WEOLOGY 



membrane is attached, while the triangular surface between the ridges gives inser- 

 tion to part of the Pronator quadratus. This crest separates the volar from the 

 dorsal surface, and gives attachment to the interosseous membrane. 



Surface. The volar surface (fades volaris; anterior surface) is concave in its 

 upper three-fourths, and gives origin to the Flexor pollicis longus; it is broad and flat 

 in its lower fourth, and affords insertion to the Pronator quadratus. A prominent 

 ridge limits the insertion of the Pronator quadratus below, and between this and 

 the inferior border is a triangular rough surface for the attachment of the volar 

 radiocarpal ligament. At the junction of the upper and middle thirds of the 

 volar surface is the nutrient foramen, which is directed obliquely upward. 



The dorsal surface (fades dorsalis; posterior surface) is convex, and smooth in 

 the upper third of its extent, and covered by the Supinator. Its middle third is 

 broad, slightly concave, and gives origin to the Abductor pollicis longus above, 

 and the Extensor pollicis brevis below. Its lower third is broad, convex, and 

 covered by the tendons of the muscles which subsequently run in the grooves on 

 the lower end of the bone. 



The lateral surface (fades lateralis; external surface} is convex throughout its 

 entire extent. Its upper third gives insertion to the Supinator. About its center is 

 a rough ridge, for the insertion of the Pronator teres. Its lower part is narrow, and 

 covered by the tendons of the Abductor pollicis longus and Extensor pollicis brevis. 



The Lower Extremity. The lower extremity is large, of quadrilateral form, 

 and provided with two articular surfaces one below r , for the carpus, and another 

 at the medial side, for the ulna. The carpal articular surface is triangular, concave, 

 smooth, and divided by a slight antero-posterior ridge into two parts. Of these, 

 the lateral, triangular, articulates with the navicular bone; the medial, quadri- 

 lateral, with the lunate bone. The articular surface for the ulna is called the ulnar 

 notch (sigmoid cavity) of the radius; it is narrow, concave, smooth, and articulates 

 with the head of the ulna. These two articular surfaces are separated by a promi- 

 nent ridge, to which the base of the triangular articular disk is attached ; this disk 

 separates the wrist-joint from the distal radioulnar articulation. This end of the 

 bone has three non-articular surfaces volar, dorsal, and lateral. The volar surface, 

 rough and irregular, affords attachment to the volar radiocarpal ligament. The 

 dorsal surface is convex, affords attachment to the dorsal radiocarpal ligament, 

 and is marked by three grooves. Enumerated from the lateral side, the first 

 groove is broad, but shallow, and subdivided into two by a slight ridge ; the lateral 

 of these two transmits the tendon of the Extensor carpi radialis longus, the medial 

 the tendon of the Extensor carpi radialis brevis. The second is deep but narrow, 

 and bounded laterally by a sharply defined ridge ; it is directed obliquely from above 

 downward and lateralward, and transmits the tendon of the Extensor pollicis 

 longus. The third is broad, for the passage of the tendons of the Extensor indicis 

 proprius and Extensor digitorum communis. The lateral surface is prolonged 

 obliquely downward into a strong, conical projection, the styloid process, which 

 gives attachment by its base to the tendon of the Brachioradialis, and by its apex 

 to the radial collateral ligament of the wrist-joint. The lateral surface of this 

 process is marked by a flat groove, for the tendons of the Abductor pollicis longus 

 and Extensor pollicis brevis. 



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

 which is thickest along the interosseous border and thinnest at the extremities except over the 

 cup-shaped articular surface (fovea) of the head where it is thickened. The trabecula3 of the 

 spongy tissue are somewhat arched at the upper end and pass upward from the compact layer of 

 the shaft to the fovea capituli; they are crossed by others parallel to the surface of the fovea. 

 The arrangement at the lower end is somewhat similar. 



Ossification (Figs. 217, 218). The radius is ossified from three centers: one for the body, 

 and one for either extremity. That for the body makes its appearance near the center of the bone, 

 during the eighth week of fetal life. About the end of the second year, ossification commences 



