THE MUSCLES OF MASTICATION. 497 



being carried in the same direction. For the better adaptation of the 

 articular surfaces, and the greater security of the joint, a biconcave 

 inter articular cartilage, thin or perforated at its centre, and thicker 

 at its margins, is interposed between the condyle and the glenoid fossa, 

 and is carried with the condyle, in all the movements of the jaw, 

 especially in the backward and forward movements, in the lateral 

 movements, and in extreme depression of the jaw, as in yawning. 

 This latter motion is checked by the pterygo-maxillary ligament. 

 Owing to the slight sliding movement of the cartilage, the axis of 

 motion of the lower jaw is not at the joint, but a little below it, in a 

 line with the grinding surfaces of the teeth. 



The force employed in moving the lower upon the upper jaw is mus- 

 cular, and the agents immediately concerned are the muscles of mas- 

 tication. In opening the mouth, the lower jaw partly descends by its 

 own weight ; but it is also drawn downwards by that portion of the 

 digastric muscle which ascends from the sides of the hyoid bone, and 

 is inserted into the hinder surface of the front part of the lower jaw. 

 The platysrna myoides, a muscle of the neck r may also assist in draw- 

 ing the jaw down, and so likewise do the genio-hyoid and mylo-hyoid 

 muscles, which ascend to it from the hyoid bone, this bone being fixed 

 by the sterno-hyoid and omo-hyoid muscles, which ascend to it from 

 the sternum and the scapulae, and also by the stylo-hyoids and the 

 hinder portion of the digastrics, which descend to it from the styloid 

 processes, and the inner part of the mastoid processes of the temporal 

 bone. The external pterygoid muscles also draw the jaw forwards, 

 and so aid in its opening. 



The closure of the jaw is- accomplished by muscular effort only ? the 

 muscles concerned being the most powerful of those of the head and 

 face. The chief of these are the temporal muscles, which descend 

 from the temporal fossae at the sides of the skull ; each arises from the 

 frontal, parietal, temporal, and sphenoid bones, pas-ses beneath the 

 zygomatic arch, and is attached to the so-called coronoid process, at 

 the upper and anterior end of the ascending part of the lower jaw, 

 about an inch and a half in front of the condyle or joint. The lever- 

 age with which these muscles act, is greater than, if they had been at- 

 tached nearer to the condyles; their action is- like that of a lever of 

 the third order, in which strength is, to a certain extent, sacrificed to 

 rapidity of motion. Another muscle of mastication, on each side, is 

 the masseter, a very thick and powerful muscle, which descends from 

 the lower border of the zygomatic arch and neighboring part of the 

 malar bone, and is inserted into the outer surface of the lower jaw, 

 near its angle, both on its ascending and horizontal part. Each of 

 these muscles consists of a superficial part, the fibres of which are di- 

 rected downwards and backwards, and of a deep part, the fibres of 

 which descend obliquely forwards; whilst, therefore, the whole muscle 

 closes the jaw, the superficial part can draw this bone a little forwards, 

 and the deeper part, slightly backwards. On the inner side of each 

 ascending portion of the jaw, between it and the cavities of the mouth 

 and pharynx, are two other strong muscles, named the external and 

 internal pterygoids, which proceed from the so-called pterygoid pro- 



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