332 HANDBOOK OF PHYSIOLOGY. 



The quantity of the enzyme determines the rapidity of the action but 

 not the amount; a small quantity will digest as much as a large quantity 

 but will take longer. The enzymes are not used up in the course of their 

 activity, as far as can be seen, and do not seem to undergo any change in 

 their composition. They are classified either according to the chemical 

 nature of their action, or according to the class of substances on which 

 they act ; the former classification is more logical, but the latter is more 

 convenient and more generally used. 



The food is first of all received into the mouth, and is subjected to the 

 action of the teeth and tongue, being at the same time mixed with the first 

 of the digestive juices the saliva. It is then swallowed, and, passing 

 through the pharynx and cesophagus into the stomach, is subjected to the 

 action of the gastric juice the second digestive juice. Thence it passes 

 into the intestines, where it meets with the bile, the pancreatic juice, and 

 the intestinal juices, all of which exercise an influence upon the portion 

 of the food not already absorbed from the stomach. By this time most 

 of the food is digested, and the residue of undigested matter leaves the 

 body in the form of faces by the external opening of the bowel. 



The Mouth is the cavity contained between the jaws and inclosed 

 by the cheeks laterally, the lips anteriorly; behind, it opens into the 

 pharynx by the fauces, and is separated from the nasal cavity above, by 

 the hard palate in front, and the soft palate behind, which forms its roof. 

 The tongue forms the lower part or floor. In the jaws are contained the 

 teeth, and when the mouth is closed these form its anterior boundaries. 

 The whole of the cavity of the mouth is lined with stratified epithelium, 

 of which the superficial layers are squamous. This epithelium is contin- 

 uous at the lips with that of the skin anteriorly, and posteriorly with 

 that of the pharynx. The mucous membrane itself, varying in thickness 

 in various parts, and consisting of a fine areolar connective, in which is 

 found adenoid tissue in considerable amount, is provided with numerous 

 small tubular glands lined with columnar epithelium, and resembling in 

 structure the mucous salivary glands, to be presently described. Into 

 the buccal cavity open the ducts of the salivary glands, which are three 

 in number on either side. 



In the mouth, then, the food is subjected to the action of the -teeth, 

 or is masticated, and is mixed with saliva. These processes of mastica- 

 tion and insalivation must be considered more in detail. 



Mastication. The act of chewing, or mastication, is performed by 

 the biting and grinding movement of the lower range of teeth against the 

 upper. The simultaneous movements of the tongue and cheeks assist 

 partly by crushing the softer portions of the food against the hard palate 

 and gams, and thus supplementing the action of the teeth, and partly by 

 returning the morsels of food to the action of the teeth, again and again, 



