THE BONES OF THE UPPER LIMB 191 



at the eni towards which the medullary foramen and the canal to which 

 it leads are directed, are the last to show signs of ossification, hut they 

 are the first to join the shaft. The only exception to this rule occurs 

 in the fibula. 



The Radius. 



The radius is the external bone of the forearm (which is 

 assumed to be in a position of supination). It is parallel with, 

 and shorter than, the ulna, and extends from the elbow to the 

 wrist. It is a long bone, and is divisible into a shaft and two 

 extremities. 



The upper extremity, which is small, presents a head and neck. 

 The head is disc-shaped, and covered by cartilage, both on its upper 

 surface and circumference. The upper surface, at its centre, 

 presents a depression which articulates with the rounded portion 

 of the capitellum of the humerus in flexion of the elbow-joint. 

 Around this depression the surface is convex, especially on the 

 inner side, and this portion glides on the inner grooved part of the 

 capitellum. The circumferential cartilage is deeper on the inner 

 aspect than elsewhere, and this portion articulates with the small 

 sigmoid cavity of the ulna, whilst the remainder plays within the 

 orbicular ligament. The constricted portion below the head is 

 called the neck. It is cylindrical, and its upper part is embraced 

 by the orbicular ligament, whilst beyond this on the outer aspect 

 it gives insertion to a few fibres of the supinator radii brevis. The 

 upper extremity presents several nutrient foramina for branches 

 of the radial recmrent and interosseous recurrent arteries. 



The shaft increases in size from above downwards, and is curved, 

 the convexity being directed outwards and slightly backwards. 

 This curve imparts elasticity to the bone, and guards it against 

 the shocks to which it is so much exposed from the fact that it 

 supports the hand. The shaft is triangular, and presents superiorly, 

 on its antero-intemal aspect just below the neck, an oval eminence, 

 called the bicipital tuberosity. This is divided vertically into two 

 parts, a rough posterior portion which gives insertion to the tendon 

 of the biceps, and a smooth anterior part which is sepsirated from 

 that tendon by a bursa. Below the bicipital tuberosity the shaft 

 presents three borders and three surfaces. The anterior border 

 extends from the lower cind anterior part of the bicipital tuberosity 

 * to the anterior border of the styloid process. In its upper third 

 it crosses the shaft obliquely downwards and outwards, this portion 

 of it being called the anterior oblique line. This line limits ex- 

 ternally the insertion of the supinator radii brevis, and internally 

 the origin of the flexor longus poUicis, whilst its prominent edge 

 gives origin to the thin radial portion of the flexor sublimis 

 digitorum. The internal or interosseous border commences at the 

 lower and back part of the bicipital tuberosity, and near the lower 

 extremity of the shaft it divides into two ridges, which pass to 



