The Radio-Ulnar Joints, between the extremities of the opposing surfaces of the radius and ulna. These joints are both gliding joints, and by their means the radius is enabled to turn round on the ulna, causing pronation and supination of the hand. (Pronation = palm downwards; supination = palm upwards.)

Fig. 8. Muscles of the Upper Arm.

Fig. 8. - Muscles of the Upper Arm.

Superior Radio-Ulnar Joint, between the head of the radius and the radial notch on the outer surface of the ulna.

Ligaments

Orbicular ligament, a ring of strong tendinous fibres attached to the extremities of the radial notch on the ulna and encircling the head of the radius; the lower edge of the ring is smaller than the upper, so that the radius is as it were suspended by its head.

Accessory Ligament : Oblique ligament, a thin band from the outer surface of the coronoid process of the ulna to the radius, where it is attached just below the bicipital tuberosity.

The synovial membrane lines the joint, and is continuous with that of the elbow-joint.

Inferior Radio-Ulnar Joint, between the inner surface of the inferior end of the radius and the outer surface of the inferior end of the ulna; the inferior surface of the lower end of the ulna is also included in the joint by means of the articular disc of cartilage which excludes the ulna from the wrist-joint.

Muscles Acting On The Radio-Ulnar Joints

Action.

Muscle.

Origin.

Insertion.

Nerve-supply.

Supination

Brachioradialis

(supinator longus)

From the upper two-thirds of the external supracondyloid ridge of the humerus

The styloid process of the radius

Musculospiral

Supinator brevis

From the external condyle of the humerus ; the external lateral and orbicular ligaments ; the triangular surface of the ulna just below the radial notch and the deep fascia

The upper part of the shaft of the radius, reaching from the neck to the oblique line, and from the anterior border of the bicipital tubercle round to the posterior border

Posterior interosseous

Pronation

Pronator radii teres

From the common flexor tendon on the internal condyle of the humerus, the lower part of the ridge above the inter-muscular septa and the deep fascia, and a slip from the inner side of the coronoid process of the ulna

The rough oval impression half-way down the outer surface of the shaft of the radius

Median

Pronator quadratus

From the lower fourth of the ventral surface of the ulna

The outer border of the lower fourth of the ventral surface of the radius

Anterior interosseous