The =accessory carpal= bone (Os carpi accessorium, pisiform) is situated behind the ulnar carpal bone and the outer part of the distal end of the ulna. It is discoid and presents for description two surfaces and a circumference. The =internal surface= is concave and forms the outer wall of the carpal groove. The =external surface= is convex and rough; a smooth groove for the outer tendon of the flexor carpi externus crosses its anterior part obliquely downward and slightly forward. The =anterior border= bears two facets; the upper one is concave and articulates with the back of the outer facet on the distal end of the radius; the lower one is convex and articulates with the ulnar carpal bone. The remainder of the circumference is rounded and rough. The accessory does not directly bear weight, and may be regarded as a sesamoid bone interposed in the course of the tendons of the middle and external flexors of the carpus, which it enables to act at a mechanical advantage. The posterior border furnishes attachment to the transverse carpal ligament, which completes the carpal canal for the flexors of the digit. THE FIRST CARPAL BONE The =first carpal= bone (Os carpale primum, trapezium) is a small inconstant bone, commonly about the size and shape of a pea, which is situated in the lower part of the internal lateral ligament behind the second carpal bone. This bone appears to be absent on both sides in about half of the cases; in a good many subjects it is present on one side only. In size it varies from a minute nodule to a discoid mass 10 to 12 mm. in length. In exceptional cases it articulates with both the second carpal and the second (inner) metacarpal bone, in other cases with the former only, but in the majority of specimens no articular facet is present. THE SECOND CARPAL BONE The =second carpal= bone (Os carpale secundum, trapezoid) is the smallest constant bone of the lower row, and is irregularly hemispherical in shape. The =superior= or =proximal surface= is a convex facet which is continued upon the =posterior= or =volar surface= and articulates with the posterior part of the radial carpal. The =external surface= faces obliquely outward and forward, and bears three facets for articulation with the third carpal bone. The =anterior= or =dorsal= and the =internal surface= bear a tuberosity to which the lateral ligament is attached. The =inferior= or =distal surface= is articular and consists of a large flattened facet for the inner (second) metacarpal bone, and a small one for the large (third) metacarpal bone. Some specimens have a small facet on the lower part of the posterior surface which articulates with the first carpal bone. [Illustration: FIG. 54.—CARPAL BONES OF HORSE, WITH ADJACENT ENDS OF RADIUS AND METACARPUS; ANTERIOR VIEW. THE ACCESSORY AND FIRST CARPAL BONES ARE NOT SHOWN. ] [Illustration: