328 STUDIES IN ADVANCED PHYSIOLOGY. 



proteids. Although first discovered by Corvisart the pro- 

 teid-digesting nature of pancreatic juice was finally worked 

 over and established with its present exactness by Ku'hne 

 in 1867. The intestinal juice was first secured in a pure 

 form by Thiry in 1865. 



THE DIGESTIVE AGENTS. 



We have now to consider the various changes, chemical 

 and physical, by means of which the solid and usually in- 

 soluble food is converted into a liquid form, suitable to be 

 absorbed from the walls of stomach and intestine and passed 

 into the blood. The process of digestion begins, of course, 

 in the mouth. No attention is here paid to peculiar changes 

 helpful in the process of digestion which have been brought 

 about by the cooking or baking of the intended food. Thus 

 it is a familiar fact that the crust of bread is more digest- 

 ible than the inner part, the process of baking having 

 changed the starch in question into a form which is more 

 easily soluble. The first step in the digestion, as far as the 

 body is concerned, is that of mastication. This is merely 

 a mechanical process by means of which the food is broken 

 up and so enabled to be swallowed. During the process of 

 mastication, however, the food is mixed with the first of 

 the digestive fluids, called the "saliva." The salivary 

 ducts of the parotid and submaxillary glands open at the 

 base of the molar teeth and are so arranged that bits of 

 saliva are introduced whenever the molars are inserted into 

 a morsel of food. In addition to this, of course, a good 

 deal of saliva flows into the front part of the mouth from 

 the sublingual gland, and is there more or less thoroughly 

 mixed by the rotating action of the tongue. 



1. Saliva and Salivary Digestion. Saliva is a clear, 

 transparent liquid when filtered. When taken from the 

 mouth it is somewhat cloudy, owing to the fact that it has 

 solid substances in it: bits of food and epithelial cells which 

 have been detached from the mucous membrane of the 



