THE ULNA 



155 



articular disc is attached, and to the anterior and posterior borders, the anterior 

 and posterior radio-ulnar ligaments respectively. The anterior surface is raised 

 into a prominent area for the anterior ligament of the wrist-joint. The lateral 

 surface is represented by the styloid process, a blunt pyramidal eminence, to the 

 base of which the hrachio-radialis is inserted, whilst the tip serves for the attach- 

 ment of the radial (external) collateral ligament of the wrist. Its lateral surface 

 is marked by two shallow furrows for the tendons of the abductor pollicis longus 

 and extensor pollicis hrevis. The posterior surface is convex, and marked by three 

 prominent ridges separating three furrows. The posterior annular ligament is 

 attached to these ridges, thus forming with the bone a series of tunnels for the 

 passage of tendons. 



The most lateral is broad, shallow, and frequently subdivided by a low ridge. The lateral 

 subdivision is for the extensor carpi radialis longus, the medial for the extensor carpi radialis 

 brevis. The middle groove is narrow and deep for the tendon of the extensor pollicis longus. 

 The most medial is shallow and transmits the extensor indicis proprius, the extensor digitorum 

 communis, the dorsal branch of the interosseous artery, and the dorsal interosseous nerve. 

 When the radius and ulna are articulated, an additional groove is formed for the tendon of the 

 extensor quinti digiti proprius. 



Ossification.— The radius is ossified from a centre which appears in the middle of the shaft 

 in the eighth week of intra-uterine life and from two epiphysial centres which appear after birth. 

 The nucleus for the lower end appears in the second year, and that for the upper end, which 

 forms simply the disc-shaped head, in the fifth year. The head unites with the shaft at the 

 seventeenth year, whilst the inferior epiphj'sis and the shaft join about the twentieth year. 



THE ULNA 



The ulna (figs. 180, 181, 189) is a long, prismatic bone, thicker above than 

 below, on the medial side of the forearm and parallel with the radius, which it 



Fig. 184. — Upper End op Left Ulna. (Lateral view.) 



Olecranon- 



Semilunar notch 



Coronoid process 



Annular ligament 



Oblique ligament 



Flexor digitorum profundus 



Radial notch 



—Supinator 



—Interosseous membrane 



exceeds in length by the extent of the olecranon process. It articulates at the 

 upper end with the humerus, at the lower end indirectly with the carpus, and on 

 the lateral side with the radius. It is divisible into a shaft and two extremities. 

 The upper extremity is of irregular shape and forms the thickest and strongest 

 part of the bone. The superior articular surface is concave from above downward, 

 convex from side to side, and transversely constricted near the middle. It belongs 



