The ulna, thicker at its humeral than at its carpal extremity, receives the pulley-like surface of the os humeri into a concavity, the greater sigmoid cavity, which is prolonged posteriorly by a rough quadrilateral process of bone, the olecranon ; and anteriorly by a pointed pyramidal eminence, the coronoid process. The plane in which the movement is effected, observes Cuvier, is in the axis of the ulna, and not in that of the os humeri, in consequence of the obliquity of the pulley ; whence it comes that in flexion the inferior extremity of the ulna is approximated to the body. Upon the radial side of the ulna, a shallow depression, the lesser sigmoid cavity receives the head of the radius. The shaft of the bone, marked by impressions for the attachment of muscles terminates in a rounded head, which is received into a concavity at the carpal extremity of the radius, and surmounted by a process termed styloid, to which is attached the external lateral ligament of the wrist joint. The radius presents at its humeral extremity a disc-like flattened head, separated from the shaft by a constricted neck. The movement of rotation is performed in the joint between the convex border of the head of the radius and the lesser sigmoid cavity of the ulna. A prominent tubercle below the neck marks the attachment of the tendon of the biceps flexor cubiti. Below the bicipital tubercle the shaft of the bone curves outwards, and presents upon its convexity a rough surface for the insertion of the tendon of the pronator radii teres muscle. The broad flat inferior extremity, continued outwards into a styloid process, forms an articular surface for two of the carpal bones, the os scaphoides and the os semilunare. Upon the posterior surface are several grooves which lodge the tendons of the following muscles of the forearm. Proceeding from the radial to the ulnar side,
1. the extensor ossis metacarpi et primi internodii pollicis.
2. The extensor carpi radialis longior et brevior.
3. The extensor secundii internodii.
4. Extensor communis digitorum.