THE MAXILLARY BONES. 33 



the apex of the styloid process to the angle of the jaw ; the 

 internal lateral from the spine of the sphenoid to the mar- 

 gins of the inferior dental foramen; the external lateral, 

 which alone is a ligament strictly proper to the articulation, 

 reaches from the outer side and tubercle of the zygoma to 

 the outer surface of the neck of the condyle. 



The form of the articulating surfaces and the compara- 

 tive absence of retaining ligaments combine to allow of a 

 variety of movement unusual in any other than a ball and 

 socket joint. The articulation acts as a simple hinge when 

 the jaw is simply depressed, and this is the only motion 

 possible in many animals, as in typical carnivora. When, 

 however, the mouth is opened to the fullest possible extent, 

 the condyle leaves the glenoid cavity, slides forward, and 

 rests on the articular eminence, the interarticular fibro- 

 cartilage being carried forward with it. The passage of the 

 condyle on to the articular eminence, although always 

 taking place when the lower jaw is excessively depressed, 

 takes place sometimes without any depression of the lower 

 jaw, which then passes horizontally forward ; or it may take 

 place on the one side only, giving to the jaw the lateral 

 movement so useful in mastication. In the mastication of 

 food the various movements are combined, or succeed one 

 another with great rapidity ; the lateral movements are not 

 very extensive, the outer cusps of the lower teeth of one 

 side being brought to antagonise the outer cusps of the 

 upper teeth, and then being made to slide forcibly down the 

 sloping surfaces of the latter till they return to their normal 

 antagonism ; when one set of muscles is tired the same pro- 

 cess is gone through on the other side of the mouth. 



The closure of the jaw, and the rotatory and oblique 

 motions, are accomplished by four pairs of very powerful 

 muscles; these are antagonised by muscles comparatively/ 

 feeble and indirect in their application. 



The closure of the jaws is effected by the masseters and 



