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Other Terms: Metacarpal I
The metacarpal bones, like the metatarsal bones, are miniature long bones because of the presence of a medullary cavity. They are numbered one to five from lateral to medial. They all have the basic design of a rounded distal head that form the large knuckles of the hand, a narrowed shaft with longitudinally concave palmar surfaces for the attachment of interosseous muscles, and an expanded proximal base. The first metacarpal is rotated in relation to the others to orient its ventral surface medially.
The term metacarpal comes from the Greek prefix meta- denoting over or beyond and karpos. The term karpos comes from the Greek word for wrist. It apparently arises from the Greek word karphos for chaff or splinters of wood. The term is a very old one and was used by Homer.
This bone articulates distally with the proximal phalanx. The articular surface is a rounded convex surface that is more narrow transversely. The smooth articular surface spreads further onto the palmar surface than it does on the dorsal surface. Proximally, the bone articulates with the trapezium and the second metacarpal.
The bone ossifies from two centers. A primary center arises in the shaft while a secondary center follows later in the base. The primary center appears at mid-shaft during the ninth week of prenatal life. At birth the bony shaft is developed. The secondary center in the first metacarpal base arises during the second year in females and during the third year in males. This is different from the other metacarpal bones, whose secondary centers arise in their heads. The epiphysis fuses about the fifteenth year in females and seventeenth in males. There can be considerable variation in these ossification times. Because the pollical metacarpal bone ossifies a second center at its base, similar to the phalanges, some consider it a third phalanx and not a metacarpal bone.