The forearm contains two bones, instead of one as in the arm. One of these bones, the ulna, is directly continuous with the humerus; the other, the radius, is continuous with the hand. In other words, the ulna is associated with the movements of the arm, and the radius with those of the hand. The large end of the ulna articulates with the humerus and its small end is at the wrist, while the large end of the radius is articulated with the hand and its small end with the humerus.

The ulna is the bone which acts mainly as a support. It articulates with the humerus by a pure hinge-joint; hence its only motion is one of extension and flexion. It is the fixed bone and does not take part in the movements of pronation and supination, but serves as an anchoring part for the attachment of the muscles which move the radius as well as the hand. At its upper extremity it has attached to it the brachialis anticus, triceps, and anconetcs muscles, which flex and extend it.

At its upper extremity on its outer side is the lesser sigmoid cavity for the articulation of the radius. Its lower extremity ends in a head tipped with a styloid process. The ulna gradually decreases in size from above downward until its lower fourth is reached, when it is slightly enlarged to end in the head. At its lower end, the lateral aspect of the head of the ulna rests in a cavity in the radius to allow of the movements of pronation and supination (Fig. 318).

The radius is small above and gradually increases in size until its lower extremity is reached, where it is largest. Its upper portion is composed mainly of compact bone with a medullary cavity; lower down as the bone becomes larger it becomes more cancellous. Hence it does not follow that it is strongest where it is largest; on the contrary it is most often fractured at its lower extremity. About two centimetres below the head of the radius is a tubercle. The biceps tendon is inserted into its posterior portion and a bursa covers its anterior part, over which the tendon of the biceps plays. The radius is the movable bone and to it is attached the hand.

Stretched across from one bone to the other is the interosseous membrane. Most of its fibres run from the ulna upward and outward, so that the shocks received on the hand are transmitted somewhat to the ulna. On its anterior surface run the anterior interosseous artery and nerve. About 2.5 cm. (1 in.) above its lower end the artery pierces the membrane to go to the back of the wrist.

Fig. 318.   Anterior view of radius and ulna with areas of muscular attachments.

Fig. 318. - Anterior view of radius and ulna with areas of muscular attachments.