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.