i8o THE HEAD AND NECK 



areas. It gives attachment on its inner surface to the discus 

 articularis, which divides the joint cavity into an upper and a 

 lower part. Anteriorly, the capsule is deficient, and the tendon 

 of the external pterygoid gains insertion into the articular disc. 



Laterally, the capsule is strengthened by the temper o- 

 mandibular (external lateral) ligament, the fibres of which are 

 directed downwards and backwards to the lower part of the neck 

 of the condyle. The obliquity of these fibres helps to prevent 

 the condyle from passing backwards and fracturing the tym- 

 panic plate, when a blow falls upon the chin. 



In the movements of protrusion and retraction of the man- 

 dible, the condyle and the articular disc move together over the 

 surface of the mandibular fossa. When the mouth is opened 

 the articular disc and the condyle both move forwards, but, at 

 the same time, the condyle rotates around a transverse axis, 

 which passes through the lower attachment of the temporo- 

 mandibular (external lateral) ligament. The latter becomes 

 tense when the mouth is widely opened and the condyle rests 

 on the summit of the articular tubercle (eminentia articularis). 

 In this position the mandible can easily be dislocated either by 

 a blow on the chin or even by the forcible contraction of the 

 external pterygoids. The method adopted to reduce this dis- 

 location first depresses the condyle so as to disengage it from 

 the front of the articular tubercle, and then, by partial closure 

 of the mouth, the condyle is made to pass back into the man- 

 dibular fossa. Considerable force is sometimes required to 

 overcome the spasm of the muscles of mastication. 



The spheno-mandibular (internal lateral) ligament is placed 

 at some distance from the joint and gives it no support. At- 

 tached above to the spine of the sphenoid, the ligament descends 

 to gain attachment to the margin of the mandibular foramen. 

 The internal maxillary artery and the inferior alveolar (dental) 

 vessels and nerve intervene between the ligament and the ramus 

 of the mandible. 



At the symphysis the mandibLe is strengthened by the 

 mental protuberance, and, when fractured, it usually gives way 

 at the margin of this elevation. The fracture passes through 

 the large canine socket and is usually compound, as the mucous 

 membrane of the mouth is so firmly attached to the bone that 

 it is almost always torn. The inferior alveolar (dental) nerve 

 may be implicated in the callus and cause neuralgic pain, which 

 niay be referred over the distribution of the buccinator and 



