Ligaments.—There is only one ligament in immediate relation with the temporo-maxil lary joint, namely, the external lateral. This is a small ligament, broader above than below, situated at the outside of the joint ; attached above to the tubercle situated at the point of divergence of the roots of the zygoma ; and directed downwards and backwards to the outside of the neck that supports the condyle of the lower jaw. Midway in its passage from the one to the other it is attached by its inner aspect to the interarticular fibro-cartilage.
The internal lateral ligament is a long thin slip extending from the spinous process of the sphenoid, and the neighbouring parts of the temporal bone to the fore part of the lip of the inferior dental canal. It lies behind the external pterygoid muscle, by the origin of which its cranial attachment is concealed, and it is separated from the temporo-maxillary joint by a considerable space through which pass the internal maxillary artery and vein, giving off their middle meningeal and inferior dental branches, which indeed are conducted, the former to the foramen spinosum of the sphe noid, the latter to the inferior dental canal, by the ligament in question, Numerous stray fibres of ligamentous tissue strengthen the synovial sacs of the temporo maxillary joint, forming a kind of capsular liganzent.
That process of the cervical fascia, which is called the styloanaxillary ligament, is gene rally enumerated as one of the ligaments of this joint. It extends from the styloid process to the lower part of the ramus of the jaw, separating the parotid from the submaxillary gland, and affording attachment to the stylo glossus muscle. The condyle of the lower jaw depends for the maintenance of its normal apposition to the temporal bone much more upon the masseter, temporal and pte•goid muscles than upon these small ligaments ; by these, however, its astero-posterior gliding is indifferently tethered.
Muscles.— The external pterygoid-muscle, proceeding backwards and outwards from its origin, is mainly inserted into the front of the neck that supports the maxillary condyle, the upper part of it, however, is inserted into the interarticular fibro-cartilage, which thereby is drawn forward along with the condyle when this muscle acts. This insertion is tendinous.
Motions of the faint. — The temporo-maxil lary joint admits °fa ginglymoid motion in the vertical direction, by which the mouth is opened and shut. This motion most of ne
cessity always take place in the joints of both sides at the same instant. It also admits of a horizontal antero-posterior gliding motion, in which the joint of one side only may be mainly concerned. In the human subject the front teeth of the lower jaw, in most cases, are not ex actly opposed to those of the upper jaw—that is, the summits of the one set are not applied to the summits of the other—in the ordinary. position of the mouth, either when at rest or engaged in mastication. The lower incisor teeth are usually posterior to the upper. But when we bite with the front teeth we bring the upper and lower set into apposition by thrusting forward the lower jaw : in this act both joints are similarly concerned. We can also execute a grinding motion from side to side, and this is done by thrusting forward one condyle whilst the other merely revolves on the axis of its neck.
The jaw is elevated or closed by the tem poral, masseter, and pterygoid muscles. The pterygoid, chiefly the external, are the agents in protruding it. These latter are antagonised by the elevating and also by the depressing muscles. The chief depressor of the lower jaw is the digastric, as is clearly shown by its comparative anatomy, but all those which ex tend from the chin to the hyoid bone are capable of, and occasionally do assist in per forming this act.
The majority of the muscular fibres that elevate the jaw arrive at their insertion into it from before backwards ; thus the masseter has a kind of twist in the arrangement of its fibres, so that those which arise most anteriorly are inserted very conspicuously furthest back, whilst the remainder proceed directly downwards or slightly forwards ; a considerable portion of those of the temporal, namely, those which arise from the anterior part of the temporal fossa, run backwards to their insertion into the coronoid process. The use of this arrangement seems, upon a careful consideration of the mechanics of the question, to be the application of the elevating or closing force in a more favourable direc tion, not, as might seem at first sight, the protrusion of the lower jaw—that is amply effected by the pterygoideus externus.