88 



ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 117. — The left ulna seen from the outer surface (§). 



Fig. 118. — The left ulna seen from behind (§). 



Fig. 119. — The left ulna seen from in front (§). 



Fig. 121. — The left radius seen from in front (-f). 



Fig. 122. — The left radius seen from the inner side (•§). 



Fig. 123. — The left radius seen from behind (§). 



Fig. 124. — The upper extremities of the radius and ulna seen from above and somewhat in front (-£). 



Fig. 125. — The lower extremities of the radius and ulna seen from below (-f). 



The inferior extremity of the bone is rounded and is also called the capitulum. Its radial 

 side presents an articular surface for the radius, the articular circumference (Fig. 119), and a 

 pointed process projecting beyond the capitulum, the styloid process (Figs. 117 to 119). The 

 distal surface of the capitulum and the articular circumference are both covered by cartilage. 



The center for the diaphysis of the ulna appears in the third fetal month; the centers for the epiphyses 'appear 

 after birth, that for the lower epiphysis not being present until the sixth year. The upper epiphysis has two centers 

 for the olecranon (only the apex of which is formed by the epiphysis) and one for the coronoid process. There is also 

 a special center for the styloid process. 



THE RADIUS. 



The radius (Figs. 121 to 125) is the outer of the two bones of the forearm, and in contrast 

 to the ulna, it is narrow and thin above and broad and thick below. 



Its superior extremity (Fig. 124) is formed by the disc-like head of the bone, a distinct 

 constriction below the head being designated as the neck, which portion of the bone is cylindrical. 



dorsal border. 



dorsal border. 



volar border ^ 



T e e volar border* 



radius.. 



Fig. 120. — Transverse section through the bones of the forearm, taken at about the middle of their length (schematized). 



The upper surface of the head exhibits a depressed articular surface for the capitulum of the 

 humerus, and the upper circumference of the margin of the head is termed the articular cir- 

 cumference (Fig. 124). 



Below the neck the upper portion of the volar surface presents a strong, rough, marked 

 projection, the tuberosity (Figs. 121, 122, and 124), for the insertion of the biceps muscle. 



The shaft of the radius, like that of the ulna, is shaped like a three-sided prism, and the 

 three surfaces are arranged in a similar manner, so that volar, dorsal, and lateral surfaces, and 

 volar and dorsal borders, and an interosseous ridge may be recognized (Fig. 120). The interosseous 



