THE ARTICULATION OF THE MANDIBLE. 



215 



Carotid 

 canal 



Spine of 

 sphenoid 



Spheno- 

 mandibular 

 ligament 



Capsule 



The temporo-mandibular articulation from 

 behind. 



rated with the front of the capsule. The stylo-maxillary ligament is a bundle of 

 fibres of the cervical fascia running from the styloid process to the angle of the jaw. 

 Movements. — These occur on both sides of the meniscus, which slides for- 

 ward and backward on the articular eminence. They may be divided into those 

 of opening and closing the mouth and of grinding the teeth. In the former, 

 as the mouth begins to open, the meniscus and 

 the head of the jaw move forward, the condyle 

 at the same time advancing on the former as 

 the lower jaw turns on a transverse axis pass- 

 ing through the neck in both halves of the jaw. 

 This continues as the mouth opens wider, the 

 meniscus descending onto the articular eminence, 

 and probably, when the movement is extreme, 

 rising a little on the other side. This has been 

 graphically demonstrated on the living by an 

 apparatus bearing luminous points at the sym- 

 physis, the condyle, and the angle of the jaw, 

 which were photographed as the mouth opened 

 to various widths.^ It was shown that the for- 

 ward movement of the meniscus occurs even in 

 a very slight opening of the mouth. The angle 

 of the jaw moves forward at the very beginning 

 of the act, but soon passes backward. The 

 point on it describes some very complex curves. 

 Grinding movements, in which the mouth is not 

 opened, must occur chiefly between the skull and 

 the meniscus ; just what occurs below the latter 

 is uncertain. The lower jaw can be thrust for- 

 ward evenly, as the meniscus of each side de- 

 scends onto the articular eminence ; but in ordinary motions it seems to advance 

 on one side and perhaps to rece*de on the other. Spee ' has shown that the opposed 

 crowns of the molars (and apparently of the premolars also) fall on the arc of a cir- 

 cle that touches the front of the condyle, drawn, when projected on a plane, from a 

 centre on the crest of the lachrymal bone. This allows the teeth of the lower jaw to 

 slide on those of the upper, which the joint would not allow were the line between 



the teeth a straight one. To this may be added 

 that the inferior incisors rest against the lingual 

 surfaces of the superior, and that the tendency 

 of the edges of the former to make a transverse 

 arch, increased by the wearing away of the outer 

 corners of the lower lateral incisors, implies an 

 alternate rising and falling of either side of the 

 jaw in grinding movements with the mouth closed, 

 though the axis, in the main antero-posterior, can- 

 not be a fixed one. It must be remembered in 

 this connection and in the mechanics of the jaw 

 throughout that the range of variations is great, 

 and that there is frequently a want of symmetry 

 in the joints. This want of precise working is in- 

 creased by the laxity of the ligaments and the num- 

 ber of muscles acting on various parts of the jaw. 

 Development. — The tympanic portion of 

 the temporal being at birth nothing but the ring, 

 it is evident that the joint belongs solely to the squamous portion, and is always 

 bounded by the fissure of Glaser. At this age the glenoid fossa is nearly fiat and 

 the eminentia articularis but slightly raised. Even after birth the joint below the 

 meniscus is very slight, so that but little motion can occur in it, while the meniscus 



' Luce : Boston Medical and Surgical Journal, July 4, 1889. 

 * Arch, fiir Anat. and P'.iys., Anat. Abtheil., 1890. 



Fig. 249. 



Capsule 



Extertial 

 pterygoid 



Interarticular 

 fibro-cartilage 

 Ext. lateral 



ligament 

 Condyle of 



jaw 



Transverse section of right temporo-man- 

 dibular articulation from behind. 



