200 HUMAN PHYSIOLOGY 



their agreeable qualities. The paths of the impulses 

 which go from the brain to the stomach at such times to 

 excite the secreting cells are in the vagus nerves. 



Some foods which are of the highest nutritive value 

 eggs, for example may be introduced into the stomach 

 of a dog and no secretion will start if the dog has not had 

 the opportunity to enjoy the meal. This by itself 

 might lead to the belief that no gastric juice is produced 

 except under the stimulus of pleasure but this is not 

 actually the case. Some foods produce the reaction by 

 their effect on the lining of the stomach. This has been 

 shown to be true of meats. It is claimed that the 

 dextrins formed in salivary digestion have the same 

 desirable action. 



Compounds which have the power to excite the gastric 

 glands directly are known as secretagogues. We are 

 quite sure that meat contains bodies deserving this name 

 but there is much disagreement concerning the right of 

 other things, such as alcohol and condiments, to the 

 title. If the dextrins are secretagogues the fact is of 

 interest because it indicates a connecting link between 

 the salivary and the gastric process. A successful sali- 

 vary digestion would prepare the way for an adequate 

 gastric secretion. Foods, like eggs which contain no 

 secretagogues when undigested, are thought to yield 

 something of the kind when their constituents have 

 undergone some cleavage in the stomach. 



Summarizing the causes of gastric secretion we may 

 say that it is induced (1) by pleasurable feelings or (2) 

 by certain stimulating substances in the food. What- 

 ever the means by which it is at first excited it is main- 

 tained in the later hours of the gastric digestion by 

 secretagogues arising as by-products of the process 

 itself. The formation of such compounds will cease 

 only when the stomach is empty, so the continuance of 

 the flow until that time is assured. 



Functions of the Acid. The hydrochloric acid of the 

 gastric juice is necessary to the type of digestion which 



