THE PANCREATIC JUICE 709 



partly of an inhibitory, partly of a secretory nature, 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 ipsojacto 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 stimulation of the peripheral end of the nerve 

 evoked a flow of pancreatic juice. He obtained the same results by stimu- 

 lating 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 there- 

 fore 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 connections 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 ' 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 0-4 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 



