THE RADIUS 219 



Structure. The long, narrow medullary cavity is enclosed in a strong wall of compact tissue 

 which is thickest along the interosseous border and dorsal surface. At the extremities the compact 

 layer thins. The compact layer is continued onto the back of the olecranon as a plate of close 

 spongy bone with lamella? parallel. From the inner surface of this plate and the compact layer 

 below it trabeculae arch forward toward the olecranon and coronoid and cross other trabeculse, 

 passing backward over the medullary cavity from the upper part of the shaft below the coronoid. 

 Below the coronoid process there is a small area of compact bone from which trabeculae curve 

 upward to end obliquely to the surface of the semilunar notch which is coated with a thin layer of 

 compact bone. The trabecula? at the lower end have a more longitudinal direction. 



Ossification (Figs. 215, 216). The ulna is ossified from three centers : one each for the body, the 

 inferior extremity, and the top of the olecranon. Ossification begins near the middle of the body, 

 about the eighth week of fetal life, and soon extends through the greater part of the bone. At birth 

 the ends are cartilaginous. About the fourth year, a center appears in the middle of the head, 

 and soon extends into the styloid process. About the tenth year, a center appears in the olecranon 

 near its extremity, the chief part of this process being formed by an upward extension of the body. 

 The upper epiphysis joins the body about the sixteenth, the lower about the twentieth year. 



Articulations. The ulna articulates with the humerus and radius. 



The Radius. 



The radius (Figs. 213, 214) is situated on the lateral side of the ulna, which 

 exceeds it in length and size. Its upper end is small, and forms only a small part 

 of the elbow-joint; but its lower end is large, and forms the chief part of the wrist- 

 joint. It is a long bone, prismatic in form and slightly curved longitudinally. It 

 has a body and two extremities. 



The Upper Extremity (proximal extremity}. The upper extremity presents a 

 head, neck, and tuberosity. The head is of a cylindrical form, and on its upper 

 surface is a shallow cup or fovea for articulation with the capitulum of the humerus. 

 The circumference of the head is smooth; it is broad medially where it articulates 

 with the radial notch of the ulna, narrow in the rest of its extent, which is embraced 

 by the annular ligament. The head is supported on a round, smooth, and con- 

 stricted portion called the neck, on the back of which is a slight ridge for the inser- 

 tion of part of the Supinator. Beneath the neck, on the medial side, is an eminence, 

 the radial tuberosity; its surface is divided into a posterior, rough portion, for the 

 insertion of the tendon of the Biceps brachii, and an anterior, smooth portion, on 

 \vhich a bursa is interposed between the tendon and the bone. 



The Body or Shaft (corpus radii). The body is prismoid in form, narrower 

 above than below, and slightly curved, so as to be convex lateralward. It presents 

 three borders and three surfaces. 



Borders. The volar border (margo wlaris; anterior border) extends from the lower 

 part of the tuberosity above to the anterior part of the base of the styloid process 

 below, and separates the volar from the lateral surface. Its upper third is promi- 

 nent, and from its oblique direction has received the name of the oblique line of the 

 radius ; it gives origin to the Flexor digitorum sublimis and Flexor pollicis longus ; the 

 surface above the line gives insertion to part of the Supinator. The middle third of 

 the volar border is indistinct and rounded. The lower fourth is prominent, and gives 

 insertion to the Pronator quadratus, and attachment to the dorsal carpal ligament; 

 it ends in a small tubercle, into which the tendon of the Brachioradialis is inserted. 



The dorsal border (margo dorsalis; posterior border) begins above at the back of 

 the neck, and ends below at the posterior part of the base of the styloid process; 

 it separates the posterior from the lateral surface. It is indistinct above and below, 

 but well-marked in the middle third of the bone. 



The interosseous crest (crista interossea; internal or interosseous border) begins 

 above, at the back part of the tuberosity, and its upper part is rounded and indis- 

 tinct; it becomes sharp and prominent as it descends, and at its lower part divides 

 into two ridges which are continued to the anterior and posterior margins of the 

 ulnar notch. To the posterior of the two ridges the lower part of the interosseous 



