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.