FIG. 57.—LEFT METACARPAL AND DIGITAL BONES OF HORSE, INTERNAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ] THE LARGE METACARPAL BONE This (Os metacarpale tertium) is a very strong long bone, placed vertically between the carpus above and the first phalanx below. It consists of a shaft and two extremities. The =shaft= (Corpus) is semicylindrical, and presents two surfaces and two borders. The =anterior= or =dorsal surface= is smooth, convex from side to side, and nearly straight in its length. The =posterior= or =volar surface= is somewhat convex from side to side and, with the small bones, forms a wide groove which lodges the suspensory ligament. On either side of its upper two-thirds it is roughened for the attachment of the small metacarpal bones. The =nutrient foramen= occurs at the junction of the upper and middle thirds. The lower third is wider and flattened. The borders are rounded. The =proximal extremity= (Basis) bears an undulating articular surface adapted to the lower row of carpal bones. The greater part supports the third carpal bone; the oblique outer part, separated from the preceding by a ridge, articulates with the fourth, and a small facet for the second is usually found at the postero-internal angle. On either side is a notch separating two small facets which articulate with the proximal ends of the small metacarpal bones. Toward the inner side of the anterior surface is the =metacarpal tuberosity=, into which the extensor carpi radialis is inserted. The posterior surface is roughened for the attachment of the suspensory ligament. The =distal extremity= (Trochlea s. Capitulum) presents an articular surface for the first phalanx and the proximal sesamoid bones, which is composed of two condyles, separated by a sagittal ridge; the inner condyle is slightly the larger. On either side is a small fossa, surmounted by a tubercle, for the attachment of the lateral ligaments of the fetlock joint. The large metacarpal is one of the strongest bones in the skeleton. The compact substance is specially thick in front and internally. The medullary canal extends further toward the ends than in most of the long bones of the horse and there is little spongy bone. THE SMALL METACARPAL BONES These are situated on either side of the posterior surface of the large metacarpal bone, and form the sides of the metacarpal groove. Each consists of a shaft and two extremities. The =shaft= (Corpus) is three-sided and tapers to the distal end. It is variably curved, convex toward the middle line of the limb. The anterior surface is flattened and is rough, except in its lower part; it is attached to the large metacarpal bone by an interosseous ligament, except near the distal end. The abaxial surface is smooth and rounded from side to side above, grooved below. The axial surface is smooth and concave from edge to edge, except below, where it forms a rounded edge.