The radio-ulnar joints are three : the upper one is an example of a pivot joint, and in it the disk-shaped head of the radius rotates in a circle formed by the lesser sigmoid cavity of the ulna internally and the orbicular ligament in the other three quarters.
The middle radio-ulnar articulation is simply an interosseous membrane, the fibres of which run downward and inward from the radius to the ulna.
The inferior radio-ulnar joint is formed by the disc-shaped lower end of the ulna fitting into the slightly concave sigmoid cavity of the radius. Below, the cavity of this joint is shut off from that of the wrist by a triangular fibro-cartilage. The move ments allowed at these three articulations are called pronation and supination of the radius. The head of that bone twists, in the orbicular ligament, round its central vertical axis for about half a circle. Below, however, the whole lower end of the radius circles round the lower end of the ulna, the centre of rotation being close to the styloid process of the ulna. The radius, there fore, in its pronation, describes half a cone, the base of which is below, and the hand follows the radius.
Comparative Anatomy. In pronograde Mammals the forearm is usually permanently pronated, and the head of the radius, instead of being circular and at the side of the upper end of the ulna, is transversely oval and in front of that bone, occupying the same place as the coronoid process of the ulna in Man. This type of elbow, which is adapted simply to support and progression, is best seen in the Ungulata; in them both lateral ligaments are attached to the head of the radius, and there is no orbicular liga ment, since the shape of the head of the radius does not allow of any supination. The olecranon process of the ulna forms merely a posterior guide or guard to the joint, but transmits no weight. No better example of the maximum changes which the uses of support and prehension bring about can be found than in con trasting the elbow of the Sheep or other Ungulate with that of Man. Towards one or other of these types the elbows of all Mammals tend. It may be roughly stated that, when pronation and supination to the extent of a quarter of a circle are possible, an orbicular ligament appears.
The intercarpal joints are gliding articulations, the various bones being connected by palmar, dorsal, and a few interosseous ligaments, but only those connecting the first row of bones are complete, and so isolate one joint cavity from another. That part of the intercarpal joints which lies between the first and second rows of carpal bones is called the transverse carpal joint, and at this a good deal of the movement which seems to take place at The carpo-metacarpal articulations are, with the exception of that of the thumb, gliding joints, and continuous with the great intercarpal joint cavity. The carpo-metacarpal joint of the thumb is the best example of a saddle-shaped joint in Man. It allows forward and backward and lateral movement, and is very strong. The metacarpo-phalangeal joints are condyloid joints like the wrist, and are remarkable for the great thickness of the palmar ligaments of their capsules. In the four inner fingers these are joined together by the transverse metacarpal ligament.
The interphalangeal articulations are simple hinges surrounded by a capsule, of which the dorsal part is very thin.
Comparative Anatomy. The wrist joint of the lower Mammals allows less lateral movement than does that of Man, while the lower end of the ulna is better developed and is received into a cup-shaped socket formed by the cuneiform and pisiform bones. Unless there is pretty free pronation and supination, the triangu lar fibro-cartilage is represented by an interosseous ligament, which may be continuous above with the interosseous membrane between the radius and ulna. In most Mammals the wrist is divided into two lateral parts, as in the human foetus, but free pronation and supination seem to cause the disappearance of the septum.