NERVOUS SYSTEM UPON SECRETION 183 



directly. The view that the acid produces its effect by absorption 

 into the blood is then negatived by Pawlow, from theoretical con- 

 siderations, as well as from the fact that injection of acid into the 

 rectum was without effect upon pancreatic secretion. 



It did not occur, however, to the discoverers of the secretion 

 of the pancreas as a specific result of the presence of acids in the 

 duodenum, that there was a third hypothesis, namely, that the 

 acid might awaken an internal secretion in the duodenal cells, 

 and that the substance so secreted might travel in the blood stream 

 to the pancreatic cells and set them in activity. 



This view did occur to Bayliss and Starling, who, on testing 

 it experimentally, found it to be the correct one, and so not only 

 brilliantly supplemented the work of the St. Petersburg school 

 on pancreatic secretion, but made a new departure in our know- 

 ledge regarding secretory processes, and opened up a new field 

 to research which is now being explored for other secretions. 



THE CHEMICAL MECHANISMS OF SECRETION CHEMICAL 

 EXCITANTS OF SECRETION OR HORMONES 



Pancreatic Secretion and Secretin. The apparently local char- 

 acter of the reaction when acid was placed in the intestine, 

 described in the preceding section, led Bayliss and Starling to 

 experimentation upon the subject, from the view that there might 

 here be an extension of the local reflexes, the action of which in 

 movements of the intestinal wall these observers had already 

 investigated. It was soon found, however, that the phenomenon 

 was one of an entirely different order, and that the secretion of 

 the pancreas is normally called into action not by nervous agency 

 at all, but by a chemical substance formed in the mucous membrane 

 of the upper parts of the small intestine under the influence of 

 acid, and carried thence by the blood stream to the gland cells 

 of the pancreas. To the active substance the name secretin has 

 been given by the authors. 



In the earlier experiments of Bayliss and Starling, dogs were 

 used, but in a later research other animals were used (rabbit, cat, 

 and monkey), and it was demonstrated that the reaction is a general 

 one for all vertebrates. 



The animals received an injection of morphia previous to the 



