THE DIGESTIVE JUICES 88 



THE SECRETION OF PANCREATIC JUICE 



One of the most effective ways of producing a flow of the juice 

 is to introduce acid into the duodenum, and no doubt the acid of the 

 gastric juice is the normal stimulus for the pancreatic flow. This flow 

 still occurs when all the nerves supplying the duodenum and pancreas 

 are cut, and it was held by Popielski and by Wertheimer and Le 

 Page that it must be due to a local reflex, the centres being situated 

 in the scattered ganglia of the pancreas and of the solar plexus. 

 Starling and Bayliss, however, pointed out that it cannot be a 

 nervous reflex, since it occurs after extirpation of the solar plexus, 

 and destruction of all nerves passing to an isolated loop of intestine. 

 Moreover, atropine does not paralyse the secretory action. It must 

 therefore be due to direct excitation of the pancreatic cells by a 

 substance or substances conveyed to the gland from the bowel by the 

 blood-stream. 



The exciting substance is not acid ; injection of 0*4 per cent, of 

 hydrochloric acid into the blood-stream has no influence on the 

 pancreas. The substance in question must be produced in the 

 intestinal mucous membrane under the influence of the acid. This 

 conclusion was confirmed by experiment. If the mucous membrane 

 of the duodenum or jejunum is exposed to the action of 0'4 per cent, 

 hydrochloric acid, a body is produced which, when injected into the 

 blood-stream in minimal doses, produces a copious secretion of pan- 

 creatic juice. This substance is termed secretin. It is associated with 

 another substance which lowers arterial blood-pressure. The two 

 substances are not identical, since acid extracts of the lower end of 

 the ileum produce a lowering of blood-pressure, but have no excitatory 

 influence on the pancreas. 



Secretin is split off from a precursor, pro-secretin, which is present 

 in relatively large amounts in the duodenal mucous membrane, and 

 gradually diminishes in amount throughout the intestine until it 

 entirely disappears in the ileum. Pro-secretin can be dissolved 

 out of the mucous membrane by normal saline solution. It has no 

 influence on the pancreatic secretion. Secretin can be split off from 

 it by boiling or by treatment with acid. 



What secretin is chemically, we do not yet know. It is soluble in 

 alcohol and ether. It is not a protein, but probably is an organic 

 substance of low molecular weight. It is, moreover, the same sub- 

 stance in all animals, and not specific to different kinds of animals. 



Whether there are any secretory nerves for the pancreas is at 

 present doubtful. Pawlow thought he had discovered them in the 



G2 



