大多角骨 - Os trapezium
The trapezium bone (greater multangulate, os multangulum majus) may be distinguished by a deep groove on its volar surface. It is situated at the radial side of the carpus, between the scaphoid and the first metacarpal bone.
The superior surface is directed upward and medialward; medially it is smooth, and articulates with the scaphoid; laterally it is rough and continuous with the lateral surface.
The inferior surface is oval, concave from side to side, convex from before backward, so as to form a saddle-shaped surface for articulation with the base of the first metacarpal bone.
The dorsal surface is rough.
The volar surface is narrow and rough. At its upper part is a deep groove, running from above obliquely downward and medialward; it transmits the tendon of the Flexor carpi radialis, and is bounded laterally by an oblique ridge. This surface gives origin to the Opponens pollicis and to the Abductor and Flexor pollicis brevis; it also affords attachment to the transverse carpal ligament.
The lateral surface is broad and rough, for the attachment of ligaments.
The medial surface presents two facets; the upper, large and concave, articulates with the trapezoid; the lower, small and oval, with the base of the second metacarpal.
The trapezium articulates with four bones: the scaphoid proximally, the first metacarpal distally, and the trapezoid and second metacarpal medially.
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