TEMPORO-MAXILLARY ARTICU L A TION.—Under this heading I propose to give a brief account of the jointing of the lower maxilla to the cranium. This joint can not, of course, be identified out of the verte brate. The mandibles and maxillae of the ar Liculata are not homologous with the jaws of vertebrate animals, and the slit of their mouths is placed vertically not horizontally.
In the human subject, as in all mammalia, this articulation takes place between the squamous portion of the temporal (squamosal) and the inferior maxillary bones. The description of the anatomy of the human temporo-maxillary joint comprises, therefore, that of the diarthro dial articular surfaces of those bones, and also of the following parts : viz., the interarticular fibro-cartilage, the two synovial sacs, the ex ternal and internal lateral and other liga ments, and part of the insertion of the external pterygoid muscle.
articular condyle of the lower jaw is sub-cylindroid in form, its length from side to side being greater than its antero-pos terior measurement. The axis of this c3linder, however, is set neither directly from side to side, nor yet exactly level, its inner part being posterior to and below its outer; so that if the axes of the two condyles were produced inwards till they met, they would form an angle in doing so, which would point down wards and backwards,—reaching the anterior margin of the foramen magnum. The carti lage that encrusts this condyle extends further down behind than in front.
The surface to which this condyle is (mediately) articulated forms a part of the glenoid fossa, and is situated just below the base of the zygomatic process of the temporal bone, between its roots. It has such form as, in respect of the set of its axis, and its tranverse measurement, pretty accurately fits the condyle. It has, however, a greater antero-posterior extent, in accordance with the capability which this joint normally pos sesses, in addition to the usual ginglymoid motion, of an antero-posterior gliding to and fro of the condyle—a kind of normal disloca tion. The glenoid fossa of the temporal bone
contains a portion of the parotid gland, as well as the condyle of the inferior maxilla ; the former occupies that portion of it which is posterior to the glasserian fissure, whilst that part which is anterior to the glasserian fissure, is lined with cartilage for the articulation of the latter. In man there is usually no bony ridge bounding this articular surface pos teriorly, such as is generally found in the lower mammalia.
The ridge that bounds it anteriorly is formed by the inferior root of the zygoma, and the cartilage is continued some way on to this ridge. Externally it is limited by a tubercle on the zygoma, which serves for the attach ment of the external lateral ligament of the joint.
Interarticular fibro cartilage. — This is a thin disc or meniscus, oval or sub-oblong in form, concave below and convex above, thus capping the condyle, thicker at the edges than in the middle, where it is not unfrequently thinned away even to perforation. It differs from all the other fibro-cartilages in having muscular fibres inserted into it; namely, a portion of those of the external pterygoid. This insertion takes place along its anterior border, and by it the fibro-cartilage is made to join in that normal amount of dislocation for wards, which as above stated, the condyle of the lower maxilla is capable of, as well as in the abnormal accidental dislocation of the jaw. It is further held in its place by the fibres of the external lateral ligament, and by the synovial bursae of the joint.
Synovial Bursa'.— These are two in num ber, one above the interarticular fibro-carti lage and the other below it. The upper one is the larger and slacker, for it alone is con cerned in the antero-posterior gliding of the joint. The two sacs, of course, communicate and are reduced to one when the interarti cular fibro-cartilage is perforated in the centre.