MECHANICAL PHENOMENA. 3 



of which are under the control of the will. The lower max- 

 illary bone may be dropped and raised, may be thrown 

 forward and drawn backward, and may be moved from 

 side to side. Ordinary mastication in the human being is 

 a combination of all these movements. The lower jaw 

 is lowered and raised in the ordinary biting movements, and 

 moved from side to side when the food is being chewed. 

 Combined with both of these may be more or less well-marked 

 forward and backward movements of the jaw. The food 

 is kept between the teeth and the act of mastication made 

 more effective by the simultaneous action of the muscles 

 of the tongue, cheeks, and lips. The cheeks, and more 

 especially the tongue, aid also in gathering together the 

 food in the mouth and forming it into a bolus preparatory 

 to the act of swallowing. 



The muscles concerned in the movements of the lower 

 jaw are the following. The masseter, temporal, and internal 

 pterygoid raise the jaw. The digastric is the chief depressor 

 of the lower maxilla, aided at times by the mylo-hyoid and 

 genio-hyoid muscles. The^ jaw is thrown forward by the 

 simultaneous contraction of the external pterygoids. When 

 these muscles move singly, side-to-side movements are pro- 

 duced. The jaw is retracted by contraction of the tem- 

 poral muscle. The muscles of mastication receive their 

 nerve supply from the inferior maxillary division of the 

 fifth cranial nerve with the exception of the genio-hyoid, 

 which is supplied by the hypoglossal nerve. 



3. Deglutition. It seems to be essential for the proper 

 performance of the act of deglutition that the mass to be 

 swallowed be moist. Dry material can either not be swallowed 

 at all or aj, best with difficulty. While certain substances 

 may therefore be 'swallowed immediately, it is necessary for 

 others that they remain in the oral cavity until they have, 

 been thoroughly mixed witfh saliva, or, in people of improper 

 dietary habits; until they have been moistened by admixture 

 with a mouthful of water, tea, or other liquid. 



