INTRODUCTION 3 



of events leading to the response of an effector at a 

 distance from the point stimulated may take place by 

 way of an elaborate nervous mechanism or by means 

 of a humoral transmission. For example, when a tasty 

 food is taken into the mouth, saliva is secreted by the 

 salivary glands although the food stimulus is at a dis- 

 tance from the glands. As is well known, this effector 

 response is brought about by the stimulation of sensory 

 nerve endings in the mouth, the nervous impulse thus 

 started passing by way of the brain to the salivary 

 glands. 



Bayliss and Starling's classical investigation of se- 

 cretin offers perhaps the most convincing evidence 

 of the humoral transmission of a stimulus. These 

 workers found that acids acting upon the intestinal 

 mucosa of a denervated loop of intestine caused the 

 pancreas to secrete. Since the injection of acids into 

 the portal vein did not induce a similar pancreatic ac- 

 tivity, it was concluded that the acid acted upon some 

 component of the intestinal mucosa to produce a sub- 

 stance, termed secretin, which was carried by way of 

 the circulation to the pancreas and there caused the 

 effector response. 



The validity of this assumption was established by 

 the observation that when intestinal mucosa is ground 

 with dilute hydrochloric acid, and then boiled to 

 coagulate the soluble proteins, an extract results which 

 upon injection into an animal induces the pancreas to 

 secrete. Such experiments yield irrefutable proof of 

 the secretin hypothesis, although to date the chemical 

 nature of secretin remains an open problem. 



The type of reasoning employed in connection with 

 the above mentioned experiments on secretin is com- 



