292 CHEMICAL MECHANISMS OF SECRETION 



the nerve cells and fibres in the pancreas itself ; just as it is im- 

 possible to do so in the case of tracing towards the periphery the 

 seat of action of any drug or other active substance for example, 

 to exclude an action of adrenalin upon nerve cells or endings upon 

 the muscular walls of small arteries rather than upon the muscle 

 cells directly. But it has been shown that the excitatory effect 

 upon the pancreatic secretion is still obtained after the gland has 

 been cut off, as far as is experimentally possible from its anatomical 

 relationships, from connection with nervous mechanisms, both 

 central and peripheral. The sensitiveness of the pancreas renders 

 practically impossible the experiment of perfusion of whipped blood 

 containing secretin through the excised gland. 



Certain physical and chemical properties of secretin solution 

 have also been investigated by Bayliss and Starling and W. A. 

 Osborne, as well as the properties of the pancreatic juice secreted 

 as a result of the action of secretin ; the results are summarised 

 in the following conclusions : 



1. The secretion of the pancreatic juice is normally evoked 

 by the entrance of acid chyme into the duodenum, and is pro- 

 portional to the amount of acid entering (Pawlow). This secre- 

 tion does not depend on a nervous reflex, and occurs when all 

 the nervous connections of the intestine are destroyed. 



2. The contact of the acid with the epithelial cells of the 

 duodenum causes in them the production of a body (secretin), 

 which is absorbed from the cells by the blood- current, and is carried 

 to the pancreas, where it acts as a specific stimulus to the pancreatic 

 cells, exciting a secretion of pancreatic juice proportional to the 

 amount of secretin present. 



3. This substance, secretin, is produced probably by a process 

 of hydrolysis from a precursor (prosecretin) present in the cells, 

 which is insoluble in water and alkalies and is not destroyed by 

 boiling alcohol. 



4. Secretin is not a ferment. It withstands boiling in acid, 

 neutral, or alkaline solutions, but is easily destroyed by active 

 pancreatic juice or by oxidising agents. It is not precipitated 

 from its watery solutions by tannic acid, or alcohol and. ether. 

 It is destroyed by most metallic salts. It is slightly diffusible 

 through parchment paper. 



5. The pancreatic juice obtained by secretin injection has no 

 action on proteins until " enterokinase " is added. It acts on 



