THE BONES OF THE UPPER LIMB I93 



the anterior and posterior margins of the sigmoid cavity. At its 

 commencement it is round and indistinct, and immediately below 

 the bicipital tuberosity it gives attachment to the oblique ligament. 

 Over the rest of its extent it is sharp and wiry for the attachment 

 of the interosseous membrane, which is also connected with the 

 posterior of the two lower di\asions. The posterior border extends 

 from the back of the bicipital tuberosity to the prominent radial 

 tubercle about the centre of the posterior border of the lower 

 extremity. In its upper third it crosses the shaft obliquely down- 

 wards and outwards, this portion of it, which is prominent, being 

 called the posterior oblique line. This line limits the insertion of 

 the supinator radii brevis above, and the origin of the extensor 

 ossis metacarpi poUicis below. 



The anterior surface is situated between the anterior and internal 

 or interosseous borders. In the upper two-thirds it is concave, 

 and gives origin to the flexor longus pollicis. In the lower 

 third it is flat and expanded, and this portion gives insertion 

 to the pronator quadratus, except close to the anterior border of 

 the lower extremity, where it gives attachment to the anterior 

 ligament of the wrist- joint. The anterior surface presents the 

 medullary foramen about the junction of the upper and middle 

 thirds. The direction of the canal to which it leads is upwards, 

 and it gives passage to the medullary branch of the anterior inter- 

 osseous artery. The portion of bone between the anterior oblique 

 line, the lower part of the front of the neck, and the bicipital tuber- 

 osity gives insertion to a portion of the supinator radii brevis 

 The external surface is situated between the anterior and posterioi 

 borders. It is convex from above do%vnwards, and from side to 

 side. In its upper third it gives insertion to the supinator radii 

 brevis ; at its centre there is a rough impression, fully an inch 

 long, for the insertion of the pronator radii teres ; and below this 

 it supports the tendons of the extensores carpi radialis longioi 

 et brevior, and is crossed obliquely by the tendons of the extensor 

 ossis metacarpi pollicis and extensor brevis pollicis (primi int^modii 

 pollicis). The external surface also supports the brachio-radialis, 

 which is inserted into its lower extremity close to the base of the 

 styloid process. The posterior surface is situated between the 

 posterior and internal or interosseous borders. Above the posterior 

 oblique line it is covered by the supinator radii brevis, which takes 

 insertion into its outer half. Below the posterior oblique line it 

 is concave over about the middle third, where it gives origin from 

 above downwards to the extensor ossis metacarpi pollicis and 

 extensor brevis pollicis. The lower third is broad and convex, 

 and it supports the tendons of the extensor longus pollicis, extensor 

 communis digitorum, and extensor indicis. 



The lower extremity is large and pentagonal, as viewed from 

 below. Externally it presents the styloid process, internally the 

 sigmoid cavity, and inferiorly the carpal articular surface, the latter 

 two being covered by cartUage. The styloid process projects down-- 



13 



