4. =Extensor carpi obliquus= (M. abductor pollicis longus et extensor pollicis brevis; oblique extensor of the metacarpus; extensor metacarpi obliquus).—This is a small muscle which curves obliquely over the distal half of the radius and the carpus. _Origin._—The external border and adjacent part of the anterior surface of the radius (the attachment area beginning at a point above the middle of the bone and extending down to its lower fourth). _Insertion._—The head of the inner (second) metacarpal bone. _Action._—To extend the carpal joint. _Structure._—The muscle is pennate and has a flat belly which curves downward, forward, and inward over the distal part of the radius. The tendon continues the direction of the muscle, crossing obliquely over the tendon of the extensor carpi radialis; it then occupies the oblique groove at the distal end of the radius, and crosses the inner face of the carpus. It is provided with a =synovial sheath=. [Illustration: FIG. 192.—CROSS-SECTION OF MIDDLE OF RIGHT FOREARM OF HORSE. _a_, Accessory cephalic vein; _b_, cutaneous branch of median nerve; _c_, extensor carpi obliquus; _d_, posterior radial artery; _e_, satellite vein; _f_, median nerve; _g_, _g′_, ulnar vessels; _h_, ulnar nerve; _i_, cephalic vein; _k_, tendon of ulnar head of deep flexor; _l_, interosseous artery; _m_, extensor carpi radialis; _n_, anterior digital extensor; _o_, lateral digital extensor; _p_, flexor carpi externus; _q_, _q_, _q_, humeral heads of deep digital flexor; _r_, radial head of same; _s_, superficial digital flexor; _t_, flexor carpi medius; _u_, flexor carpi internus; _v_, radius; _w_, skin; _x_, anterior radial artery. (After Ellenberger, in Leisering’s Atlas.) ] _Relations._—Superficially, the skin and fascia, the lateral extensor, and the common extensor; deeply, the radius, the extensor carpi radialis, the carpal capsule, and the internal lateral ligament. _Blood-supply._—Interosseous and anterior radial arteries. _Nerve-supply._—Radial nerve. B. FLEXOR DIVISION 1. =Flexor carpi internus= (M. flexor carpi radialis s. radialis volaris; internal flexor of the metacarpus).—This muscle lies on the inner surface of the forearm, immediately behind the inner border of the radius. _Origin._—The flexor (internal) epicondyle of the humerus, below and behind the lateral ligament. _Insertion._—The proximal end of the internal (second) metacarpal bone. _Action._—To flex the carpal joint and to extend the elbow. _Structure._—The muscle has a short tendon of origin, which is succeeded by a somewhat flattened, fusiform belly. The tendon of insertion begins near the distal fourth of the radius. It is provided with a synovial sheath which begins two or three inches (ca. 5 to 8 cm.) above the carpus and extends almost to the insertion of the tendon.