23 



is absorbed into the blood and provokes the secretion of more intestinal 

 juice lower down in the canal, in preparation for the reception of the 

 advancing mass of food. 



We see that, from the entry of the food into the stomach until 

 its passage through the ileo-caecal valve, there is a continuous chain of 

 chemical reflexes, and that the process in any section of the alimentary 

 canal calls forth the activity of the digestive apparatus in the immediately 

 following section. In the stomach this chemical mechanism or reflex is 

 associated with, and probably subordinated to, a nervous reflex mechan- 

 ism. In the rest of the alimentary canal the chemical mechanism seems 

 sufficient to account for the secretion of all the digestive juices which are 

 demanded by the food. Whether there is in addition a chain of nervous 

 processes, the evidence at present before us is not sufficient to decide, 

 though we ourselves have been unable to obtain any satisfactory evidence 

 of their existence. The mechanisms I have described suffice to explain in 

 large measure the adaptation of the digestive processes to variations in 

 the quality of the food supplied. As a rule the more indigestible the 

 foodstuff, the longer will it remain in the stomach ; the greater, there- 

 fore, will be the secretion of acid gastric juice, which is the stimulus 

 setting free the chain of processes below the pyloric sphincter. 

 Increased secretion of gastric juice will be attended automatically 

 with increased secretion of the other digestive juices. 



There is, however, evidence of a more specific adaptation of certain 

 of the digestive juices to ,the nature of the foodstuffs. Experiments 

 carried out in Pawlow's laboratory have shown that the saliva poured 

 into the mouth varies in consistence and other qualities, according to the 

 nature of the food or other substances introduced into the mouth. Thus 

 introduction of sand into a dog's mouth evokes profuse secretion of 

 watery saliva ; meat provokes secretion of thick, viscid saliva; bread of 

 thin saliva, chiefly from the parotid gland, and so on. The same results 

 can be evoked by showing the dog these substances i.e., by the psychic 

 reproduction of previous actual stimulation of peripheral sense organs. 



In the stomach the psychic secretion, which is so important in 

 inaugurating the digestion of the food, is apparently unaffected by altera- 

 tions in diet, any variations which may be caused being sufficiently 

 explained by the different digestibilities and therefore different length of 

 stay of the foodstuffs in the stomach. In the case of the pancreas, how- 

 ever, according to Walther and Vasilieff, there is an accurate adaptation 

 of the composition of the juice to the nature of the food, the dog that has 

 been on a bread diet secreting more amylopsin, while the dog on a proteid 

 diet secretes a larger amount of trypsin or trypsinogen. These results 

 have been controverted by Popielski, according to whom the composition 

 of pancreatic juice is determined solely by the strength of the stimulus 

 which the pancreas receives. Further experimental work is still required 

 on the subject. If Walther's contentions are supported, it will be interest- 

 ing to determine whether the adaptation of the pancreatic activity to the 

 nature of the food is nervous in character as imagined by Pawlow, or 



