THE INTESTINE 61 



involved an important modification of our conception as to the 

 empire of the nervous system. The production of secretin is, 

 in fact, the best authenticated examjple of internal secretion 

 which can be quoted. The hitherto most oft-cited examples, 

 such as those of the adrenal body and the thyroid gland, 

 are, in the opinion of the present writer, more distinctly 

 hypothetical. 



Beyond all doubt secretin is a powerful excitant of the 

 pancreatic secretion, but its specific nature is denied by some 

 authors. Bayliss and Starling admit that secretin acts to a 

 small degree on the secretion of bile, and other authors state 

 that it acts on the secretion of saliva and the gastric and 

 intestinal juices. Again, while Bayliss and Starling affirm 

 that secretin can only be obtained from the upper part of the 

 small intestine, others assert that they have found this sub- 

 stance, though only in small amount, in other parts of the 

 intestinal tract, and even in lymphatic glands. 



Although Bayliss and Starling showed that the depressor 

 substance in extracts of the intestinal mucous membrane is 

 independent of secretin, and that a secretin solution can be 

 obtained free from the depressor substance, yet this has not 

 hindered Popielski and others from urging that the flow of 

 pancreatic juice after injection of intestinal extracts is due to 

 the lowering of the blood-pressure and consequent anaemic 

 stimulation of nerve centres. 



Meig shows that secretin does not produce its effects by 

 acting on the nerve endings in the intestinal epithelium, and 

 the last-named author, as well as Wertheimer and Le Page, 

 state that they obtained a secretion of pancreatic juice by 

 injecting acid into an isolated loop of jejunum whose nerve 

 communications were intact, and even when the venous blood 

 of this loop is diverted, and the thoracic duct is tied off. Fleig 

 concludes that the mechanism of secretion of pancreatic juice 

 after the injection of acid into the upper part of the small 

 intestine is, under normal conditions, of a double character : 

 (1) Secretin calls forth secretion of pancreatic juice by direct 

 action on the cells of the pancreas. (2) Acids, independently 

 of the formation of secretin, cause a secretion of pancreatic 

 juice by reflex nervous action. 



These two secretions are said by Sawitsch to present quite 

 different characters. The latter is thick, opalescent, rich in 



