230 PHYSIOLOGY OF ALIMENTATION. 



decreased. But not until all the acid in the duodenum has 

 been neutralized will the production of pancreatic secretin, 

 and, in consequence, the pancreatic flow, cease entirely. 



This chemical interaction between the stomach, duodenum, 

 and pancreas is further aided by the rhythmic opening and 

 closing of the pyloric sphincter, which has already been de- 

 - scribed. 1 The presence of free hydrochloric acid in the 

 stomach causes the pyloric sphincter to relax and some of 

 the gastric contents to pass into the duodenum. As soon 

 as the acid reaches this part of the intestinal canal, however, 

 the sphincter is closed reflexly, and remains closed until the 

 duodenal contents have become neutralized by being mixed 

 with pancreatic juice. As soon as this neutralization has 

 been accomplished, the pylorus relaxes a second time and a 

 fresh portion of the gastric contents enters the duodenum. 

 This in its turn calls forth a production of pancreatic secretin, 

 a secretion of pancreatic juice, and the cycle is repeated. 



6. Significance of the Physiology of the Gastric and Pan- 

 creatic Secretions. The advances made in our knowledge of 

 gastric and pancreatic activity have done much to give us an 

 experimental basis for what has hitherto been empirical in 

 medical practice and have at the same time pointed out the 

 direction which medicine must take in combating certain 

 affections of the alimentary tract. The experimental results 

 detailed above show most clearly the physiological impor- 

 tance of the appetite. Appetite is synonymous with a copious 

 secretion of gastric juice, and the necessity of this secretion 

 for a proper and rapid digestion of proteins and the evil 

 effect of its absence are manifest nowhere more clearly than 

 in those disorders which are associated with a deficient secre 1 - 

 tion from the stomach. We recognize, in consequence, the 

 importance of catering to the appetite in health, in order to 

 maintain, gastric digestion at its best, and the urgency of 

 restoring this physiological sense in those diseases in which 



1 See p. 16. 



