The masses of the hand are two - one that of the hand proper, the other that of the thumb.
The first of these is beveled from knuckles to wrist on the edge; from wrist to knuckles on the flat side, and from first to little finger from side to side. It is slightly arched across the back.
Somewhat more arched are the knuckles, concentric around the base of the thumb. The second knuckle is larger and higher than the rest; the first is lower on its thumb side, where it has an overhang, as has also the knuckle of the little finger, due to their exposed positions.
Belonging to the hand is the pyramidal mass of the first segment of the thumb, which joins on at an angle, never quite flat with the hand, and bending under it to more than a right angle with its flat surface.
The thumb may be drawn in until only its root bulges beyond the lateral line of the hand, and may be carried out to a great angle with it. In this latter position its first segment forms a triangle whose base is the side of the hand, equal to it in length; whose height is, on the palmar surface, equal to the width of the hand, and on the dorsal surface, almost as great.
On the little finger side, the form is given by the abductor muscle and the overhang of the knuckle, by which the curve of that side is carried well up to the middle of the first segment of the finger.
The pad of the palm overlaps the wrist below and the knuckles above, reaching to the middle of the first segment of the fingers.
On the back of the hand, nearly flat except in the clenched fist, the tendons of the long extensors are superficial, and may be raised sharply under the skin. They represent two sets of tendons more or less blended, so are double and have connecting bands between them.
The Hand. Muscles of the Hand, front palmar.
1 Abductor pollicis.
2 Flexor brevis pollicis.
3 Abductor transversus pollicis.
5 Annular ligament.
6 Flexor brevis minimi digiti.
7 Abductor minimi digiti.
The Hand. Muscles of Back of Hand.
1 First dorsal interossei.
2 Abductor pollicis.
3 Dorsal interossei.
4 Tendons of extensor communis digitorum.
The Hand. Wedging of the Wrist: ThumB Side.
The Hand. Wedging of the Wrist: Little Finger Side.
Morticed with the bones of the hand are the bones of the wrist: the two make one mass, and the hand moves with the wrist.
Eight bones (carpal bones) in two rows make the arch of the wrist: in size they are like deformed dice. The two pillars of this arch are seen on the palmar side, prominent under the thumb and the little finger. The latter is the heel of the hand, but the arch is thicker and a bit higher on the thumb side. Under it pass the long flexor tendons to the fingers and thumb.
The dome of the arch is seen on the back, with an apex at the trapezium under the first finger. It is crossed by the long extensor tendons of the fingers, which converge on its outer half.
Its width is twice its thickness. It is narrower both ways where it joins the arm, giving an appearance of constriction.
There is always a step-down from the back of the arm, over the wrist, to the hand.
Being solid with the hand, the wrist moves with the hand on the forearm. Its movement is like that of a boat in water; easily tipping sideways (flexion and extension) with more difficulty tilting endways (side-bending) which in combination give some rotary movement, but having no twisting movement at all. This movement is accomplished by the forearm.
The inset of this boat-shaped joint with the arm gives the appearance of constriction. The prow, under the thumb, is higher than the stern under the little finger.
When fully extended, the back of the hand with the arm makes almost a right angle; when fully flexed, the palmar surface makes almost a right angle; the total movement therefore is slightly less than two right angles.
When the wrist is fully flexed, it forms at the back a great curve over which the extensor tendons are drawn taut, so much so that the lingers can never be closed when the wrist is fully flexed. In this position the flexor tendons are raised prominently under the skin.
When hand and arm lie extended along a flat surface, it is the heel of the hand that is in contact, the arm bones being lifted from the surface.
To the four corners of the wrist are fastened muscles; two in front (flexor carpi radialis and flexor carpi ulnaris) and two behind (extensor carpi radialis and extensor carpi ulnaris, the former being double). By their contraction the wrist is moved in all directions, except twisting, which movement is produced not in the wrist but in the forearm. Only the tendons cross the wrist, the muscular bodies lying in the forearm.