The =third carpal= bone (Os carpale tertium, os magnum) is much the
largest bone of the lower row, forming more than two-thirds of the width
of the latter. It is flattened from above downward, and is twice as wide
in front as behind. The =superior= or =proximal surface= consists of two
facets separated by an antero-posterior ridge; the inner facet is
concave and articulates with the radial carpal; the outer facet—for the
intermediate carpal—is concave in front and convex behind, where it
encroaches on the posterior surface. The =inferior= or =distal surface=
is slightly undulating, and articulates almost entirely with the large
(third) metacarpal bone, but it usually bears a small oblique facet at
its inner side for the inner (second) metacarpal, and there is commonly
a non-articular depression externally. The _internal surface_ faces
backward and inward, and bears three facets for articulation with the
second carpal, between which it is excavated and rough. The =external
surface= has two facets for articulation with the fourth carpal, and is
depressed and rough in its middle. The =anterior= or =dorsal surface= is
convex and is crossed by a rough transverse ridge. The =posterior= or
=volar surface= is relatively small, and is rounded; its upper part is
encroached upon by the superior articular surface, below which it is
rough.