THE TEMPOROMANDIBULAR ARTICULATION 281 



articularis on to the upper surface of the articular disk. The lower one passes from the under 

 surface of the articular disk to the neck of the condyle of the mandible, being prolonged down- 

 ward a little farther behind than in front. The articular disk is sometimes perforated in its 

 centre; the two synovial sacs then communicate wit'.i each other. 



The nerves of this joint are derived from the auriculotemporal and masseteric branches 

 of the inferior maxillary. The arteries are derived from the temporal branch of the external 

 carotid. 



Movements. The movements possible in this articulation are very extensive. Thus, the 

 mandible may be depressed or elevated, or it may be carried forward or backward. It must be 

 borne in mind that there are two distinct joints in this articulation that is to say, one between 

 the condyle of the mandible and the articular disk, and another between the disk and the glenoid 

 fossa; when the mandible is depressed, as in opening the mouth, the movements which take place 

 in these two joints are not the same. In the lower compartment, that between the condyle and 

 the articular disk, or meniscus, the movement is of a ginglymoid or hinge-like character, the 

 condyle rotating on a transverse axis on the articular disk, or meniscus; while in the upper 

 compartment the movement is of a gliding character, the articular disk, or meniscus, together 

 with the condyle, gliding forward on to the eminentia articularis. These two movements take 

 place simultaneously the condyle and articular disk, or meniscus, move forward on the emi- 

 nence, and at the same time the condyle revolves on the articular disk, or meniscus. In the 

 opposite movement of shutting the mouth the reverse action takes place; the articular disk, 

 or meniscus, glides back, carrying the condyle with it, and this at the same time revolves back 

 to its former position. When the mandible is carried horizontally forward, as in protruding the 

 lower incisors in front of the upper, the movement takes place principally in the upper compart- 

 ment of the joint the articular disk, or meniscus, carrying with it the condyle, glides forward 

 on the glenoid fossa. This is because this movement is mainly effected by the External 

 pterygoid muscles, which are inserted into both condyle and articular disk. The grinding or 

 chewing movement is produced by the alternate movement of one condyle, with its disk, for- 

 ward and backward, while the other condyle moves simultaneously in the opposite direction; 

 at the same time the condyle undergoes a vertical rotation on its own axis on the disk in the 

 lower compartment. One condyle advances and rotates, the other condyle recedes and rotates, 

 in alternate succession. 



The mandible is depressed by its own weight, assisted by the Platysma, the Digastric, the 

 Mylohyoid, and the Geniohyoid muscles. It is elevated by the anterior part of the Temporal, 

 Masseter, and Internal pterygoid muscles. It is drawn forward by the simultaneous action of 

 the External pterygoid and the superficial fibres of the Masseter; and it is drawn backward by 

 the deep fibres of the Masseter and the posterior fibres of the Temporal muscles. The grinding 

 movement is caused by the alternate action of the two External pterygoids. 



Surface Form. The temporomandibular articulation is quite superficial, situated below the 

 base of the zygoma, in front of the tragus and external auditory meatus, and behind the posterior 

 border of the upper part of the Masseter muscle. Its exact position can be at once ascertained 

 by feeling for the condyle of the mandible, the working of which can be distinctly felt in the 

 movements of the mandible in opening and shutting the mouth. When the mouth is opened 

 wide, the condyle advances out of the glenoid fossa on to the eminentia articularis, and a depres- 

 sion is felt in the situation of the joint. 



Applied Anatomy. Genuine dislocation of the mandible is almost always forward. Croker, 

 King, and Theim, however, have reported posterior displacement. Dislocation is caused by vio- 

 lence or muscular action. When the mouth is open, the condyle is situated on the eminentia 

 articularis, and any sudden violence, or even a sudden muscular spasm, as during a convulsive 

 yawn, may displace the condyle forward into the zygomatic fossa. The displacement may be 

 unilateral or bilateral, according as one or both of the condyles is displaced. The latter of the 

 two is the more common. The articular disk adheres to the condyle until it passes over the 

 eminentia articularis, but at this point remains behind. 



Sir Astley Cooper described a condition which he termed " subluxation." It occurs princi- 

 pally in delicate women, and is belived by some to be due to the relaxation of the ligaments, 

 permitting too free movement of the bone. Others believe it is due to displacement of the articular 

 disk. Still others attribute the symptoms to gouty or rheumatic changes in the joint. In close 

 relation to the condyle of the mandible is the external auditory meatus and the tympanum; 

 any force, therefore, applied to the bone is liable to be attended with damage to these parts, or 

 inflammation in the joint may extend to the ear, or, on the other hand, inflammation of the middle 

 ear may involve the articulation and cause its destruction, thus leading to ankylosis of the joint. 

 In children, arthritis of this joint may follow the exanthemata, and in adults it occurs as the 

 result of some constitutional conditions, as rheumatism or gout. The temporomandibular 

 joint is also occasionally the seat of osteoarthritis , leading to great suffering during efforts of mas- 

 tication. A peculiar affection sometimes attacks the neck and condyle of the mandible, consisting 

 in hypertrophy and elongation of these parts and consequent protrusion of the chin to the opposite 

 side. 



