HAND, TILE. The genus homo, or MAN, takes rank in the classification of mam mals as a distinct order, BIMANA, in consequence of man being the only animal pos sessing two hands. At first sight, it might be considered that four-handed animals—the monkeys, apes, and their allies, which are placed by zoologists in the order QUADEU mAnk—were superior to those which possess only two hands, but this is far from being the case. None of these four hands are adapted to the variety of actions which the human hand is capable of performing, and they are all, in some degree, required for support and locomotion; so that while in the higher forms of the quadrumana the extremities present an approximation in structure to those of man, in the lower they gradually tend to resemble the ordinary quadrupedal type. " That," says envier, "which constitutes the hand, properly so called, is the faculty of opposing the tliuml> to the other fingers, so as to seize upon the most minute objects—a faculty which is car ried to its highest degree of perfection in man, in whom the whole anterior extremity is free, and can be employed in prehension." The peculiar prehensile power of the human hand is chiefly dependent upon the length, power, and mobility of the thumb, which can be brought into exact opposition to the extremities of all the nngers,whether separately or grouped together.
Before describing the hand itself, we must say a few words on the upper extremity generally, of which the hand may be regarded as•the essential part.
The general arrangement of the bones of the arm will be readily understood by a reference to fig. 1. The general plan of the osseous framework of the upper and lower limb is very similar. The humerus or arm-hone corresponds to the femur or thigh-bone; the lower end of the humerus is connected with the two bones of the forearm, the radius and the ulna, which correspond with the two bones of the leg. Then come the carpal bones, the metacarpal bones, and the phalanges, just as we have tarsal bones, meta tarsal bones, and phalanges in the foot.
In fig. 2 (which we copy from Humphry's Human Foot and human Hand,) we have diagram showing the way in which the bones of the hand are arranged. The carpal bones (3 to 10 in the figure) are eight in number, and are arranged in the wrist in two rows. The first or upper row consists practically of three bones (3, 4, 5), the fourth (6)
being regarded as belonging to the class of sesamoid bones (q.v.), and the second row of four bones (7, 8, 9, 10); so that, excluding the pisiform bone (6), the carpal and the tarsal bones correspond in number. As we commonly term the palm the front of the hand, The thumb becomes conventionally the outer, and the little finger the inner digit; but according to the rules of comparative anatomy, and in order to compare the hand and foot, we ought to reverse these terms. The outer (3) of the carpal bones of the first row supports (through the intervention of 7 and 8) the bones of the thumb and fore Enger (t and nO, and constitutes with them the outer division of the hand. The inner 15) of the carpal bones bears the little, and the next (the ring) finger (v and iv), and con stitutes with them the inner division of the hand; while the middle one (4) bears the middle finger (m), and belongs to the middle division of the hand. We likewise see from this figure, and likewise from fig. 1, that the two outer bones (3 and 4) are connected with the radius, while the inner bone (5) is connected (indirectly by a thick ligament) with the ulna.
It is unnecessary for us to enter into any anatomical details regarding the individual carpal bones. Collectively, they are so arranged that the carpus presents a dorsal con vex surface, upon which the tendons of the extensor muscles of the fingers play, and a palmar concave surface on which the tendons of the flexor muscles lie. The several bones are joined to one another—each bone being united to three or more others—by a large extent of surface, and are girded together by strong ligamentous bands. The wrist is thus as strong as if it had been constructed of one solid piece of bone, while the slight gliding movements which occur between the several bones give it an elasticity which serves to break the shocks that result from falls upon the hand. The uppermost surface of the first row of carpal bones is convex, and this convex surface is received into a wide cup or socket, formed by the lower articular surface of the radius, and by a ligament passing from that bone to the ulna.