THE ULNA 125 



and i)()sterior Ijurders. Its upper fourth is marked off by an oblique ridge run- 

 ning from the lesser sigmoid cavity to the posterior border. The surface above the 

 line receives the insertion of the anconeus ; . from the line itself a few fibres of 

 supinator brevis arise. The surface below the oblique line is subdivided by a 

 vertical ridge; the portion lying between this ridge and the posterior border is in 

 relation with the extenmr carpi uhiari><. Between this line and the interosseous 

 border the following muscles arise in order from above downwards: extensor oms 

 metararpi pollich, the crtrnsor secundi internodii pjollicis, and the extensor indicis. 



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

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

 groove into which an interarticular cartilage is inserted. That part of the head 

 adjacent to the groove is semilunar in shape and plays upon the interarticular 

 cartilage which excludes it from the wrist-Joint. The margin of the head is also 

 semilunar, and is received into the sigmoid cavity of the radius. The styloid 

 process projects from the inner and back part of the bone, and appears as a con- 

 tinuation of the posterior liorder. To its extremity the internal lateral ligament is 

 attached, and its posterior surface is grooved for the passage of the tendon of the 

 exteii-<nr carpi ulnaris. 



Muscles. — The following are attached to the ulna: — 



Triceps. Flexor carpi ulnaris. 



Anconeus. Extensor ossis metacarpi poUicis. 



Brachialis anticus. Extensor secundi internodii pollicis. 



Pronator quadratus. Extensor indicis. 



Flexor sublimis digitorum. Pronator radii teres. 



Flexor profundus digitorum. Supinator brevis. 



Flexor longus pollicis. Extensor carpi ulnaris. 



Ligaments: — 



Internal lateral of elbow. Interosseous membrane. 



Anterior of elbow. Anterior radio-ulnar. 



Posterior of elbow. Posterior radio-ulnar. 



Orbicular. Internal lateral of Avrist- joint. 



Oblique. Interarticular fil^ro-cartilage. 



Blood-supply. — The nutrient vessel enters the shaft near the middle of the 

 anterior surface; it is derived from the anterior interosseous trunk, and is directed 

 towards the proximal end. The upper extremity receives branches from the 

 anterior and posterior ulnar recurrent and from the interosseous recurrent. The 

 lower end receives twigs from the anterior and posterior interosseous arteries. 



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

 appears near the middle of the shaft in the eighth week of embryonic life. At 

 birth the greater portion of the olecranon process is cartilaginous. During the 

 fourth year a nucleus appears for the distal epiphysis. The cartilaginous ole- 

 cranon is mainly ossified from the shaft, and in the course of the tenth year a 

 scale-like epiphysis appears at its summit. This unites during the sixteenth year. 

 The distal epiphysis consolidates about the eighteenth year. 



