THE DELICATE ADJUSTMENTS OF HAND AND ARM

LIKE the knee joint, the elbow is a hinge and though less ./complicated than the knee it presents some difficulties, as three bones enter into its formation. These are the humerus

above, and the radius and ulna below. The two latter form the bones of the forearm and the radius is on the thumb side and the ulna on the side of the little finger. The object of this joint is not only to allow the forearm to be bent but also to allow it to rotate on itself so that the palm of the hand may be turned up or down. This is accomplished by making the bone on the thumb side of the forearm (the radius) articulate both with the humerus and with the ulna.

The upper end of the radius is a thick, circular disc, saucer-shaped on top and with both its top and sides covered with smooth cartilage. The saucer fits exactly on to a round knob on the lower end of the humerus, and the rim is surrounded by a sleeve of thick ligament with a very smooth inner surface, thus enabling rotation to occur very easily. This sleeve of ligament binds the radius to the ulna and helps to prevent damage to the joint.

The main function of the elbow, however, is to allow the arm to be bent so that we can feed ourselves. It must also be a strong joint so that we can carry things. These two necessities have been fulfilled by making the upper end of the ulna into a hook that looks like the curve on the top of a question mark. The more or less cylindrical lower end of the humerus fits into the hook horizontally and there is a very strong muscle holding it down into the hook and preventing its dislocation forwards. This arrangement allows the arm to be bent forwards but prevents bending backwards and in addition the hook prevents the elbow from being ‘pulled out’ of joint when, for example, a heavy weight is carried.

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