MUSCLES OF MASTICATION l6/ 



The inferior maxilla is a single bone. Its body is horizontal, of a 

 horseshoe shape, arid in the alveolar cavities in its superior border, are 

 the lower teeth. Below the teeth, both externally and internally, are 

 surfaces for the attachments of the muscles concerned in the various 

 movements of the jaw and for one of the muscles of the tongue. 



Temporo-maxillary Articulation. In man the articulation of the 

 lower jaw with the temporal bone is such as to allow an antero-posterior 

 sliding movement and a lateral movement, in addition to the movements 

 of elevation and depression. The condyloid process is convex, with an 

 ovoid surface, the general direction of its long diameter being trans- 

 verse, and slightly oblique from without inward and from before back- 

 ward. This process is received into a cavity of corresponding shape in 

 the temporal bone the glenoid fossa which is bounded anteriorly 

 by a rounded eminence called the eminentia articularis. 



Between the condyle of the lower jaw and the glenoid fossa, is an 

 oblong interarticular disk of nbro-cartilage. This is thicker at the 

 edges than in the centre. It is pliable and is so situated that when the 

 lower jaw is projected forward, making the lower teeth project beyond 

 the upper, it is applied to the convex surface of the eminentia articularis 

 and presents a concave surface for articulation with the condyle. One 

 of the uses of this cartilage is to present constantly a proper articulating 

 surface on the articular eminence and thus permit the antero-posterior 

 sliding movement of the lower jaw. It is also important in the lateral 

 movements of the jaw, in which one of the condyles remains in the 

 glenoid cavity and the other is projected, so that the bone undergoes 

 a slight rotation. 



Muscles of Mastication. To the lower jaw are attached certain 

 muscles by which it is depressed, and others by which it is elevated, 

 projected forward, drawn backward and moved from side to side. The 

 following are the principal muscles concerned in the production of these 

 varied movements : 



MUSCLES OF MASTICATION 



Muscles that depress the lower jaw 



MUSCLE ATTACHMENTS 



Digastric . Mastoid process of the temporal bone lower border of the 



inferior maxilla near the symphysis, with its central tendon 

 held to the side of the body of the hyoid bone. 



Mylo-hyoid .... Body of the hyoid bone mylo-hyoid ridge on the internal sur- 

 face of the inferior maxilla. 



Genio-hyoid . . ... Body of the hyoid bone inferior genial tubercle on the inner 



surface of the inferior maxilla, near the symphysis. 



Platysma myoides . . . Clavicle, acromion and fascia anterior half of the body of 



the inferior maxilla, near the inferior border. 



