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.