Turning movement as distinguished from rotary movement (flexion to each corner in rotation) is not present in the wrist, but is produced by the radius or turning bone of the forearm. Movement in the wrist is confined to flexion and extension (about one right angle) and side-bending (a little more than half a right angle, in the average hand); these two combined produce some rotary movement.
In movements of the wrist to extreme positions, the hand and fingers almost always participate, on account of association of tendons and muscular action; and in these positions it is practically always separation and hooking of the fingers that is produced.
The movement of the hand reflects itself as far as the shoulder, through the biceps muscle, which aids in turning the radius. In all movement but turning, the wrist can act alone. Turning, to nearly two right angles, is carried out by the radius. Further movement of any kind must be performed by elbow or shoulder.
At the elbow it is the hinge movement that is important, wherefore the large size of the ulna or hinge bone, and the small size of the radius. At the wrist it is the turning movement that is important, wherefore the radius forms two-thirds of the joint, the ulna one-third.
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