216 



THE SKELETON. 



with the middle third of the bone is the nutrient canal, directed obliquely 

 upwards and inwards. 



The posterior surface, directed backwards and outwards, 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 downwards 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 Supi- 

 nator 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 attach- 

 ment from above downwards to part of the Supinator Brevis, the Extensor 

 Ossis Metacarpi Pollicis, the Extensor Secundi Internodii Pollicis, and the Ex- 

 tensor 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 Digi- 

 torum muscle; its lower fourth is subcutaneous. 



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

 articulation 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 downwards, and plays on the surface of the triangular 

 fibro-cartilage, which separates this bone from the wrist-joint ; the remaining 

 portion, directed outwards, 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 attach- 

 ment 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 ex- 

 tremity, and one for the olecranon (Fig. 

 159). Ossification commences near the 

 middle of the shaft about the fifth 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 

 , ~s^ , year, ossific matter appears in the ole- 



A&e&rfat 4*y^{*&^\JotiuS}t*ftat 2Oylf J . ^.\ , ,. 



cranon 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. 



Fig. 159. Plan of the Development of the 

 Ulna. By Three Centres. 



Agpears at M&^ 



