Each of the four fingers has three bones (phalanges, soldiers). Each phalanx turns on the one above, leaving exposed the end of the higher bone. There are no muscles below the knuckles; but the fingers are traversed by tendons on the back, and are covered on the front by tendons and skin pads.
The middle finger is the longest and largest, because in the clasped hand it is opposite the thumb and with it bears the chief burden. The little finger is the smallest and shortest and most freely movable for the opposite reason. It may move farther back than the other fingers, and is usually held so, for two reasons; one is that the hand often "sits" on the base of the little finger; the other is that being diagonally opposite the thumb it is twisted farther backward in any outward twisting movement, and so tends to assume that position.
Sectional Views 1 1 First finger between knuckle and second joint
2 Between second and third joint
3 Last joint at nail
The skin pads are of approximately the same length, as necessary when the finger is tightly closed, but the segments are of different lengths; so the creases are not opposite the joints.
In the first finger the creases are beyond the knuckle, opposite the middle joint, and short of the last; in the second finger they are beyond the knuckle, beyond the second joint, about opposite the last; in the third finger they are beyond the knuckle, beyond the second. The other positions vary in different individuals.
The joints of the fingers are built like shallow saddle joints; that is, one reaches up on the sides, the other reaches down on the front and back.
In every case it is the more distant bone that turns on the convex end of the nearer bone, leaving the end of the latter exposed in flexion.
On the palmar surface, when the fingers are straight, the palm extends beyond the knuckles half way to the next joint; but when the fingers are bent, a portion bends with them, and belongs with them; so that when bent the fingers on the palmar side start from the knuckle.
Thus when straight the fingers have three pads; when bent they have four.
When curled close, the ends of the fingers just cover the heads of their first phalanges; that is, they lie with their tips against the knuckles, supporting them. This is a mechanical necessity in fitting the fingers into the fist.
Thus the two outer segments are longer than the first, but when measured from the back of the knuckle, the first segment is equal in length to the latter two.
Opposite the three bones of the finger are four skin pads; the pads therefore smaller.
The first joint is about equal to the last two, measuring from back of the knuckle (though the bone itself is shorter). When the three joints are bent to form three sides of a square, the four pads fill in the quarters of it. Three of the grooves between them are diagonals, with two other grooves irregularly placed.
The blow with the fist falls on the knuckle of the second finger, which is the longest, strongest, and in line with the radius.
The more tightly it is clenched, the more it is arched across the knuckles.
The bones of the second row lie in the same plane.
The thumb lies against the first finger, or across the second.
The hand, open, is an implement.
When driven forward, the second knuckle, as the most prominent, becomes the point of impact; but in clenching it is braced by the entire fist, bone, tendon and knuckle.
When driven directly forward, the second knuckle is in line with the wrist and the radius, making a straight battering ram.
There is no muscular covering for the knuckles; only the tendons, which are half blended with them, and roughened skin.
In clenching, this skin is tightly stretched, and by contact with objects is hardened, so that in other positions it is wrinkled.
The end of the metacarpal bone is a round dome, over which fits the socket of the first phalanx. The dome is protected on the sides by square projecting flanges, which are matched by the sides of the socket. They are in the first finger set at a slight diagonal, so that there is an overhang of the phalanx, serving to protect the joint in lateral blows.
Tendons of the extensor digitorum communis Dorsal interosseous muscles
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