176 



DIGESTION 



teeth, only at a very variable time ranging from the seventeenth to the 

 twenty-fifth year. 



Mastication. This is the first of the digestive processes if we 

 neglect prehension of the food by the hands and lips. It is an habitual 

 voluntary process carried on by a neuro-muscular mechanism, and once 

 started goes on almost reflexly. 



The masticatory muscles may be considered briefly in the following 

 classes: Those which raise the lower jaw, those which move it laterally 



FIG. 83 



The teeth of a seven-year-old child. The permanent teeth are already formed, and are > 

 waiting for positions in the jaws. (Litch.) \ 



and forward, those which lower it, and those which keep the food in 

 place between the opposed sets of teeth. (1) The elevators of the lower 

 jaw are the masse ter, the temporal, and the internal pterygoid. The 

 masseter connects the molar process and the zygoma with the angle and 

 ramus and coronoid process of the jaw with powerful contractile fibers,, 

 its action being, therefore, to draw the jaw both forward and backward 

 as well as upward on the upper maxilla. The temporal muscle fills com- 

 pletely the temporal fossa of the skull and its fibers converge thence to 

 the coronoid process of the jaw. By its contraction the latter is drawn 

 powerfully upward, while at the same time the movement is somewhat 



