HAND. The human hand is composed of 27 bones, namely, eight bones of the carpus or wrist arranged in two rows of four each, the row next the fore-arm containing the scaphoid, the semi-lunar, the cuneiform and the pistform, and that next the metacarpus, the trapezium, the trapezoid, the os magnum and the unci form. The metacarpus consists of the five bones which form the palm, the first being that of the thumb, the others that of the fingers in succession. Lastly, the fingers 'proper contain 14 bones called phalanges, of which the thumb has but two all the other digits having three each. These bones are jointed so as to admit of a variety of movements, the most char acteristic being those by which the hand is flexed backward, forward and sideways, and by which the thumb and fingers are moved in different ways.
The chief muscles which determine these movements are the which pass down the forearm, are attached by tendons to the phalanges of the fingers and serve to flex or bend the fingers; and the °extensors') for ex tending the fingers. There are two muscles which flex all the fingers except the thumb. The thumb has a separate long and short flexor. There is a common extensor for the fingers which passes down the back of the forearm and divides at the wrist into four tendons, one for each finger, each being attached to all three phalanges. The forefinger and little finger have, in addition, each an extensor of its.own, and the thumb has both a short and a long extensor. The tendons of the muscles of the hand are interlaced and bound together by bands and aponeurotic fibres, and from this results a more or less complete unity of action.
It is sometimes difficult to make a movement w:th a single finger without the others taking part in it, as in executing instrumental music, for instance; but practice gives to these move ments perfect independence.
Of all the movements of the hand the op position of the thumb to the other fingers, alone or united, especially characterizes the human hand. This action of the thumb results from its length, from the first metacarpal bone not being placed on the same plane as the other four, as is the case in the monkey, and from the action of a muscle— the long flexor of the thumb— peculiar to the human hand. This muscle com pletes the action of the other motor of the thumb and permits man to hold a pen, a graver or a needle; it gives to his hand the dexterity necessary in the execution of the most delicate work. Properly speaking then, the hand, with its highly specialized muscles, belongs to man alone. It cannot be considered, as in the ape, as a normal organ of locomotion. It is essen tially the organ of touch and prehension. It molds itself to a body to ascertain its form; it comes to the aid of the eye in completing or rectifying its impressions. The functions of touch devolve principally on its anterior or palmar face, the nervous millz abounding specially at the ends of the fingers. A layer of adipose tissue very close in texture protects, without lessening its power or its delicacy, the network of muscles, vessels and nerves with which this remarkable organ is equipped.