936 THE EXTERNAL SECRETIONS 



extremely sensitive epithelium of this organ to cease secreting. Later 

 on Pawlow conceived the idea of permitting the cardiac fibers of the 

 vagus to degenerate before attempting to test the power of this nerve 

 upon the pancreas. In all these cases, the stimulation of the distal end 

 of this nerve, 3 or 4 days after its division, evoked a copious flow of 

 pancreatic juice, but in spite of the fact that the existence of these 

 reflex paths has been thoroughly established, no definite statements 

 can be made at this time regarding the relative importance of the 

 nervous and chemical stimuli. At all events, the impulses from the 

 mouth as well as those of psychic origin seem to play a less important 

 part in the secretion of pancreatic juice than in that of gastric juice. 



Inasmuch as a formation of pancreatic juice also takes place 

 after the pancreas has been completely isolated from the central 

 nervous system, Popielski^ came to the conclusion that this organ em- 

 braces a local nervous mechanism which is sufficient for its activity. 

 The claim was then made by Dolinsky^ and Pawlow that the active 

 factor involved in this secretion is the acid of the gastric juice which 

 exerts its peculiar action as soon as the chyme is discharged into the 

 duodenum. In substantiation of this contention Popielski proved 

 later on that the injection of an acid into a segment of the duodenum 

 gives rise to a copious flow of pancreatic juice even after both vagi 

 nerves and the ramifications of the solar plexus have been cut. This 

 fact immediately disposed of the contention that the acid stimulates 

 the duodenal mucosa and thereby sets up certain afferent impulses 

 which are finally relayed to the pancreas by way of the corresponding 

 fibers of the vagus system. On repeating these experiments, Bayliss 

 and Starling^ discovered that a secretion of pancreatic juice also results 

 if the acid is introduced into a loop of duodenum which had previously 

 been isolated from the rest of the body by dividing all its nervous 

 connections. This result showed very clearly that a chemical stimulus 

 must be at work which affects the pancreatic cells through the blood 

 stream. In further substantiation of this view these authors found 

 that an acid, when injected into the portal vein, remains without effect, 

 while an acid infusion of the mucous membrane of the duodenum, when 

 neutralized and injected into the same channel, gives rise to a copious 

 flow of pancreatic juice. These tests were then controlled by the use 

 of similar extracts of other organs, but none of these yielded positive 

 results. 



The conclusion to be derived from these data is that the activity 

 of the pancreas is also under the control of a hormone which originates 

 in the mucosa of the duodenum and upper small intestine. This agent 

 is called secretin. It should be noted, however, that it is not present 

 as such in these cells, but is held in them in an inactive form to which 

 the name of pro-secretin has been given. In consequence of the 



1 Centralbl. fiir Physiol., x, 1896, 405, and xix, 1906, 801. 



2 Dissertation, St. Petersburgh, 1894. 



3 Jour, of Physiol., xxviii, 1902, 325. 



