_Structure._—This muscle consists of three principal heads. The =humeral head= (Caput humerale) constitutes the bulk of the muscle. It is marked by tendinous intersections, and is separable into three secondary heads. The =ulnar head= (Caput ulnare, ulnaris accessorius) is much smaller, and is at first superficially situated between the outer and middle flexors of the carpus, the =radial head= (Caput radiale, radialis accessorius) is the smallest, and is not always present; it is situated on the distal two-thirds of the posterior surface of the radius, under the humeral head. Each of these heads is provided with a tendon. The principal tendon—that of the humeral head—begins about three or four inches (8 to 10 cm.) above the carpus. It is soon joined by the small tendon of the ulnar head, which begins about the middle of the forearm. The tendon of the radial head fuses with the principal tendon close to the carpus. The conjoined tendon passes downward through the carpal canal, being included in the =carpal synovial sheath= with the superficial flexor tendon, as previously described. Continuing downward, it is joined about the middle of the metacarpus by a strong fibrous band, the so-called =inferior= or =subcarpal check ligament= (Caput tendineum). This is a direct continuation of the posterior ligament of the carpus. Below this the tendon passes through the ring formed by the perforatus, then in succession over the sesamoid groove, the inferior sesamoidean ligaments, and the tendon surface of the third sesamoid, to its insertion (Figs. 150 and 151). Its terminal part is much widened. From the distal third of the metacarpus to the distal end of the second phalanx it is inclosed in the =digital synovial sheath= described in connection with the perforatus. The =navicular bursa= or =bursa podotrochlearis= is found between the tendon and the third sesamoid or navicular bone. The terminal part of the tendon is bound down by the fibrous sheet described with the fascia. [Illustration: FIG. 198.—DIGIT OF HORSE, LATERAL VIEW. _1_, Large metacarpal bone; _2_, distal end of small metacarpal bone; _3_, fetlock joint; _5_, first phalanx; _6_, pastern joint; _7_, second phalanx; _8_, coffin joint; _9_, third phalanx; _11_, lateral cartilage; _11′_, upper (subcutaneous) border of cartilage; _12_, suspensory ligament; _13_, branch of _12_ to extensor tendon; _14_, deep flexor tendon; _15_, superficial flexor tendon; _15′_, branch of _15_; _16_, posterior annular ligament of fetlock; _17_, proximal annular or vaginal ligament of digit; _18_, distal annular ligament or reinforcing sheath of deep flexor tendon; _19_, lateral ligament of coffin joint; _21_, tendon of common or anterior extensor; _22_, tendon of lateral extensor; _23_, digital vein. (After Ellenberger-Baum, Anat. für Künstler.) ]