THE PANCREATIC JUICE 755 



in which the inhibitory predominate, and by the further fact that the 

 pancreas is extremely susceptible to alterations in its blood supply, so that 

 any stimulation of the vagus which caused inhibition of the heart would 

 ipso facto prevent the effect of simultaneous excitation of secretory fibres 

 from making its appearance. Pawlow noticed that if in an animal with a 

 permanent fistula the vagus on one side were cut and left for four days in 

 order to allow the cardio -inhibitory fibres to degenerate, repeated stimula- 

 tion of the peripheral end of the nerve evoked a flow of pancreatic juice. 

 He obtained the same results by stimulating this nerve below the point at 

 which it had given off its cardio-inhibitory fibres, in animals in which the 

 reflex inhibitions from the operation itself were prevented by total section 

 of the medulla. Under certain circumstances he obtained also secretion 

 on stimulation of the splanchnic nerves, and therefore concluded that these 

 two nerves splanchnics and vagi were the efferent channels in the reflex 

 secretion set up by the introduction of the acid into the duodenum. It 

 was shown later however independently both by Popielski, a pupil of 

 Pawlow, and by Wertheimer, that the injection of acid into a loop of small 

 intestine was followed by secretion of juice even after section of both vagi 

 and destruction of the sympathetic ganglia at the back of the abdominal 

 cavity. On repeating these experiments Bayliss and Starling found that a 

 secretion of juice was produced even when the acid was introduced into a 

 loop of the small intestine entirely freed from any possible nervous connec- 

 tions with the rest of the body. It was evident therefore that the stimulus 

 or message from the intestine to the pancreas which causes the secretion of 

 the latter must be carried, not by the nervous system, but by the blood 

 stream. Since the injection of acid into the portal vein was without effect 

 on the pancreas, it was concluded that something must be produced in the 

 epithelial cells of the gut under the influence of acid, and that this product 

 of the epithelial cells -was absorbed in the blood stream and was the active 

 agent in exciting the pancreas. On pounding up some scrapings of the 

 intestinal mucous membrane with dilute hydrochloric acid and filtering, and 

 injecting the filtrate, a copious flow of pancreatic juice was produced. This 

 chemical messenger or hormone from the intestine to the pancreas is called 

 ' secretin,' or c pancreatic secretin ' to distinguish it from possible other 

 members of the same class. It is produced in the mucous membrane from 

 a precursor pro-secretin. The latter has not been isolated, but that it is 

 present in the mucous membrane is shown by the fact that secretin can be 

 extracted by the action of acids from mucous membrane which has been 

 killed by heat or by the prolonged action of alcohol. 



Secretin itself is not a ferment. In order to prepare it the mucous 

 membrane is ground up with sand, boiled with 04 per cent, hydrochloric 

 acid, and then neutralised while boiling by the cautious addition of sodium 

 hydrate. The coagulable proteins are in this way precipitated, and the 

 filtered solution contains the secretin. It is not precipitated by the ordinary 

 alkaline reagents, and diffuses slowly through animal membranes Though 

 stable in acid solutions, it is very rapidly destroyed in alkaline or neutral 





