Joints and Ligaments

The freely movable joints (true diarthrosis) are classified as follows :— ( ) Gliding joints (Arthrodia), in which the articular surfaces are flat, as in the carpal and tarsal bones.

(2) Hinge joints (Ginglymus), such as the elbow and inter phalangeal joints.

(3) Condyloid joints (Condylarthrosis), allowing flexion and extension as well as lateral movement, but no rotation. The meta carpo-phalangeal and wrist joints are examples of this.

(4) Saddle-shaped joints (Articulus sellaris), allowing the same movements as the last with greater strength. The carpo-meta carpal joint of the thumb is an example.

(5) Ball and socket joints (Enarthrosis), allowing free move ment in any direction, as in the shoulder and hip.

(6) Pivot-joint (Trochoides), allowing only rotation round a longitudinal axis, as in the radio-ulnar joints.

Embryology.

Joints are developed in the mesenchyme, or that part of the mesoderm which is not concerned in the forma tion of the serous cavities. The synarthroses may be looked upon merely as a delay in development. The diarthroses represent an arrest of development at an earlier stage, for a part of the original embryonic tissue remains as a plate of round cells, while the neighbouring two rods chondrify and ossify. This plate may become converted into fibro-cartilage, in which case an amphiar throdial joint results, or 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 re sult. 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. In the human foetus the joint cavities are all formed by the tenth week of intrauterine life.

Joints of the Axial Skeleton.

The bodies of the vertebrae except those of the sacrum and coccyx are separated, and yet connected, by the intervertebral disks. These are formed of alter nating concentric rings of fibrous tissue and fibro-cartilage, with an elastic mass in the centre. 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 ligaments 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 vertebrae 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 con tiguous thoracic vertebrae and the disk between, and ligaments strengthen each articulation. At the junction of the ribs with their cartilages the periosteum simply becomes perichondrium and binds the two structures together. Where the cartilages, however, join the sternum, or where they join one another, diar throdial joints with synovial cavities are established. In the case of the second rib this is double, and in that of the first usually wanting.