116 THE HUMAN MECHANISM 



This is a reason for introducing the soup early at a course 

 dinner. Meat extracts and meat juices are the most effective 

 food constituents for this purpose ; milk and water are far 

 less effective, while most foods, notably bread, white of eggs, 

 etc., have no such effect at all. 



3. Stimulation of the stomach itself by the products of protein 

 digestion. Although the mere contact of most foods with the 

 lining of the stomach does not evoke a secretion of gastric 

 juice, yet it is known that after digestion has been begun 

 by the action of the " psychic " secretion, certain of the 

 products of protein digestion arouse a second secretion ,by 

 acting directly on the lining of the stomach. This second 

 secretion increases in amount as the first (or " psychic " ) 

 secretion diminishes, and continues throughout the remaining 

 period of gastric digestion. 



To sum up : The secretion of the gastric juice is initiated 

 by a complicated series of nervous processes connected with 

 the enjoyment of the food while it is being taken and masti- 

 cated ; this is aided to some extent by direct stimulation of 

 the lining of the stomach by a few food constituents, notably 

 the extractives of meat. The gastric juice thus secreted acts 

 upon the proteins of the food and produces from them diges- 

 tive products which directly stimulate the stomach to secrete 

 and, in fact, maintain the secretion to the end of the period 

 of gastric digestion. Without the "psychic" secretion pro- 

 teins are not digested fast enough to induce sufficient sub- 

 sequent secretion ; without the stimulus of the products of 

 protein digestion the " psychic " secretion does not suffice to 

 complete the digestion of a hearty meal a labor which may 

 require four or five hours. 1 



1 What we have called the "psychic" secretion is probably an uncondi- 

 tioned reflex from the mouth, reenforced by a conditioned reflex involving the 

 action of the cerebrum ; the stimulation by the products of protein digestion 

 and possibly that by meat extracts, on the other hand, is probably due to 

 a hormone (p. 89) liberated in the mucous membrane of the pyloric region, 

 thence passing into the blood, and so stimulating the gastric glands to secrete. 



