The metacarpal bones are numbered from without inwards. The or that of the thumb, is the shortest of all and likewise the thickest. Its carpal extremity will likewise serve to distinguish this bone; it wants the cuneiform shape, and is rather wider on its palmar than its dorsal surface. It has no arti tarlar facets on its sides, being articulated with the trapezium alone by means of a surface which is concave from before backwards, and convex from side to side; the body of this bone is flatter on its palmar and dorsal surfaces than any of the others.
The second metacarpal bone is the longest; it, however, exceeds the third by a very slight difference. It is further distinguished from the third by the diminutive size of the articular facet on the radial side of its car al extremity.
The third metacarpal bone, though shorter than the second, is manifestly thicker and stronger; this excess of developenient being attributable to its affording insertion to one of the most powerful muscles of the hand,— namely, the adductor pollicis.
The fourth and fifth metacarpal bones are shorter and in every way smaller than the pre ceding ones. The fifth is shorter and somewhat thicker than the fourth : it has no articular facet on the ulnar side of its carpal extremity, but presents a prominent tubercle in that situation fctr the insertion of the extensor carpi ulnaris.
The structure of the metacarpal bones is the same as that of the long hones in general. are two points of ossification for each metacarpal bone, one for the body and the carpal extremity, the other for the digital extremity. The first metacarpal bone, however, according to Cruveilhier, oilers an exception to this, inasmuch as its carpal extremity is developed from a point of ossifica tion distinct from that of the body. In some instances there are three points of ossification for each metacarpal bone. The bodies of the metacarpal bones are completely ossified at birth. Between the second and third years appear the points for tire inferior extremity in the four inner bones and the superior extremity in the first, but the complete fusion of the extremities with the shafts does not take place till near the twentieth year.
III. Fingers (digiti; Fr. les doigts ; Germ. die Finger). The fingers dill'er strikingly from the toes as regards their length, to which, in deed, is due their greater 'nobility. They are
numbered in proceeding from the radial to the ulnar side of the hand. All except the thumb are composed of three phalanges, the superior or metacarpal, the middle, and the inferior or lingual: in the thumb the middle phalanx is absent. The fingers differ considerably in length; the thumb is by far the shortest, and the middle finger is the longest. Next in length is the ring finger, then the index, and last arid least the little finger.
The metacarpi' phalanges have the following general characters :-1st, a body slightly con cave from above downwards on the palmar surface, and convex on the dorsal; 2d, a supe rior or metacarpal extremity more expanded than the inferior, hollowed into an articular surface for the head of the metacarpal bone; and 3d, an inferior extremity, having a pulley like surface for articulation with the middle phalanx. The metacarpal phalanges are the longest.
The middle phalanges present the same cha racters as the preceding as regards the body. The superior extremity has a pulley-like articu lar surface, conve.r transversely ; that of the inferior extremity being concave in the same direction.
The ungual phalanges are readily distin guished by the inferior or ungual extremity, which is rough, non-articular, horseshoe-shaped, with the convexity directed downwards. It is this part of the bone which supports the nail. The superior extremity is articulated with the middle phalanx by a pulley-like surface, con cave transversely. The ungual phalanx of the thumb is considerably larger than that of any of the other fingers.
In point of structure and development the phalanges scarcely differ from the metacarpal bones. There are two points of ossification, one for the body and inferior extremity, the other for the superior extremity. This last is late in making its appearance, not until between the third and seventh year, while the ossifica tion of the body commences at an early period of intrauterine life.
Although the perfect prehensile baud is pecu liar to man and the Quadrumana, the inferior segment of the anterior extremity will be found to possess many interesting analogies through out the inammiferous series. On this point we refer to the articles OSSEOUS SYSTEM (Comp. Anat.) and SKELETON.