The _costo-transverse ligaments_ bind the ribs to the transverse processes of the thoracic vertebrae. The _superior costo-transverse ligament_ binds the neck of the rib to the transverse process of the vertebra above; the _middle_ or _interosseous_ connects the back of the neck to the front of its own transverse process; while the _posterior_ runs from the tip of the transverse process to the outer part of the tubercle of the rib. The inner and lower part of each tubercle forms a diarthrodial joint with the upper and fore part of its own transverse process, except in the eleventh and twelfth ribs. At the junction of the ribs with their cartilages no diarthrodial joint is formed; the periosteum simply becomes perichondrium and binds the two structures together. Where the cartilages, however, join the sternum, or where they join one another, diarthrodial joints with synovial cavities are established. In the case of the second rib this is double, and in that of the first usually wanting. The _mesosternal joint_, between the pre- and mesosternum, has already been given as an example of a symphysis. _Comparative Anatomy._--For the convexity or concavity of the vertebral centra in different classes of vertebrates, see SKELETON: _axial_. The intervertebral disks first appear in the Crocodilia, the highest existing order of reptilia. In many Mammals the middle fasciculus of the stellate ligament is continued right across the ventral surface of the disk into the ligament of the opposite side, and is probably serially homologous with the ventral arch of the atlas. A similar ligament joins the heads of the ribs dorsal to the disk. To these bands the names of anterior (ventral) and posterior (dorsal) _conjugal ligaments_ have been given, and they may be demonstrated in a seven months' human foetus (see B. Sutton, _Ligaments_, London, 1902). The _ligamentum nuchae_ is a strong elastic band in the Ungulata which supports the weight of the head. In the Carnivora it only reaches as far forward as the spine of the axis. The JAW JOINT, or _temporo-mandibular articulation_, occurs between the sigmoid cavity of the temporal bone and the condyle of the jaw. Between the two there is an interarticular fibro-cartilage or meniscus, and the joint is surrounded by a capsule of which the outer part is the thickest. On first opening the mouth, the joint acts as a hinge, but very soon the condyle begins to glide forward on to the eminentia articularis (see SKULL) and takes the meniscus with it. This gliding movement between the meniscus and temporal bone may be separately brought about by protruding the lower teeth in front of the upper, or, on one side only, by moving the jaw across to the opposite side.