PHYSIOLOGY OF THE DIGESTIVE GLANDS 473 



the main stomach was stimulated mechanically, as by placing inert 

 objects such as a piece of sponge or sand in the stomach, no secretion 

 occurred. Evidently, therefore, the stimulus is dependent upon some 

 chemical quality of the food. 



By introducing various foods it was found that there is considerable 

 difference in the degree to which they can excite the secretion. Water, 

 egg white, bread and starch, were all found to have very little if any 

 effect. On the other hand, when protein that had been partly digested 

 by means of pepsin and hydrochloric acid was introduced into the 

 stomach, it immediately called forth a secretion. The conclusion is that 

 the partly digested products, even of insipid food, are capable of directly 

 exciting the secretion. These include proteoses and peptones, and it 

 was, therefore, of great interest to find that a solution of commercial 

 peptone is also an effective stimulus. This is a result of deep significance, 

 for it indicates that the food which has been partially digested by the 

 appetite juice will serve as a stimulus to continued secretion. 



The psychic juice has been aptly called the "ignition juice, " because 

 by producing partial digestion it serves to ignite the process of gastric 

 secretion. Experimental evidence of its great importance in gastric 

 digestion was secured by Pavlov in experiments in which he placed 

 weighed quantities of meat attached to threads in the stomach through 

 a gastric fistula, and after some time removed them and determined by 

 the difference in weights the extent to which they had become digested. 

 It was found that when the appetite juice was excited by sham feeding 

 at the same time that food was placed directly in the stomach, its diges- 

 tion was much more rapid than in cases in which it was placed in the 

 stomach without the animal's knowledge, as when he was asleep. 



Other foods having a direct stimulating effect on the gastric secre- 

 tion are meat extracts and, to a certain extent, milk. This effect of meat 

 extract is interesting in connection with the practice of taking soup as 

 a first or early stage in dining. It not only excites the appetite juice, 

 but also serves as a direct stimulus to the gastric secretion. 



As to the nature of the mechanism by which this direct secretion takes 

 place, it was shown by Popielski 12 that the secretion still occurs after all 

 the nerves proceeding to the stomach are cut. Evidently, therefore, it 

 is independent of the extrinsic nerve supply of the viscus. As a result 

 of his experiments Popielski concluded that the secretion must depend 

 on a local reflex mediated through the nerve structures present in the 

 walls of the stomach itself. Another explanation of the result has, 

 however, in recent years been given more credence by the experiments of 

 Bayliss and Starling on the influence of hormones on the secretion of 

 pancreatic juice (cf. page 460). Eclkins 1 " suggested that a similar 



