154 



THE SKELETON. 



and terminates below at the middle of the head of the ulna. Its two middle fourths 

 are very prominent ; its lower fourth is smooth and rounded. This border gives 

 attachment to the interosseous membrane, and separates the anterior from the pos- 

 terior surface. 



The anterior surface, much broader above tha below, is concave in the upper 

 three-fourths of its extent, and aifords attachment to the Flexor profundus digi- 

 torum ; its lower fourth, also concave, is covered by the Pronator quadratus. The 

 lower fourth is separated from the remaining portion of the bone by a prominent 

 ridge, directed obliquely from above downward and inward; this ridge (the obUijt/c 

 or Pronator ridge] marks the extent of attachment of the Pronator quadratus. 

 At the junction of the upper with the middle third of the bon'e is the nutrient 

 canal, directed obliquely upward and inward. 



The posterior surface, directed backward and outward, is broad and concave 

 above, somewhat narrower and convex in the middle of its course, narrow, smooth, 

 and rounded below. It presents, above, an oblique ridge, which runs from the 

 posterior extremity of the lesser sigmoid cavity, downward to the posterior border ; 

 the triangular surface above this ridge receives the insertion of the Anconeus 

 muscle, whilst the upper part of the ridge itself affords attachment to the Supinator 

 brevis. The surface of bone below this is subdivided by a longitudinal ridge, 

 sometimes called the perpendicular line, into two parts : the internal part is smooth, 

 and covered by the Extensor carpi ulnaris ; the external portion, wider and rougher, 

 gives attachment from above downward to part of the Supinator brevis, the Exten- 

 sor ossis rnetacarpi pollicis, the Extensor longus pollicis, and the Extensor indicis 

 muscles. 



The internal surface is broad and concave above, narrow and convex below. 

 It gives attachment by its upper three-fourths to the Flexor profundus digitorum 

 muscle : its lower fourth is subcutaneous. 



The Lower Extremity of the ulna is of small size, and excluded from the artic- 

 ulation of the wrist-joint. It presents for examination two eminences, the outer 



and larger of which is a rounded, articular eminence, 

 termed the head of the ulna, the inner, narrower and 

 more projecting, is a non-articular eminence, the 

 styloid process. The head presents an articular facet, 

 part of which, of an oval or semilunar form, is directed 

 downward, and articulates with the upper surface of 

 the interarticular fibro-cartilage which separates it from 

 the wrist-joint ; the remaining portion, directed out- 

 ward, is narrow, convex, and received into the sigmoid 

 cavity of the radius. The styloid process projects from 

 the inner and back part of the bone, and descends a 

 little lower than the head, terminating in a rounded 

 summit, which affords attachment to the internal lat- 

 eral ligament of the wrist. The head is separated from 

 the styloid process by a depression for the attachment 

 of the triangular interarticular fibro-cartilage ; and 

 behind, by a shallow groove for the passage of the 

 tendon of the Extensor carpi ulnaris. 



Structure. Similar to that of the other long bones. 

 Development. By three centres : one for the shaft, 

 one for the inferior extremity, and one for the olec- 

 ranon (Fig. 102). Ossification commences near the 

 middle of the shaft about the eighth week, and soon 

 extends through the greater part of the bone. At 

 birth the ends are cartilaginous. About the fourth 

 year a separate osseous nucleus appears in the middle 



of the head, which soon extends into the styloid process. About the tenth year 

 ossific matter appears in the olecranon near its extremity, the chief part of this 



Olecranon. 



Appears *_/^aEfe,_JbwM shaft at 

 10th year, jf^^ 16th year. 



I Shaft 



" 



Appears at 

 4th year. 



Joins shaft at 

 20th year. 



Inferior extremity. 



F:G. 102. Plan of the develop- 

 ment of the ulna. By three centres. 



