260 THE SKELETON. 



radius, and terminates below at the posterior part of the base of the styloid process ; 

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

 but well marked in the middle third of the bone. 



The internal or interosseous border commences above at the back part of the 

 tuberosity, where it is rounded and indistinct, becomes sharp and prominent as 

 it descends, and at its lower part divides into two ridges, which descend to the 

 anterior and posterior margins of the sigmoid cavity. This border separates the 

 anterior from the posterior surface, and has the interosseous membrane attached 

 to it throughout the greater part of its extent. 



The anterior surface is narrow and concave for its upper three-fourths, and 

 gives attachment to the Flexor longus pollicis muscle ; it is broad and flat for its 

 lower fourth, and gives attachment to the Pronator quadratus. A prominent ridge 

 limits the attachment of the Pronator quadratus below, and between this and the 

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

 ligament of the wrist-joint. At the junction of the upper and middle third of this 

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



The posterior surface is rounded, convex, and smooth in the upper third of its 

 extent, and covered by the Supinator brevis muscle. Its middle third is broad, 

 slightly concave, and gives attachment to the Extensor ossis metacarpi pollicis 

 above, the Extensor brevis pollicis 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 external surface is rounded and convex throughout its entire extent. Its 

 upper third gives attachment to the Supinator brevis muscle. About its centre is 

 seen a rough ridge, for the insertion of the Pronator radii teres muscle. Its lower 

 part is narrow, and covered by the tendons of the Extensor ossis metacarpi pollicis 

 and Extensor brevis pollicis muscles. 



The Lower Extremity of the radius is large, of quadrilateral form, and provided 

 with two articular surfaces one at the extremity, for articulation with the carpus, 

 and one at the inner side of the bone, for articulation with the ulna. The carpal 

 articular surface is of triangular form, concave, smooth, and divided by a slight 

 antero-posterior ridge into two parts. Of these, the external is large, of a triangular 

 form, and articulates with the scaphoid bone ; the inner, smaller and quadrilateral, 

 articulates with the semilunar. The articular surface for the ulna is called the 

 sigmoid cavity of the radius ; it is narrow, concave, smooth, and articulates with 

 the head of the ulna. The circumference of this end of the bone presents three 

 surfaces an anterior, external, and posterior. The anterior surface, rough and 

 irregular, affords attachment to the anterior ligament of the wrist-joint. The 

 external surface is prolonged obliquely downward into a strong, conical projection, 

 the styloid process, which gives attachment by its base to the tendon of the 

 Supinator longus, and by its apex to the external lateral ligament of the wrist- 

 joint. The outer surface of this process is marked by a flat groove, which runs 

 obliquely downward and forward, and gives passage to the tendons of the Extensor 

 ossis metacarpi pollicis and the Extensor brevis pollicis. The posterior surface is 

 convex, affords attachment to the posterior ligament of the wrist, and is marked by 

 three grooves. Proceeding from without inward, the first groove is broad but 

 shallow, and subdivided into two by a slightly elevated ridge: the outer of these 

 two transmits the tendon of the Extensor carpi radialis longior, the inner the 

 tendon of the Extensor carpi radialis brevior. The second, which is near the 

 centre of the bone, is a deep but narrow groove, bounded on its outer side by a 

 sharply-defined ridge ; it is directed obliquely from above, downward and outward, 

 and transmits the tendon of the Extensor longus pollicis. The third, lying most 

 internally, is a broad groove, for the passage of the tendons of the Extensor indicia 

 and Extensor communis digitorum. 



Structure. Similar to that of the other long bones. 



Development (Fig. 203). By three centres : one for the shaft and one for each 

 extremity. That for the shaft makes its appearance near the centre of the bone 



