The bones of the forearm articulate at their extremities. The body of the radius is united to the body of the ulna by a short, thick, interosseous ligament; the fibres of this ligament, though short, do not prevent the production of some movements at the articulations of the bones. The radius so crosses the ulna that above, it is in front and external to the latter, while below, it is internal. This bone is flattened from front to back, and slightly convex anteriorly. Its superior extremity is formed, externally, of a portion which represents the head of the radius in man; internally, by another portion which represents half of the coronoid process of the ulna, which, in the human being, belongs exclusively to the latter (see p. 39, the encroachment of the radius on the ulna). This extremity is surrounded with a vertical articular surface which is placed in contact with a small cavity which is hollowed out on the ulna (the lesser sigmoid cavity); and presents at its superior aspect a surface which articulates with the inferior extremity of the humerus. The shaft of the bone has on its internal border rugosities analogous to the imprint of the pronator radii teres of the human skeleton; these rugosities, indeed, give insertion to a muscle of the same function, and bearing the same name. The inferior extremity, broader than the superior, is hollowed on its external aspect by a small cavity which receives the inferior extremity of the ulna; its inferior surface (concave) articulates with the carpus; its anterior surface (the homologue of the posterior surface of the corresponding extremity of the human radius) presents grooves which serve for the passage of the tendons of the muscles which pass from the forearm to the back of the hand. (For the names of the muscles whose tendons pass in these grooves, see Fig. 29.) The ulna is furnished at its superior extremity with an olecranon process, which is more prominent than that of the human ulna; this process is compressed laterally, and its internal surface is hollowed; there we also find a great sigmoid cavity, and a coronoid process situated at the internal part of the anterior surface, a process which, as we have previously shown, it shares with the radius. The shaft of the bone, prismatic and triangular, diminishes in thickness as it approaches the lower extremity, which articulates with the corresponding extremity of the radius. In the dog, the ulna terminates inferiorly in a blunt point, without enlargement, analogous to the head of the human ulna; in the cat, by a head which is prolonged into a styloid process, by which it articulates with a portion of the carpus.