A MANUAL OF ANATOMY 



is called the supinator ridge, and it gives origin to a part of the 

 supinator radii brevis. The two lips form by their meeting the 

 commencement of the external or interosseous border. The anterior 

 part of this surface superiorly receives the bicipital tuberosity of 

 the radius, with the tendon of insertion of the biceps, in pronation, 

 whilst the posterior part gives origin to fibres of the supinator 

 radii brevis. 



The lower extremity is small, and presents a head and styloid 

 process. These are separated behind by a groove for the tendon 

 of the extensor carpi ulnaris, and below by a rough pit which gives 

 attachment to the apex of the triangular interarticular fibro-cartUage. 

 The outer aspect of the head is convex, and covered by cartilage for 

 articulation with the sigmoid cavity of the radius, a portion of 

 the synovial membrane, called membrana sacciformis, intervening. 

 The inferior surface, also covered by cartilage, is flat, and is 

 related to the upper surface of the triangular interarticular fibro- 

 cartilage, with the intervention of another portion of the mem- 

 brana sacciformis. 



The styloid process, of small size and subcutaneous, projects 

 downwards from the posterior and inner part of the head (mainly 

 from the back part), and it terminates in a round tip which 

 gives attachment to the internal lateral ligament of the wrist- 

 joint. 



The lower extremity presents several nutrient foramina for 

 branches of the anterior and posterior 

 interosseous arteries. 



Articulations. — Superiorly with the 

 trochlea of the humerus, and the inner 

 aspect of the head of the radius ; inferiorly 

 with the sigmoid cavity of the radius, and 

 the triangular interarticular fibro-cartilage, 

 the latter structure separating it from the 

 cuneiform or pyramidal bone. 



Structure. — This is similar to that of 

 long bones. 



Ossification. — The ulna ossifies in cartilage from 

 one primary, and two secondary, centres. The 



primary centre appears at the middle of the 

 shaft in the eighth week, and from it the shaft, 

 coronoid process, and greater part of the olecra- 

 non process are ossified. At birth the shaft and 

 ■ coronoid process are ossified, but the greater part 

 of the olecranon and the lower extremity are 

 cartilaginous. The secondary centre for the lower 

 extremity appears in the fourth year, and from it 

 the head and styloid process are ossified. The 



secondary centre for the upper extremity appears in the tenth year, and may 

 involve only a thin scale on the summit of the olecranon process, or it may 

 afford ossification to a third, or even a half, of that process. The upper 

 epiphysis joins about the sixteenth year, and the lower about the twentieth 

 year. 



Appears in the loth Year, 

 and joins about i6 



Appears in the 8th Week 

 (intra-uterine) 



Appears in the 4th Year, 

 and joins about 20 



Fig. 112. — Ossification 

 OF THE Ulna. 



