In opening this joint in the manner already described, it will be seen how extensive is its synovial membrane. It extends some distance on the palmar and dorsal surfaces of the neck of the os magnum, and sends two processes between the bones of the first row (between the scaphoid and lunar, and the lunar and cuneiform), and three processes between those of the second row, (one on each side of the os magnum,) and one between the trapezium and trapezoid.
Motions of the carpal articulations.—An examination of the dissected carpus will at once show how limited are the motions between any two of the carpal bones of each row. The movement of one row upon the other, however, is more extensive, hut only in the direction of flexion and extension, the former being con siderably greater in consequence of the less resistance of the dorsal ligaments. Solidity and strength, a power of resistance to violence which might easily occasion fracture, were the carpus one solid bone, are gained by the mini ber of small bones of which it is composed, the arthrodial form of its articulations, and the strong ligaments by which the motions of these joints are restricted.
'1r/fru/a/ion cf the pisifinnit bone.—The pisiform bone is so little connected with the mechanism of the carpus that its articulation with the cuneiform bone demands a separate consideration. A plane oval surface on the posterior part of the pisiform is articulated with a corresponding one on the palmar aspect of the cuneiform, and several strong ligaments strengthen the joint. Two lateral ligaments pass from the pisiform to the cuneiform bone, the internal, which is also anterior, being of considerable strength. This bane is further connected to the unciform by strong ligament ous fibres; and a strong bundle, which bears the same relation to the tendon of the flexor carpi ulnaris as the ligamentum patellae does to the tendon of the rectus fernoris, extends to the carpal extremity of the fifth metacarpal bone. This joint is provided with a loose synovial membrane ; its motions are those of gliding in the directions of the axis of the articular surfaces.
Carpo-metacarpal join ts.—These are very strong articulations, and, with the exception of the first and fifth, enjoy a very limited extent of motion. The four internal ones are nearly planiform arthrodia', restricted on the palmar and dorsal surfaces by strong and short liga ments ( pahnar and dorsal ligaments), the latter being much better developed. The second
metacarpal bone is articulated with the trapezoid in an extremely firm manner: its painter liga ment extends from the extremity of the meta carpal bone to the trapezium internal to the ridge, and covered by the tendon of the radial flexor of the wrist. There are three dorsal ligaments, an external attached to the trapezium, and an internal to the os magnum. These two ligaments are oblique in their direction ; the third or middle one is vertical and attached to the trapezoid. The third metacarpal bone is arti culated with the as magnum : here we find three strong palmar ligaments, an external one which extends obliquely outwards to the trape zium, an internal one which passes in front of the carpal extremity of the fourth metacarpal bone, adhering to it, and inserted into the unciform and the fifth metacarpal bone, and a middle one which passes vertically to the os magnum. This joint has two dorsal ligaments, both inserted into the os magnum. Thefiurth metacarpal bone is articulated with the radial portion of the inferior articular surface, and with a very small portion of the os magnum; it has a single palmar and dorsal ligament. The fifth metacarpal bone is articulated with the outer part of the inferior surface of the unciform; this surface is convex transversely and concave from before backwards, while that on the metacarpal bone is convex from before backwards and concave transversely. The proper ligaments of this joint are very feeble, being merely a few fibres attached to the ante rior and posterior surfaces of the synovial mem brane. The joint, however, is protected in front by the prominence of the unciform pro cess, which descends a little below the line of the articulation, and limits the forward motion of the carpal extremity of the bone ; and pos teriorly it is strengthened by the tendon of the extensor carpi ulnaris, while its motion ulnad is restricted by the strong internal palmar liga ment of the third metacarpal bone, which we have already described as passing from that bone to the fifth metacarpal and the miciform bones. The fifth carpo-metacarpal articulation approaches in many particulars to the first ; it has agreater latitude of motion than the three immediately preceding it, and its articular sur faces very much resemble those of the first.