Trapezoid - Os trapezoideum
The trapezoid (lesser multangulate, os multangulum minus) is the smallest bone in the distal row. It may be known by its wedge-shaped form, the broad end of the wedge constituting the dorsal, the narrow end the volar surface; and by its having four articular facets touching each other, and separated by sharp edges.
The superior surface, quadrilateral, smooth, and slightly concave, articulates with the scaphoid.
The inferior surface articulates with the proximal end of the second metacarpal bone; it is convex from side to side, concave from before backward and subdivided by an elevated ridge into two unequal facets.
The dorsal and volar surfaces are rough for the attachment of ligaments, the former being the larger of the two.
The lateral surface, convex and smooth, articulates with the trapezium.
The medial surface is concave and smooth in front, for articulation with the capitate; rough behind, for the attachment of an interosseous ligament.
The trapezoid articulates with four bones: the scaphoid proximally, second metacarpal distally, trapezium laterally, and capitate medially.
This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from http://www.bartleby.com/107/).
General Anatomy > Bones; Skeletal system > Appendicular skeleton > Bones of upper limb > Free part of upper limb > Bones of hand > Carpal bones > Trapezoid