_Origin._—(1) The front of the distal extremity of the humerus, just external to the coronoid fossa; (2) the external tuberosity on the proximal extremity of the radius, the external lateral ligament of the elbow, and the external border of the radius at the junction of its proximal and middle thirds; (3) the external surface of the shaft of the ulna; (4) the fascia of the forearm. [Illustration: FIG. 190.—MUSCLES OF LEFT THORACIC LIMB OF HORSE, FROM ELBOW DOWNWARD, ANTERIOR VIEW. _a_, Extensor carpi radialis; _g′_, superficial pectoral muscle. (After Ellenberger-Baum, Anat. für Künstler.) ] _Insertion._—(1) The extensor (or pyramidal) process of the third phalanx; (2) the anterior surface of the proximal extremities of the first and second phalanges. _Action._—To extend the digital and carpal joints, and to flex the elbow joint. _Structure._—The muscle is a compound one, representing the common extensor, together with vestiges of the proper extensors of the digits. Usually at least =two heads= may be distinguished. The principal or humeral head (Caput humerale) arises from the front of the extensor epicondyle of the humerus in common with the extensor carpi. Its belly is fusiform, and terminates in a point near the distal third of the radius. The tendon appears on the surface of the muscle about the middle of the belly, the arrangement being pennate. The tendon passes downward through the outer of the two large grooves on the front of the distal end of the radius, and over the capsule of the carpal joint. Passing down over the front of the metacarpus, it inclines gradually inward, reaching the middle line of the limb near the fetlock. A little below the middle of the first phalanx it is joined by the branches of the suspensory ligament, and the tendon thus becomes much wider. Two =synovial membranes= facilitate the play of the tendon. The proximal one is a =sheath= which begins about three inches (ca. 7 to 8 cm.) above the carpus, and terminates at the proximal end of the metacarpus. At the fetlock a =bursa= occurs between the tendon and the joint capsule, but otherwise the two are adherent. The smaller head, arising chiefly from the radius and ulna, is often divisible into two parts (Fig. 443). The larger of these (Caput radiale, muscle of Phillips)[59] arises from the outer tuberosity and outer border of the radius, and from the lateral ligament of the elbow joint. The flat belly is succeeded by a delicate tendon, which accompanies the principal tendon over the carpus (included in the same sheath), and then passes outward to fuse with the tendon of the lateral extensor. Usually a slip is detached which is inserted on the proximal extremity of the first phalanx, or ends in the fascia here. The smaller and deeper division (Caput ulnare, muscle of Thiernesse)[60] is usually somewhat difficult to isolate. It arises from the ulna close to the interosseous space. It has a small rounded belly and is provided