en. XXXTL] succus ENTERICUS 495 



that the secretion cannot be reflex, since it occurs after extirpation of 

 the solar plexus, and destruction of all nerves passing to an isolated 

 loop of jejunum. 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. So many of the connections between 

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

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

 compare to the postman. 



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 jejunum or duodenum 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 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 proteid, 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 this discovery is an isolated instance of chemical sym- 

 pathy between different organs, or whether it is only one of a class of 

 similar mechanisms, must be left to the future. It certainly shows 

 that some revision of, or addition to, Pawlow's work is necessary. In 

 none of Pawlow's experiments on the pancreas was a possible expul- 

 sion of acid from the stomach into the duodenum excluded. 



The Succus Entericus. 



Succus entericus has been obtained free from other secretions by 

 means of a fistula. Thiry's method is to cut the intestine across in 

 two places; the loop so cut out is still supplied with blood and 

 nerves, as its mesentery is intact ; this loop is emptied, one end is 



