492 DIGESTION 



sphincter is as follows: When acid is placed in the duodenum through 

 a fistula, the sphincter will not open ; when the alkaline secretions of 

 the liver and pancreas are excluded from the duodenum by the ligation 

 of the bile and pancreatic ducts the evacuation of the stomach is delayed ; 

 if acid is excluded by suturing the pylorus to the intestine below the 

 duodenum, the evacuation of the stomach is hastened. Water and egg 

 white may leave the stomach independently of any acid reflex control of 

 the pylorus. By observations made through a duodenal fistula, it has 

 been found that, after a quantity of water has been swallowed, most if 

 not all of it very soon enters the duodenum in a more or less continuous 

 stream. It is no doubt on this account that drinking contaminated water 

 is especially dangerous on an empty stomach. 



The nervous pathway through which these acid reflexes take place has 

 been shown to be the myenteric plexus. Indeed, the whole mechanism 

 is quite analogous with that which we shall see occurs in the intestine 

 during peristalsis: the stimulus, that is, the acid, causes a contraction 

 of the gastric tube behind.it and a dilatation in front. 



Fig. 158. Outlines of shadows in abdomen obtained by exposure to x-rays 2 hours after 

 feeding with food containing bismuth subnitrate. The food in A was lean beef, and in B boiled 

 rice. The smaller size of the stomach shadow and the much greater total area of the intestinal 

 shadows in B than in A show that carbohydrate leaves the stomach earlier than protein. (From 

 Cannon.) 



Rate of Emptying- of Stomach 



The relationship of these facts to the rate at which different foodstuffs 

 leave the stomach is very readily explained. The method for investigat- 

 ing this problem, which again we owe to Cannon, consists in feeding ani- 

 mals with a strictly uniform amount of different foods made up, as 

 nearly as possible, of equal consistency and containing bismuth subni- 

 trate in the proportion of 5 gin. to each 25 c.c. By feeding such mix- 

 tures to cats previously starved for twenty-four hours, and examining 

 the abdomen by the x-ray at regular intervals, the shadows cast by the food 

 after passage into the intestine can be outlined on tracing paper, and 



