CHEMICAL MECHANISMS OF SECRETION 187 



membrane of the lower end of the ileum possesses when in- 

 travenously injected no exciting effect upon the pancreatic 

 secretion. 



With regard to the seat of action of secretin, Bayliss and Starling 

 have traced it as far as possible towards the periphery, and con r 

 elude that it acts in all probability directly as a chemical excitant 

 upon the secreting cells of the pancreas. It is impossible with our 

 present experimental methods to exclude a possible action upon 

 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 the 

 anatomical relationships, from connection with nervous mecha- 

 nisms, both central and peripheral. The sensitiveness of the 

 pancreas renders practically impossible the experiment of per- 

 fusion 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, taken from Bayliss and Starling's 

 paper : 



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, 



