THE ULNA. 



213 



posterior surface is subdivided by a faint longitudinal ridge, the bone betwee 

 which and the interosseous crest furnishes origins for the abductor pollicis longus, 

 extensor pollicis longus, and extensor indicis proprius muscles, in order proximo- 

 distally. The surface of bone between the dorsal margin and the afore-mentioned 

 longitudinal line is smooth and overlain by the extensor carpi ulnaris muscle, which, 

 however, does not arise from it. 



The distal extremity of the ulna presents a rounded head (capitulum ulnae), 

 from which, on its medial and dorsal aspect, there projects distally a cylindrical 

 pointed process called the styloid process. To the extremity of this latter is attached 

 the ulnar collateral ligament of the carpus, whilst on the volar aspect it has 

 connected with it the antero-medial portion of the capsule of the wrist-joint. The 

 antero-lateral half of the circumference of the head is furnished with a smooth 

 narrow convex articular surface, which fits into the ulnar notch of the radius. 

 Its distal surface, flat and semilunar in shape, and separated from the root of 

 the styloid process by a well-marked groove, rests on the upper surface of the 

 triangular articular disc of the wrist, the apex of which is attached to the groove 

 just mentioned. The margins of the head, to the volar side and dorsal to the 

 radial articular surface, have attached to them the volar and dorsal distal radio- 

 ulnar ligaments. The dorsal and medial surface of the styloid process is channelled 

 by a groove which separates it from the dorsal surface of the head, and extends 

 proximally some little way upon the dorsal surface of the distal end of the body. 

 In this is lodged the tendon of the extensor carpi ulnaris muscle. The pro- 

 portionate length of the ulna to the body height is as 1 is to 6'26-6*66. 



Nutrient Foramina. A foramen, having a proximal direction, for the nutrient artery of the 

 body opens on the volar surface of the bone from two to three inches distal to the tuberosity. 

 Vascular canals of large size are seen proximal and dorsal to the radial notch, just dorsal to 

 the notched lateral border of the semilunar notch. At the distal end of the bone similar openings 

 are seen in the groove between the styloid process and the distal articular surface of the head. 



Connexions. The ulna articulates proximally with the trochlea of the humerus. On the 

 lateral side it is in contact with the radius at both proximal and distal ends, the proximal radio- 

 ulnar articulation being formed by the head of the radius and the radial notch of the ulna, the 

 distal radio-ulnar joint comprising the head of the ulna, which fits into the ulnar notch of the 

 radius. Between these two joints the bodies of the bones are united by the interosseous membrane. 

 The distal surface of the head of the ulna does not articulate with the carpus, but rests on 

 the proximal surface of the interposed articular disc. The ulna is superficial throughout 

 its entire extent. Proximally the olecranon 

 process can be readily recognised, particularly 

 when the elbow is bent, as in this position the 

 olecranon is withdrawn from the olecranon 

 1 fossa of the humerus in which it rests when the 

 joint is extended. Distal to this the subcutane- 

 ous triangular area on the back of the olecranon 

 can be easily determined, and from it the 

 posterior border of the bone can readily be 

 traced along the line of the " ulnar furrow " 

 to the styloid process. With the hand 

 .supine this latter process can be felt to the 

 medial side and slightly behind the wrist. 

 When the hand is pronated, the distal end 

 1 of the radius rolls round the distal extremity 

 of the ulna, and the antero-lateral surface of 

 the head of the latter bone now forms a well- 

 l marked projection on the dorsum of the wrist 

 ^in line with the cleft between the little and 

 ' ring fingers. 



Ossification. The ulna is ossified 

 from one primary and two or more secondary 

 centres. The centre for the body appears 

 early in the second month of foetal life. At 

 birth the body and a considerable part 

 of the proximal extremity, including the 

 coronoid process, are ossified, as well as part 

 of the distal extremity. The olecranon and 

 'the distal surface of the head and the styloid process are cartilaginous. About ten 

 years of age a secondary centre appears in the cartilage at the proximal end of the bone, and 



Fuses with shaft about 16 years 



Appears about 10 years 



Appears about 6 years 



Fuses with shaft 20-23 years 



At Birth. About 12 years. About 16 years. 

 FIG. 205. THE OSSIFICATION OP THE ULNA. 



