The large pastern is a short stout bone placed between the small pastern below and the fetlock joint above. Its superior extremity is larger than the inferior, and presents two shallow depressions separated by a groove, into which the central ridgre and two condyles, already spoken of as existing on the lower end of the canon, are fitted to form a joint of considerable extent of motion. The lower extremity is small, and divided by a superficial groove into two condyles. Sesamoid Bones (10, fig. 302). - These are two small floating bones situated behind the inferior extremity of the canon. They are somewhat triangular in form, with their bases directed downward, and are closely united one to the other. In front they are covered with cartilage, and articulate with the condyles of the canon bone, thus forming part of the fetlock joint. Behind they are covered with fibro-cartilage, and by apposition form a groove over which the great flexor tendon of the foot plays like a rope over a pulley. On the outer sides, from the apex downward, a roughened groove gives attachment to the two branches of the suspensory ligament. The under surface of the base is also roughened, and from it proceed short strong ligaments (inferior sesamoid), which attach the bones to the posterior surface of the large and small pasterns. OS Coronae (fig. 305), small pastern, or second phalanx, is a short bone placed between the large pastern and the os pedis, or foot-bone. One-half of it is enclosed by the hoof, and the rest forms the region of the coronet. Its upper extremity presents two shallow depressions, with which the two condyles of the lower end of the large pastern articulate. The lower end, like that of the suffraginis, is divided by a shallow groove into two condyles, by which the bone articulates with the os pedis. The tendon of the extensor pedis muscle is attached to the anterior surface, and that of the flexor pedis perforatus to the inner and outer part of the superior border behind. At the upper and posterior part it is flattened and covered by fibro - cartilage, thus forming a smooth surface over which the tendon of the flexor perforans freely plays in its course towards the foot-bone.
Fig. 303. - Metacarpal Bones (Posterior View).
1 Outer Small Metacarpal Bone. - Inner Small Metacarpal Bone. 3 Large Metacarpal Bone. 4 Nutritive Foramen. 5 Articular Condyles. 6 Intervening Ridge.
Fig. 304. - Os Suffraginis or Large Pastern Bone.