric juice, 

 and incidentally of exposing the mixture to the pyloric 

 wall, thus favouring the formation of the gastric hormone 

 already described. The cardiac sac meanwhile, by exert- 

 ing a constant pressure upon its contents, keeps the gastric 

 mill supphed. 



As the stomach empties, diminution in its size affects 

 first the middle of the body, which becomes tubular in 

 shape. The part above this then diminishes until it is 

 almost emptied. The pyloric part is the last to be 

 evacuated. 



The vigour of the gastric movement varies directly with 

 the amount of HCl present, this acid, in fact, providing 

 the stimulus to peristalsis. 



As to the cause of the gastric movements, it is not 

 certain how far they are myogenic and how far they are 

 to be ascribed to Auerbach's plexus. 



The Pyloric Sphincter 



The pylorus remains firmly contracted during the whole 

 of digestion except at regularly recurring intervals of 

 momentary duration, during which it opens and allows a 

 small part of the gastric contents to be squirted through. 

 When this has happened it immediately closes again. 



