The second metacarpal bone (metacarpal bone of the index finger) is the longest, and its base the largest, of all the metacarpal bones.
Human anatomy
Its base is prolonged upward and medialward, forming a prominent ridge.It presents four articular facets, three on the upper surface and one on the ulnar side:
Of the facets on the upper surface:
the intermediate is the largest and is concave from side to side, convex from before backward for articulation with the lesser multangular;
the lateral is small, flat and oval for articulation with the greater multangular;
the medial, on the summit of the ridge, is long and narrow for articulation with the capitate. [Source: Wikipedia 
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