as most of the articulation at the wrist is formed by the radius and the inter-articular fibro-cartilage. The head of the ulna is at the lower extremity of the bone and articulates on the outer side with the radius and below with the triangular fibro-cartilage. From its inner side projects the styloid process. The olecranon process forms the upper extremity and presents anteriorly an articular surface, the greater sigmoid cavity, for articulation with the trochlea of the humerus, where it fits into the olecranon fossa during extension. The same articulating surface also covers the coronoid process, a smaller projection below and in front of the olecranon, which fits into the coronoid fossa during flexion. Continuous with the greater sigmoid cavity on the outer side is the lesser sigmoid cavity for articulation with the head of the radius. Under the triceps tendon, which is inserted into the olecranon, is a bursa or sac of synovial membrane, such as occurs in parts where much force is brought to bear.
The Radius.—The radius, or spoke of the wheel, is on the outer side of the forearm and gets its name from the way it turns upon the ulna in pronation. The shaft is larger below than above and is slightly curved longitudinally for greater strength. The upper extremity or head is small and has a slightly concave upper surface for articulation with the radial head of the humerus. It articulates by its sides with the lesser sigmoid cavity and is bound to the ulna by the orbicular ligament, which runs over a smooth articular surface. Below the head is the constriction of the neck with the tuberosity for the biceps tendon to the inner side below. The lower extremity is large and forms the chief part of the wrist-joint, articulating with the semilunar and scaphoid bones of the wrist. From the lower extremity the strong conical styloid process projects externally.
In the living the olecranon process of the ulna is always felt at the elbow and the posterior border of the ulna forms the prominent ridge down the forearm, leading
