The carpal bones are usually described as eight in number, namely, scaphoides, semilunare, cuneiforme, pisiforme, trapezium, trapezoides, magnum, and unciforme. But the os scaphoides is composed of two primitively distinct bones ; so likewise the os unciforme, which supports the two last metacarpal bones. Cuvier remarked that the carpus of monkeys has one bone more than that of man, and that it is situated between the scaphoid, trapezium, and os magnum, of which latter he incorrectly regarded it as a dismemberment. The os scaphoides is formed of two primitively distinct pieces, which, in the orang, "extend almost as much from the os lunare as from the radius, along the radial side of the carpus to reach the trapezium and trapezoides ; it is in great part interposed between the lunare of the proximal row and the trapezium and trapezoid of the distal row of the carpal bones. The similarity of its connections in the carpus with those of the scaphoid in the tarsus is so close that the serial homology of the two bones is unmistakable."* But the breadth of the human hand requires that the carpal bones should be arranged in two rows only; consequently we find the two last bones coalesced to form the single os scaphoides, which, with the os semilunare, and os cuneiforme, constitute a convex articulating surface applied to the inferior extremity of the radius, and to the interarticular cartilage of the ulna. The os pisiforme projects from the os cuneiforme out of the line of the first range of carpal bones. The trapezium, the trapezoides, and the magnum support each a metacarpal bone. The os unciforme, to which are articulated the fourth and fifth metacarpal bones, is composed of two pieces, which in man are confluent at an early period.
* Owen on the Archetype Skeleton, p. 167.
The digits, supported upon their metacarpal bones, rarely exceed, in mammalia, the number of ossicles contained in the distal row of the carpus. Though often reduced to one digit, as in the horse, the proper typical connections are never lost, though they may be, from many reasons, somewhat obscured. In man, the single piece representing the two first carpal bones, and called os sca-phoides, articulates by a convex head with the trapezium and the trapezoides, the former supporting the thumb, the latter the forefinger. The semi-lunare, the os magnum, and the rays of the middle finger succeed one another in a straight line. The cuneiform and pisiform bones overhang the two ossicles, which, coalesced, form the os unciforme, upon which rest the two last digits, the ring and the little finger. The thumb, the most inconstant digit in mammalia, though the most highly developed in man, has but two phalanges appended to the metacarpal bone; all the other digits have three.