c. Superiorly a quadrilateral concave surface for articulation with the seaphoid. d. On the ulnar side a very small surface, adapted to a corresponding one on the radial side of the os magnum. The palmar surface is non-articular, five-sided, slightly excavated, and rough from the insertion of ligaments. The dorsal surface, also non-articular, is of greater extent, con vex, and likewise rough.
7. Os magnum (as capitatum ; Fr. le grand os; Germ. das Kopfbein). This bone is, as its name implies, principally characterized by its excess in size over the other carpal bones, and from the number of bones with which it is connected, it may be regarded as the key-bone of the carpus. Superiorly it is in the form of a rounded head (capitulum), flattened on the ulnar side, where it articulates with the unei form bone. The superior prominent portion of this head is received into the excavation of the lunar bone, and by its radial side it articulates with the inferior hollow surface of the scaphoid. The inferior portion of the bone is cuboid, and has been called the body ; it is rough and con vex on its palmar surface, also rough but irregular on its dorsal, both these surfaces affording insertion to numerous ligaments. Inferiorly we notice an extensive articular sur face, which is adapted in the centre to the third metacarpal bone, on the radial side to the second, and by a very small portion on the ulnar side to the fourth metacarpal bone. On the ulnar side of its inferior portion it articu lates a second time with the os uuciforme by a small circular articular surface, the cir cumference of which nearly equals that of the flat surface of a split pea. Lastly, on its radial side it articulates with the trapezoid bone. Thus the os magnum articulates with seven bones ; three metacarpal bones, two carpal bones in the inferior row, and two in the supe rior row.
8. Os unciforme (from uneus, a hook, as ha matum ; Fr. l'os crochn ou 2inciforme ; Germ. das Hakenbein, oder Keilformigcr Knochen). This bone has received its name from that which allows of its being easily distinguished from all the carpal bones,—namely, the hooked process, which projects from the radial edge of its palmar surface. This process constitutes a considerable prominence on the ulnar side of the carpus (cminentia carpi ulnaris inferior), and affords insertion to the annular ligament. Its concavity looks towards the radial side of the carpus; the remainder of the palmar sur face is rough for ligamentous insertion. The dorsal surface is likewise rough, convex, and of considerable extent. This bone articulates inferiorly with the fourth and fifth metacarpal bones, on its radial side with the os magnum, and on its superior surface with the cuneiform bone.
Structure of the carpal bones.—These bones are chiefly composed of the reticular osseous tissue, to which their extreme lightness is attri butable, the surface being invested by a thin layer of compact texture, in this respect per fectly resembling the bones of the tarsus.
Development. —The carpal bones are very late in their developement; at birth they are completely cartilaginous. According to Cru veilhier, each bone is developed by a single point of ossification. The os magnum and os uneiforme are the first in which the ossifie pro cess commences, about the end of the first year ; between the third and fourth years it begins in the cuneiform, a year later in the trapezium and lunar, and between the eighth and ninth years in the seaphoid and trapezoid bones. The ossification of the pisiform does not begin till between the twelfth and fifteenth years, and Cruveilhier states that of all the bones of the skeleton it is the last in which the process of ossification is curnpleted.
Metacarpus (Germ. die illittclhand). Five bones constitute the metacarpus, the four internal ones being parallel to each other, the external one diverging outwards at an acute angle with the middle line of the hand. These bones vary in length from about two inches and a half to one inch six-eighths. They articulate inferiorly with the superior or metacarpal pha langes, and superiorly with the carpus.
Each metacarpal bone presents two extremi ties, and a shaft or body between them. The superior or carpet extremity is expanded and wedge-shaped, the broader part being towards the dorsal aspect. Three articular surfaces exist on each; one, the most extensive, on the superior or carpal surface, for articulation with a carpal bone; the other two on the radial and Winn. surfaces, adapted to the adjacent meta carpal bone or to a carpal hone. The palmar and dorsal surfaces are rough and irregular, and afford insertion to the ligaments which strengthen the earpo-metacarpal joints. The inli•ior or digital extremity is in the form of a rounded head, flattened on each side, where we notice a depression, and behind it a tubercle which affords insertion to the lateral ligament of the joint. The smooth articular surface of tire head extends further upon the palmar sur face of the bone than upon its dorsal surface, or, as in the case of the metatarsal bones, more on the side of flexion than on that of extension. The shaft or body is prismatic and slightly curved, so as to present a concavity towards the palmar surface, and a convexity to the dorsal.