DIGESTION. 



141 



Anterior belly of digastric 



Mylohyoid 



Geniohyoid 



Temporal 



Internal portion of masseter 



Internal pterygoids 



External pterygoids 



External portion of masseter 



Anterior fibers of temporal 



Posterior fibers of temporal 



Internal portion of masseter 



Digastric, mylohyoid, and geniohyoid 



Internal pterygoids 



External pterygoids 



Pterygoids, external and internal 



Temporal 



Masseter 



Depress the lower jaw and open the 

 mouth. 



Elevate the lower jaw and close the 

 mouth. 



Draw the lower jaw forward and cause 

 the lower teeth to project beyond 

 the upper. 



Draw the lower jaw back to its normal 

 position. 



Contracting alternately, draw the jaw 

 to the opposite side. 



Produce grinding movements of the 

 lower jaw. 



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, and the geniohyoid muscles, agree in having a similarity of 

 origin the hyoid bone and a common area of insertion, the anterior 

 portion of the lower jaw. Their anatomic relation is such that their 

 combined action will depress the lower jaw and open the mouth. 



The action of the elevator muscles becomes apparent when their points 

 of origin and insertion are considered. 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. After the 

 mouth has been opened, the simultaneous contraction of these muscles will 

 elevate the jaw and closes the mouth with considerable force. The power 

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

 thickness of the muscle-bundles. 



The action of the rotator muscles, the external and internal pterygoids, 

 those which give rise to the lateral movements of the jaw, depends in like 

 manner on their origin and insertion. The first arises from the outer 

 surface of the external pterygoid plate and the great wing of the sphenoid 

 bone; the second arises mainly from the inner surface of the external ptery- 

 goid plate; they are inserted into the neck of the condyle and angle of the 

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

 ing 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 produced 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 accomplished by the 

 contraction of the orbicularis oris and buccinator muscles from without and 

 the tongue muscles from within. 



The Nerve Mechanism 1 of Mastication. Mastication is a complex 

 act and involves the cooperation of a number of muscles, afferent and efferent 

 nerves, and a central mechanism by which they are excited to, and coordi- 

 nated in their activity. The central mechanism is located in the medulla 

 oblongata in the gray matter beneath the floor of the fourth ventricle. 



1 By this term is meant a combination of nerves, afferent and efferent, and nerve centers 

 which when excited to action coordinates the actions of the organs with which it is associated. 



