574 MAMMALIAN ANATOMY 



The internal surface (Fig. 442) is situated between the anterior 

 and posterior borders. It is of equal width above and below, but it 

 is so twisted on its long axis that while the upper part faces inward 

 the lower part faces backward. The surface in the main is concave 

 from above downward and is convex transversely ; it is marked for 

 most of its extent by a faint median groove. At its upper end is a 

 median oblong, roughened area, into which the tendon of the brachialis 

 anticus is inserted. The ulnar head of the flexor profundus digitorum 

 has its origin along the posterior border. 



The posterior surface (Fig. 440) is the largest of the three sur- 

 faces; it lies between the posterior and external borders, and occupies 

 the entire outer aspect of the bone. At its upper end it is wide, 

 presenting the greatest antero-posterior diameter of the bone ; at the 

 lower end it is narrow. Its anterior margin is slightly sinuous ; the 

 posterior margin is slightly emarginate. In its upper fourth it is 

 nearly flat ; a ridge, more or less marked, runs distally for a short 

 distance from the root of the posterior lip of the lesser sigmoid cavity. 

 The region contiguous to the external border, extending from the 

 upper to the lower fourth, is prominent and roughened for ligamentous 

 attachment. The remaining part of the surface is concave transversely 

 above and convex below, and the whole surface is gently convex from 

 the proximal to the distal end. Two muscles arise from this surface, 

 the extensor indicis above and behind and the extensor ossis metacarpi 

 pollicis from the whole length, above only from the front, but below 

 from the entire width. 



The Lower Extremity of the ulna is small, and consists of two 

 parts, the head and the styloid process. 



The head is separated from the shaft by a straight roughened line 

 running round the bone. This line is most clearly marked in front, 

 where it forms the lower boundary of the anterior surface of the shaft. 

 It is the remains of the suture once existing between the shaft and 

 the distal epiphysis. The circumference of the head is scarcely larger 

 than that of the terminal portion of the shaft. It is short in the 

 proximo-distal line. Its outer posterior part is produced into the 

 styloid process. The head presents anterior and posterior surfaces and 

 external and posterior borders. 



The posterior surface is the continuation of the posterior surface 

 of the shaft. It is quadrate and roughened and flat or slightly convex. 



