120 The Unity of the Organism 



The mode of operation of this secretion is tersely stated 

 by Bayliss: "Food entering the duodenum causes the pro- 

 duction of a special substance which enters the blood and 

 excites the pancreas to pour into the duodenum a digestive 

 juice." 5 



That the presence of various substances, especially acids, 

 in the duodenum, induces a flow of pancreatic juice was 

 known when Bayliss and Starling began their work in this 

 field; but up to that time the excitation of the pancreas to 

 such action was supposed to be through a nerve reflex. 

 These investigators had reported in 1902 6 that acid in the 

 duodenum is able to cause the pancreas to secrete after 

 nervous communication between the intestinal wall and the 

 pancreas is excluded. They went further and obtained an 

 extract from the duodenal mucous membrane which, being 

 injected into a vein, induced a copious flow of pancreatic 

 juice. The substance, whatever it is, which acts thus they 

 call secretm. It has been surmised by a few physiologists 

 that the effects are not due to the direct action of the secre- 

 tion on the pancreatic gland-cells, but that the influence 

 is exerted through the vaso-dilator mechanism. But this 

 surmise is negatived by the demonstration that the secretin 

 will induce the flow of pancreatic juice while it does not 

 alter the blood pressure. The case seems, then, fully estab- 

 lished, and is so clear-cut and relatively simple an instance 

 of the coordinated functioning of two wholly distinct parts 

 of the body through chemical means, that it is desirable 

 to get sharply before us the known steps in the process. 

 In the course of normal digestion, food acidulated in the 

 stomach passes into the duodenum. Here, probably in 

 virtue of its acidity, it acts upon the cells of the mucous 

 membrane in such a way as to induce them to produce a 

 substance which is discharged, not into the intestines, there 

 to take its part in digestion, but into the blood. By the 

 blood stream the substance is carried through its whole 



