284 



HUMAN ANATOMY. 



Fig. 



gSSfc 





but sometimes the front border is short and the inferior runs obhquely backward, 

 making it ahnost triangular. 



The shaft/ which presents three borders and three surfaces, steadily diminishes 

 from above downward. In the upper part the bone curves slightly backward and 

 outward (i.e., towards the radius), then inward through the greater part of its 

 extent, till at the lower quarter it again bends outward and, at the same time, for- 

 ward. T\\^ posterior border"^ is formed by the union of the two lines bounding the 

 subcutaneous surface at the back of the olecranon. Following 

 the curves just described, it runs to the back of the styloid 

 process, being very distinct in the first two-thirds, where it gives 

 origin to the aponeurosis of the flexor carpi ulnaris. The anterior 

 border''' springing from the junction of the front and inner sides 

 of the coronoid, runs down to end just above the front of the 

 styloid process. Its last quarter, which ks rough to give origin to 

 the pronator quadratus, has a backward slant. The outer or in- 

 terosseoiis border* is very sharp in the middle two-fourths of the 

 shaft, where it gives origin to that membrane. It begins above 

 by the union of two lines, which, starting from the front and 

 back of the lesser sigmoid cavity, bound a triangular depression. 

 The posterior of these lines, sharp and raised, is the supinator 

 ridge. The depression which gives origin to the supinator brevis 

 receives the bicipital tuberosity of the radius in pronation. The 

 border becomes indistinct below and is lost as it approaches the 

 head of the ulna. The anterior surface is usually concave through- 

 out, though the upper part may be convex. In the third quarter 

 a line often appears which slants downward into the front border, 

 giving origin to the upper fibres of the pronator quadratus. 

 Below this line, when present, there is a depression occupied by 

 that muscle. Above this arises the flexor profundus digitorum. 

 The nutrient foramen running upward is a little above the mid- 

 dle. The inner surface, concave at the side of the upper ex- 

 tremity and convex below, gives further origin in its upper two- 

 thirds to the last-named muscle. The posterior surface has 

 several features which are to be recognized only on a well-marked 

 bone, and are very variable. The oblique line starts from the 

 supinator ridge, or from the hind edge of the lesser sigmoid cav- 

 ity, and runs downward to the posterior border at the end of 

 the first third. It gives origin to a part of the supinator brevis, 

 and helps to mark of^ a three-sided depression running onto 

 the olecranon for the anconeus. It is sometimes the apparent 

 continuation of the supinator ridge, as in Fig. 296. The region 

 below this is subdivided by a vertical ridge of uncertain beginning 

 and end. Sometimes it springs from the interosseous border, 

 and it is usually lost below in the hind one. The extensor carpi 

 ulnaris springs from the surface internal to it, which is some- 

 times a deep gutter. External to the vertical ridge are areas 

 for the extensor ossis metacarpi pollicis, extensor longus pol- 

 licis, and extensor indicis from above downward in the order 

 named. 



The lower extremity of the ulna consists of the head and 

 the styloid process. The head " is a rounded enlargement pro- 

 jecting forward and outward, presenting an articular surface on 

 the outer side, which passes onto the front and the back, making 

 at least two-thirds of a circle, around which the radius swings. A ridge marks 

 the upper border of this surface, which overhangs the lower. The latter is rounded, 

 so that the lateral articular surface continues without real interruption into the 

 inferior, which is separated from the wrist-joint by the triangular fibro-cartilage. 

 The under side of the articular surface is somewhat kidney-shaped, the concavity 

 looking towards the styloid process, from which it is separated by a groove for the 



^ Corpus ulnae. ^ Margo dorsalis. ^ Margo volaris. ^Crista interossea. 'Capitulum. 



Longitudinal section 

 of ulna. 



