258 



THE SKELETON. 



At the junction of the upper with the middle third of the bone 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 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, concave, and 

 gives origin to (occasionally is merely covered by) the Extensor carpi ulnaris ; the 

 external portion, wider and rougher, gives attachment from above downward to 

 part of the Supinator brevis, the Extensor ossis metacarpi 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 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 outward, 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 lateral 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. 202). 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 pn> 

 cess. About the tenth year ossific matter appears in 

 the olecranon near its extremity, the chief part of 

 this process being formed from an extension of the 

 shaft of the bone into it. At about the sixteenth 

 year the upper epiphysis becomes joined, and at 

 about the twentieth year the lower one. 



Articulations. With the humerus and radius. 

 Attachment of Muscles. To sixteen : to the 

 olecranon, the Triceps, Anconeus, and one head of 

 the Flexor carpi ulnaris. To the coronoid process, 

 the Brachialis anticus, Pronator radii teres, Flexor 

 sublimis digitorum, and Flexor profundus digitorum ; 

 generally also the Flexor longus pollicis. To the 

 shaft, the Flexor profundus digitorum, Pronator quad- 

 ratus, Flexor carpi ulnaris, Extensor carpi ulnaris, Anconeus, Supinator brevis, 

 Extensor ossis metacarpi pollicis, Extensor longus pollicis, and Extensor indicis. 



Olecranon. 



Appears rt^r^jjfe.j i ns sha ft at 

 10th year, j , 16th yeaff 



Appears at 

 4th year. 



Joins shaft at 

 20th year. 



Inferior extremity. 



FIG. 202. Plan of the develop- 

 ment of the ulna. By three centres. 



