CH. XXXIV.] SECRETION OF THE PANCREAS 521 



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. So many of the connections between 

 organs are made by nerves (the telegraphic service of the body), that 

 we are apt to forget the other messenger, the blood, whom we may 

 compare to the postman. 



The exciting substance is not acid; injection of 04 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 jejunum or duodenum is exposed to the action of 0'4 per cent, 

 hydrochloric acid, a substance is produced which, when injected into 

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

 pancreatic juice, and also, but to a less extent, of bile. 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 as we descend the intestine. 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. 



Pawlow by experiments of a similar nature to those which led 

 him to the discovery of the secretory nerves of the gastric mucous 

 membrane, thought he had also discovered the secretory nerves of 

 the pancreas in the vagus, and to a less extent in the splanchnic 

 nerves. His failure to produce this result in some experiments he 

 explained by the concomitant stimulation of secreto-inhibiting fibres. 

 It is quite possible that nerves of this nature exist ; but Pawlow's 

 experiments do not prove their existence, because the passage of acid 

 chyme into the duodenum was not excluded, and so he may only 

 have been dealing with a production of secretin, the chemical 

 stimulus to pancreatic activity. 



Starling's work on secretin naturally led him and others to seek 

 for other chemical messengers employed in the regulation of the 

 activities of the body, and it has already been established that 

 secretin is by no means a solitary instance of such. The general 

 name given to these agents is that of hormone. The chemical 



