158 



THE SKELETON 



The oblique line gives attachment to a few fibres of the supinator and marks off the posterior 

 surface into two unequal parts. That above the line, much the smaller of the two, receives the 

 insertion of the anconeus. The more extensive part below is subdivided by a vertical ridge 

 into a medial portion, smooth, and covered by the extensor carpi ubiaris, and a lateral portion 

 which gives origin to three muscles, viz., the abductor pollicis longus, the extensor pollicis longu$ 

 and the extensor indicts proprius, from above downward. 



The lower extremity of the ulna is of small size and consists of two parts, the 

 head and the styloid process, separated from each other on the inferior surface by a 

 groove into which the apex of the articular disc is inserted. That part of the head 

 adjacent to the groove is semilunar in shape and plays upon the articular disc 

 which thus excludes the ulna from the radio-carpal or wrist-joint. The margin of 

 the head is also semilunar, and is received into the ulnar notch of the radius. The 

 styloid process projects from the medial and back part of the bone, and appears 

 as a continuation of the dorsal border. To its rounded summit the ulnar collateral 

 ligament of the wrist-joint is attached, and its dorsal surface is grooved for the 

 passage of the tendon of the extensor carpi ulnaris. Immediately above the 

 articular margin of the head the anterior and posterior radio-ulnar ligaments are 

 attached in front and behind. 



Fig. 187. — The Left Radius and Ulna in Pronation. (Anterior view.) 



Ossification. — The ulna is ossified from three centres. The primary nucleus appears near 

 the middle of the .shaft in the eighth week of intra-uterine life. At birth the inferior extremity 

 and the greater portion of the olecranon are cartilaginous. The nucleus for the lower end ap- 

 pears during the foiu-th year and tin; epiphy.sis joins with the shaft from the eighteenth to the 

 twentieth year. 'I'he greater [)art of the olecranon is ossified from the shaft, but an epiphysis 

 is sub.sequently formed from a nucleus whicli ai)pcars in the tenth year. 



The epiphysis varies in size, and may be cither scale-like and form a thin plate on the sum- 

 mit, or involve the upper fourth of the olc(-ranon and the corresponding articular surface. In 

 the latter case the epiphysis is probal^ly composed of two parts fused together: (1) The scale 

 on the summit of the olecranon process, and (2) the beak centre which enters into the formation 

 of the upper end of the .semilunar notch (see fig. 186). The epiphysis unites to the shaft in the 

 sixteenth or seventeenth year. 



