206 ELEMENTS OF PHYSIOLOGY 



by the palate bones, covered with mucous membrane. 

 The larger part of the roof of the mouth is formed by 

 these bones and is called the hard palate. The roof is 

 completed in the rear by the fleshy soft palate. The floor 

 of the mouth is occupied mostly by the tongue ; the lips 

 form the front wall, and the "cheeks the side walls. The 

 cheek is composed partly of a large flat muscle called 

 the buccinator ("trumpeter," because used in blowing a 

 trumpet). The contraction of the trumpeter muscles, to- 

 gether with the movements of that flat muscle called the 

 tongue, keeps the food between the teeth in the act of 

 chewing, or mastication. 



366. Salivary Glands. There are three pairs of glands 

 that secrete saliva. In structure one of these glands with 

 its duct resembles a minute bunch of grapes with a hollow 

 stem. The largest, called the parotid, is just beneath the 

 skin in front of the ear. Its duct opens into the mouth 

 in the upper jaw opposite the second molar tooth. This 

 gland swells in a disease called the mumps. The next 

 largest gland is the submaxillary ', lying within the angle 

 of the lower jaw. Its duct opens into the floor of the 

 mouth. The smallest, the sublingual, lies farther to the 

 front ; both glands of the pair lie beneath the tongue, and 

 open by a number of ducts. The fluid produced by these 

 glands becomes mixed with the mucus from the mucous 

 membrane of the mouth and is called the saliva. 



367. Saliva. This is a thin, colorless, alkaline liquid, 

 slightly sticky, and often containing air bubbles. About 

 TTMnf P ar t f the saliva is a white substance called ptyalin, 

 which has the power to change starch to malt sugar while 

 remaining unchanged itself. Hence ptyalin is a ferment. 

 Ferment is the name given to certain organic substances 

 which act chemically upon other substances, and change 

 their nature without becoming changed themselves. From 

 one to three pints of saliva are produced daily. Its flow 



