438 THE HUMAN BODY 



Pawlow's studies showed, moreover, that the psychical secre- 

 tion is not the only secretion of gastric juice which occurs during 

 the digestion of a meal. This was proved by the simple observa- 

 tion that the amount of juice secreted during the eating of a 

 "fictitious meal" is much less than that produced if the food 

 eaten enters the stomach. We must look, then, for some other 

 stimulating agency additional to the psychical one. In such a 

 search the attention turns naturally to the foods swallowed. Do 

 they serve as chemical stimuli for the production of the additional 

 secretion? It has been shown that some foods, milk and water 

 very slightly, the juices of meat more, do excite the secreting 

 mechanism somewhat, but the really effective excitant appears 

 to be something produced during the process of gastric digestion 

 itself. Thus if the taking of food is attended with pleasure, so that 

 a psychical secretion is produced, the digestive process is started 

 and itself furnishes the stimulating agent for the additional se- 

 cretion needed to complete the digestion. On the other hand, 

 food eaten under conditions not favorable to the production of a 

 psychical secretion may fail of digestion completely, through the 

 absence of all factors which may lead to an outpouring of the juice. 



Nature of the Chemical Stimulus to Gastric Secretion. It has 

 been shown that the substances mentioned in the last paragraph 

 as chemical excitants of gastric secretion do not stimulate the 

 glands directly but indirectly through a hormone, gastric secretin. 

 This hormone is apparently derived from some substance in the 

 mucous membrane of the pyloric region, which reacts with the 

 exciting substances derived from the food in such fashion as to 

 produce the hormone, which is then taken up by the blood and 

 carried to the gastric glands. 



Control of the Pancreatic Secretion. Proper regulation of the 

 outpouring of pancreatic juice requires that it begin about the 

 time food begins to pass from the stomach into the small intes- 

 tine. Since this may occur at a variable time after the eating of 

 the meal, it would seem to call for a regulating mechanism quite 

 independent of the act of eating. It has been shown that this 

 requirement is fulfilled through the action of a hormone which 

 is produced in active form during the time that food is passing 

 from the stomach into the small intestine, and only then. The 

 mucous membrane of the small intestine at its upper end contains 



