156 TEXT-BOOK OF PHYSIOLOGY. 



points on its surface. The muscles concerned in the movements 

 of mastication are presented in the following table : 



External pterygoids ^| Draw the lower jaw forward and 



External portion of masseter > cause the lower teeth to project 



Anterior fibers of temporal J beyond the upper. 



Posterior fibers of temporal ~) 



Internal portion of masseter f Draw the lower jaw back to its 



Digastric, mylohyoid, and genio- f normal position. 



hyoid J 



Internal pterygoids \ Contracting alternately, draw the 



External pterygoids J jaw to the opposite side. 



and i _en ts 



The action of the depressor muscles becomes apparent when their 

 points of origin and insertion are considered. The anterior belly 

 of the digastric, the mylohyoid, the geni^hyoid muscles, agree in 

 having a similarity of origin the hyoid bone and a common area 

 of insertion, the anterior portion of the inferior maxillary. Their 

 anatomic relation is such that their combined action will depress the 

 lower jaw and open the mouth. 



The elevator muscles arise from various points on the side of 

 the head, and are inserted into the coronoid process, ramus, and 

 internal surface of the angle of the lower jaw. When the mouth 

 has been opened, the simultaneous contraction of these muscles 

 elevates the jaw and closes the mouth with considerable force. The' 

 power of these muscles is very great, and depends on the shortness 

 and thickness of the muscle-bundles. 



The action of the rotator muscles, those which give rise to the 

 lateral movements of the jaw, depends in like manner on their origin 

 and insertion. Arising from the superior maxillary and sphenoid 

 bones, they are inserted into the neck of the condyle and angle of 

 the lower jaw respectively. When they contract, the condyle on the 

 corresponding side is drawn forward, while the opposite condyle 

 remains stationary. As a result, the symphysis of the jaw is directed 

 to the opposite side. The grinding movements of the jaw are pro- 

 duced by the coordinated action of all the groups of muscles acting 

 more or less successively. 



For the proper mastication of the food it is essential that it be 

 kept between the opposing surfaces of the teeth. This is accom- 

 plished by the contraction of the orbicularis oris and buccinator 

 muscles from without and the tongue muscles from within. 



