The =fourth carpal= bone (Os carpale quartum, unciform) is somewhat wedge-shaped, and is readily distinguished from the second by its greater size and its posterior tubercle. The =superior= or =proximal surface= articulates with the intermediate and ulnar; it is convex and curves outward, backward, and downward, encroaching on the external and posterior surfaces. The =inferior= or =distal surface= bears two inner facets for the large (third) metacarpal and an outer one for the external (fourth) metacarpal bone. The =internal surface= has two or three facets for articulation with the third carpal, between which it is excavated and rough. The =anterior= or =dorsal surface= is convex and rough. The =external surface= is small, being encroached upon by the superior articular surface. The =posterior= or =volar surface= bears a tubercle on its lower part.[21] THE CARPUS AS A WHOLE The bones of the carpus, exclusive of the accessory, form an irregular quadrangular mass, the width of which is about twice the height or the antero-posterior diameter. The =anterior= or =dorsal surface= is convex from side to side, depressed along the line of junction of the two rows, and prominent below. The =posterior= or =volar surface= is in general slightly convex, but very irregular. It forms with the accessory the =carpal groove= (Sulcus carpi), which in the recent state is rendered smooth by the posterior ligament; it is converted into the =carpal canal= (Canalis carpi) for the flexor tendons by the transverse carpal ligament, which stretches across from the accessory bone to the inner side. The =proximal surface= is widest internally and is elevated in front, concave behind; it is entirely articular and adapted to the carpal articular surface of the radius. The =distal surface= is also articular and is irregularly faceted in adaptation to the surfaces of the metacarpal bones; each of the lower bones usually articulates with two metacarpal bones, but sometimes the third does not bear on the inner metacarpal bone. The =lateral surfaces= are both irregular and rough, the internal one being the wider. With the exception of the accessory, ulnar, and second, each bone articulates with two bones of the other row. =Development.=—Each ossifies from a single center. THE METACARPUS Three =metacarpal bones= (Ossa metacarpalia) are present in the horse. Of these, only one, the third or large metacarpal bone, is fully developed and carries a digit; the other two, the second and fourth, are much reduced, and are commonly called the internal and external small metacarpal or “splint” bones. [Illustration: FIG. 56.—DISTAL ROW OF CARPAL BONES, METACARPUS, FIRST AND SECOND PHALANGES, AND PROXIMAL SESAMOID BONES OF HORSE; POSTERIOR VIEW, LEFT SIDE. The fourth (external) metacarpal bone is wrongly numbered as I. (After Schmaltz, Atlas. d. Anat. d. Pferdes.) ] [Illustration: