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.

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.

The proximal extremity or head (Basis) is relatively large. In the case of the inner bone it usually bears two facets above which support the second and third carpal bones, while the outer bone has here a single facet for articulation with the fourth carpal bone. Each has also two facets for articulation with the large metacarpal, and is elsewhere roughened for the attachment of ligaments and muscles. The inner bone may present a small facet behind for the first carpal bone.

The distal extremity (Capitulum) is usually a small nodule, which projects to a variable extent in different subjects, and is easily felt in the living animal. It is situated two-thirds to three-fourths of the way down the region.

The small metacarpal bones vary much in length, thickness, and curvature. In the majority of cases the inner bone is the longer; in other subjects the outer one is the longer or there is no material difference. Sometimes the curvature is very pronounced, so that the distal end causes a decided projection. The distal end is very variable in size and may be a mere point.

Development.—The large metacarpal bone ossifies from three centers. The proximal extremity unites with the shaft before birth, the distal extremity toward the middle of the second year. The small metacarpal bones ossify from two centers, one of which is for the proximal extremity. Their distal ends are cartilaginous at birth. Fusion of the middle part of the shaft with the large metacarpal bone is common.