General features.-The radius is the lateral bone of the forearm. It is a long bone, prismoid in form and gently curved in its long axis. The upper and lower ends are both expanded, but the latter is by much the wider of the two. The shaft increases in breadth rapidly towards the lower end, is convex to the lateral side and is concave forwards in its lower part. Examination of the shaft, therefore, will enable a given radius to be assigned to its correct side in the body.
The upper end of the radius includes the head, the neck and the tuberosity. The head is disc-shaped and bears on its upper surface a shallow cup for articulation with the capitulum of the humerus. The articular circumference of the head is smooth and is deepest on the medial side, where it articulates with the radial notch of the ulna. Its posterior aspect can readily be felt through the skin, as it lies at the bottom of a small depression which is visible in the living subject on the lateral side of the posterior aspect of the extended elbow. The neck of the radius forms a distinct constriction immediately below the head, and overhung by it, especially on the lateral side. The tuberosity of the radius is placed below the medial part of the neck. Its posterior part is roughened but its anterior part is usually smooth.
The shaft of the radius is narrow above but widens rapidly towards the lower end. As already stated, it is gently curved with the convexity directed to the lateral side. On transverse section it is somewhat triangular in outline, but only one of its three borders is sharp and easy to identify. The interosseous border forms a salient crest, except at its upper end, where it approaches the lower part of the tuberosity. At its lower end it follows the posterior margin of a small, elongated, triangular area, which constitutes an additional-medial-surface for this end of the bone. In its lower three-fourths this border gives attachment to the interosseous membrane, which connects the opposed borders of the two bones of the forearm. The anterior border can be recognized without difficulty at its upper and lower ends, but it is rounded and indefinite in the intervening region. It commences just below the anterolateral part of the tuberosity and runs downwards with an inclination towards the lateral side. This part is often described as the anterior oblique line of the radius. The lower part of the anterior border forms a sharp crest along the lateral margin of the anterior surface, which can be recognized easily through the skin. The posterior border is clearly defined in its middle third only. Above, it runs obliquely upwards and medially towards the postero-inferior part of the tuberosity. Below, it forms a rounded ridge directed towards a tubercle which marks the posterior aspect of the lower end of the bone.
The anterior (volar) surface lies between the anterior and the interosseous borders. It is slightly concave from side to side and curves forwards at its lower end. A little above its middle it is pierced by the nutrient foramen, which is directed upwards. The posterior surface, which is placed between the interosseous and the posterior borders, is generally flat but may be slightly hollowed out in its upper part. The lateral surface is gently convex in all directions. Above, owing to the obliquity of the upper ends of the anterior and posterior borders, it encroaches on the anterior and posterior aspects of the bone, and this widened portion is usually slightly roughened. A finely irregular rough surface occupies a somewhat oval area near the middle of the shaft, but below this the surface of the bone is smooth and featureless.
The lower end is the widest part of the radius, and is four-sided on transverse section. Its lateral aspect is slightly rough and projects downwards beyond the rest of the bone to form the styloid process. This projection can be felt through the skin, when the tendons which conceal it in the living body are relaxed. The inferior surface of the lower end (fig. 421) is smooth and takes part in the formation of the wrist-joint. This articular surface is divided by a faint ridge into a medial, quadrangular and a lateral, triangular area, which covers the medial side of the styloid process. The anterior surface forms a thick, prominent ridge, which can be palpated in the living subject. The medial surface is occupied by the ulnar notch, which provides a smooth strip, concave from before backwards, for articulation with the head of the ulna in the inferior radio-ulnar joint. The posterior aspect is marked by the dorsal tubercle, which is limited on its medial side by a narrow, oblique groove. A wide, shallow groove lies on the lateral side of the tubercle and is divided into two parts by a very faint vertical ridge. A similar, but continuous, groove marks the medial part of the posterior aspect.
Particular features.-The upper aspect of the head and its articular circumference are everywhere covered with hyaline cartilage. The upper margin of the head fits into the, groove between the capitulum and the trochlea, and, when the forearm is flexed it invades the radial fossa. The articular circumference articulates with the radial notch of the ulna, and in the rest of its extent is surrounded by, but gives no attachment to, the annular ligament, within which it rotates in pronation and supination. The neck of the bone is surrounded by the narrower, lower part of the annular ligament, but is separated from it by a protrusion of the synovial membrane of the superior radioulnar joint:
The rough posterior part of the tuberosity gives insertion to the biceps muscle, but its smooth anterior part is separated from the tendon by a bursa. A little below the tuberosity the oblique cord is attached to the radius.
The upper oblique part of the anterior border and a variable portion of the border below gives origin to the thin, expanded radial head of the flexor digitorum sublimis. Its conspicuous lower extremity gives attachment to the lateral edge of the extensor retinaculum (dorsal carpal ligament). The small triangular area in front of the lower end of the interosseous border and above the u1nnr notch gives insertion to the deepest fibers of the pronator quadratus.
The upper two-thirds of the anterior surface provides an extensive area for the origin of the flexor pollicis longus muscle, which conceals the nutrient foramen. The lower fourth of this surface, together with the triangular area on the lateral side of the bone, receives the insertion of the pronator quadratus. The roughened area at the middle of the lateral surface is situated at the region of maximum curvature and gives insertion to the pronator teres. Above, the surface widens at the expense of the anterior and posterior surfaces, and the elongated V-shaped area (figs. 415 and 416) provides insertion for the supinator muscle. Below the insertion of pronator teres the lateral surface is smooth and is covered by the tendons of the radial extensors of the wrist. The upper part of the posterior surface gives origin to the abductor pollicis longus, above, and the extensor pollicis brevis, below. The lower part of this surface is devoid of muscular attachments but is closely covered by the extensor pollicis brevis and longus muscles.
The styloid process of the radius projects to a lower level than the ulnar styloid, and its tip is concealed by the tendons of the abductor pollicis longus and the extensor pollicis brevis. It gives attachment by its tip to the lateral ligament of the wrist-joint. The lateral aspect of the lower end of the radius just above the styloid process receives the insertion of the brachioradialis and is crossed obliquely, from above downwards and forwards, by the tendons of the abductor pollicis longus and extensor pollicis brevis. The ridge-like anterior aspect of the lower end gives attachment to the anterior ligament of the wrist-joint. The ulnar notch is limited below by a smooth ridge to which the base of the triangular articular disc of the inferior radio-ulnar joint is attached. A small protrusion of the synovial membrane of the joint extends upwards in front of the lower end of the interosseous membrane. The lateral, triangular part of the carpal articular surface articulates with the scaphoid (navicular) and the medial, quadrangular part with the lateral part of the lunate bone. When the hand is abducted fully the whole of the lunate bone comes into contact with the radius.
Structure.–The structure of the radius is like that of the other long bones.
Ossification (figs. 417, 418).—The radius is ossified from three centers : one for the shaft, and one for each end. That for the shaft appears near the middle, in the eighth week of fetal life. About the close of the second year, ossification begins in the lower end ; and at the fifth year, in the upper end. The upper epiphysis fuses with the shaft at the age of seventeen or eighteen years, the lower about the age of twenty. An additional center sometimes appears in the radial tuberosity about the fourteenth or fifteenth year.
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