FIG. 55.—CARPAL ARTICULAR SURFACE OF RADIUS AND PROXIMAL ARTICULAR SURFACES OF CARPAL AND METACARPAL BONES, LEFT SIDE. THE ACCESSORY AND FIRST CARPAL BONES ARE NOT SHOWN. _Cr_, Radial carpal; _Ci_, intermediate carpal; _Cu_, ulnar carpal; _C2_, _C3_, _C4_, second, third, and fourth carpals; _Mc.II_, second or inner small metacarpal bone; _Mc.III_, third or large metacarpal bone; _Mc.IV_, fourth or outer small metacarpal bone; _1_, _2_, grooves for tendons of anterior extensor and extensor carpi radialis; _3_, metacarpal tuberosity. Arrows indicate relations of facets. Short arrow points to facet on ulnar carpal for articulation with accessory carpal. (After Schmaltz, Atlas. d. Anat. d. Pferdes.) ] THE THIRD CARPAL BONE 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. THE FOURTH CARPAL BONE