FIG. 51.—SAGITTAL SECTION OF UPPER PART OF RADIUS AND ULNA OF HORSE. _Cm_, Medullary cavity of ulna. ] THE INTERMEDIATE CARPAL BONE The =intermediate carpal= bone (Os carpi intermedium, semilunar, lunar) is somewhat wedge-shaped, wider in front than behind. The =superior= or =proximal surface= is saddle-shaped, and articulates with the middle facet on the distal end of the radius. The =inferior= or =distal surface= is smaller, convex in front, concave behind, and articulates with the third and fourth carpal bones. The =internal surface= has upper and lower facets for articulation with the radial carpal, and between these it is excavated and rough. The =external surface= is similar to the preceding and articulates with the ulnar carpal. The =anterior= or =dorsal surface= is rough and slightly convex. The =posterior= or =volar surface= bears a tuberosity on its lower part. THE ULNAR CARPAL BONE The =ulnar carpal= bone (Os carpi ulnare, cuneiform) is the smallest and most irregular bone of the upper row. The =superior= or =proximal surface= is concave, and fits the lower part of the outer facet on the distal end of the radius. The =inferior= or =distal surface= is oblique and undulating for articulation with the fourth carpal bone. The =internal surface= has upper and lower facets for articulation with the intermediate. The =anterior= or =dorsal= and =external surfaces= are continuous, convex, and rough. The =posterior= or =volar surface= is oblique, and bears a concave facet for articulation with the accessory carpal bone; below this is a tubercle. [Illustration: FIG. 52.—LEFT CARPAL BONES OF HORSE, WITH DISTAL END OF RADIUS AND PROXIMAL END OF METACARPUS; INTERNAL VIEW. ] [Illustration: FIG. 53.—LEFT CARPAL BONES OF HORSE, WITH ADJACENT ENDS OF RADIUS AND METACARPUS; EXTERNAL VIEW. _Ca_, Accessory carpal bone; _Cr_, radial carpal; _Ci_, intermediate carpal; _Cu_, ulnar carpal; _C1–4_, first to fourth carpals; _Mc. II_, _III_, _IV_, metacarpal bones; _1_, groove for tendon of extensor carpi obliquus; _2_, groove for lateral extensor tendon; _3_, groove for tendon of flexor carpi externus; _4_, metacarpal tuberosity. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ] THE ACCESSORY CARPAL BONE