case an amphiarthrodial joint results, or it may become absorbed in the centre to form a joint cavity, or, if this absorption occurs in two places, two joint cavities with an intervening meniscus may result. Although, ontogenetically, there is little doubt that menisci arise in the way just mentioned, the teaching of comparative anatomy suggests that, phylogenetically, they originate as an ingrowth from the capsule pushing the synovial membrane in front of them. The subject will be returned to when the comparative anatomy of the individual joints is reviewed. In the human foetus the joint cavities are all formed by the tenth week of intra-uterine life.

Anatomy

Joints of the Axial Skeleton.

The bodies of the vertebrae except those of the sacrum and coccyx are separated, and at the same time connected, by the intervertebral disks. These are formed of alternating concentric rings of fibrous tissue and fibro-cartilage, with an elastic mass in the centre known as the nucleus pulposus. The bodies are also bound together by anterior and posterior common ligaments. The odontoid process of the axis fits into a pivot joint formed by the anterior arch of the atlas in front and the transverse ligament behind; it is attached to the basioccipital bone by two strong lateral check ligaments, and, in the mid line, by a feebler middle check ligament which is regarded morphologically as containing the remains of the notochord. This atlanto-axial joint is the one which allows the head to be shaken from side to side. Nodding the head occurs at the occipito-atlantal joint, which consists of the two occipital condyles received into the cup-shaped articular facets on the atlas and surrounded by capsular ligaments. The neural arches of the vertebrae articulate one with another by the articular facets, each of which has a capsular ligament. In addition to these the laminae are connected by the very elastic ligamenta subflava. The spinous processes are joined by interspinous ligaments, and their tips by a supraspinous ligament, which in the neck is continued from the spine of the seventh cervical vertebra to the external occipital crest and protuberance as the ligamentum nuchae, a thin, fibrous, median septum between the muscles of the back of the neck.

The combined effect of all these joints and ligaments is to allow the spinal column to be bent in any direction or to be rotated, though only a small amount of movement occurs between any two vertebrae.

The heads of the ribs articulate with the bodies of two contiguous thoracic vertebrae and the disk between. The ligaments which connect them are called costo-central, and are two in number. The anterior of these is the stellate ligament, which has three bands radiating from the head of the rib to the two vertebrae and the intervening disk. The other one is the interarticular ligament, which connects the ridge, dividing the two articular cavities on the head of the rib, to the disk; it is absent in the first and three lowest ribs.

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.