THE MECHANISMS OF DIGESTION 491 



are driven against the pyloric sphincter by the peristaltic waves, instead of 

 being returned as an axial stream into the stomach, are ejected into the 

 duodenum. 



We must now consider the mechanism by which the pyloric sphincter 

 opens to permit the passage of the chyme. Bombardment by the peri- 

 staltic waves is evidently not the cause of its opening, for, as we have 

 seen, many such waves may arrive at it without this result. Since it is 

 evidently in order that the intestine may not suddenly become over- 

 whelmed with large masses of food that the pylorus only occasionally 

 opens, it might be thought that its opening depends upon the degree of dis- 

 tention of the upper part of the intestine. It is true that excessive disten- 

 tion of the upper part of the intestine does hold the pyloric sphincter 

 closed, but this can not be the physiological stimulus, because considerable 

 quantities of chyme are never found here. 



The first clue to the real nature of the mechanism was afforded by 

 observing the behavior of the sphincter when solutions are introduced 

 into the duodenum through a fistula. Acid solutions were found to 

 cause a complete inhibition of gastric evacuation, whereas alkaline solu- 

 tions had no effect. This difference indicates that acids in contact with 

 the duodenal mucous membrane reflexly excite contraction of the sphinc- 

 ter, and that it relaxes only after the acid has become neutralized 

 by mixing with the pancreatic juice and bile. 



On account of the great importance of the pyloric mechanism in insur- 

 ing that the chyme shall enter the intestine only in such quantities that 

 it can be properly acted upon by the intestinal digesting juices, it will 

 be of interest to consider briefly some of the experimental observations 

 by which this mechanism has been studied. We may consider first the 

 evidence that acid on the stomach side of the pylorus causes a relaxation 

 of the sphincter: (1) When carbohydrate food is fed, it ordinarily leaves 

 the stomach fairly rapidly, but if its acid-absorbing power is increased 

 by mixing it with sodium bicarbonate, exit from the stomach is greatly 

 delayed. (2) Proteins ordinarily leave the stomach more slowly than 

 carbohydrates, but if acid proteins are fed, their exit is much more 

 rapid. (3) If a fistula is made into the pyloric vestibule through which 

 some of the contents can be removed, it will be found that just prior' to 

 the opening of the pyloric sphincter, a distinctly acid reaction develops 

 in the food ; and furthermore if acid solutions are injected through this 

 fistula, they cause the pyloric sphincter to open, whereas alkalies retard 

 its opening. (4) A similar effect of acid in opening the sphincter can 

 be demonstrated by applying it to the pyloric mucosa of an excised 

 stomach kept alive in oxygenated Ringer's solution. 



The. evidence that acid on the duodenal side causes closure of the 



