ARTICULATION OF THE LOWER JAW. 233 



usually classed among the ligaments of the jaw, it can be considered only as an 

 accessory to the articulation. 



The Capsular Ligament forms a thin and loose capsule, attached above to the 

 circumference of the glenoid cavity and the articular surface immediately in front ; 

 below, to the neck of the condyle of the lower jaw. It consists of a few thin scat- 

 tered fibres, and can hardly be considered as a distinct ligament ; it is thickest at 

 the back part of the articulation. 1 



The Interarticular Fibro-cartilage (Fig. 159) is a thin plate of an oval form, 

 placed horizontally between the condyle of the jaw and the glenoid cavity. Its 

 upper surface is concavo-convex from 

 before backward, and a little convex 

 transversely, to accommodate itself to 

 the form of the glenoid cavity. Its 

 under surface, where it is in contact 

 with the condyle, is concave. Its cir- 

 cumference is connected to the cap- 

 sular ligament, and in front to the 

 tendon of the External pterygoid 

 muscle. It is thicker at its circum- 

 ference, especially behind, than at its 

 centre. The fibres of which it is com- 

 posed have a concentric arrangement, 

 more apparent at the circumference 



than at the Centre. Its Surfaces are FlG - 159. Vertical section of temporo-mandibular 



Ti ,. . , , . . . articulation. 



smooth. It divides the joint into 



two cavities, each of which is furnished with a separate synovial membrane. 



The Synovial Membranes, two in number, are placed, one above, and the other 

 below, the fibro-cartilage. The upper one, the larger and looser of the two, is con- 

 tinued from the margin of the cartilage covering the glenoid cavity and eminentia 

 articularis on to the upper surface of the fibro-cartilage. The lower one passes from 

 the under surface of the fibro-cartilage to the neck of the condyle of the jaw, being 

 prolonged downward a little farther behind than in front. The interarticular cartilage 

 is sometimes perforated in its centre ; the two synovial sacs then communicate with 

 each other. 



The nerves of this joint are derived from the auriculo-temporal and m'asseteric 

 branches of the inferior maxillary. The arteries are derived from the temporal 

 branch of the external carotid. 



Actions. The movements permitted in this articulation are ( very extensive. 

 Thus, the jaw may be depressed or elevated, or it may be carried forward or back- 

 ward. It must be borne in mind that there are two distinct joints in this articu- 

 lation that is to say, one between the condyle of the jaw and the interarticulav 

 fibro-cartilage, and another between the fibro-cartilage and the glenoid fossa ; when 

 the jaw 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 fibro-cartilage, the movement is of a ginglymoid or hinge-like 

 character, the condyle rotating on a transverse axis on the fibro-cartilage ; while in 

 the upper compartment the movement is of a gliding character, the fibro-cartilage, 

 together with the condyle, gliding forward on to the eminentia articularis. These 

 two movements take place simultaneously the condyle and fibro-cartilage move 

 forward on the eminence, and at the same time the condyle revolves on the fibro- 

 cartilage. In the opposite movement of shutting the mouth the reverse action takes 

 place ; the fibro-cartilage glides back, carrying the condyle with it, and this at the 

 same time revolves back to its former position. When the jaw is carried horizon- 

 tally forward, as in protruding the lower incisors in front of the upper, the move- 



1 Sir G. Humphry describes the internal portion of the capsular ligament separately as the short 

 internal lateral ligament and it certainly seems as deserving of a separate description as the external 

 lateral ligament is. 



