a) . 
a THE RADIUS. 195 
The internal or interosseous border, faint above where it les in line with the 
posterior border of the bicipital tubercle, becomes sharp and prominent in the middle 
third of the bone. Below this it splits into two faint lines, which lead to either 
side of the sigmoid cavity on the lower end of the bone, thus including between 
them a narrow triangular area into which the deeper fibres of the pronator 
quadratus muscle are inserted. To this border, as well as to the posterior of the 
two divergent lines, the interosseous membrane is attached. 
The external border (oftentimes described as the external surface) is thick and 
rounded above, but becomes thinner and more prominent below, where it merges 
with the base of the styloid process. About its middle the anterior and posterior 
oblique lines become confluent with it, and here, placed between them, is a rough 
elongated impression which marks the insertion of the pronator radii. teres muscle. 
Above this, and on the outer surface of the neck, the supinator brevis muscle is 
inserted, whilst this border below is overlain by the tendons of the brachio-radialis 
_and the extensor carpi radialis longior and brevior muscles. 
The anterior or flexor surface (facies volaris) is crossed obliquely by a line which 
runs from the bicipital tubercle above, downwards and outwards towards the 
middle of the outer border of the shaft. This, oftentimes called the anterior oblique 
line, serves for the attachment of the radial head of origin of the flexor sublimis 
digitorum muscle. Above it, the front of the bone has the fibres of the supinator 
brevis muscle inserted into it, whilst below and internal to it, extending as low as 
the inferior limit of the middle third of the bone, is an extensive surface for the 
origin of the flexor longus pollicis muscle. In the lower fourth of the bone, where 
the shaft is broad and flat in front, there is a surface for the insertion of the 
pronator quadratus muscle which also extends to the interosseous ridge. 
The extensor or posterior surface (facies dorsalis) is also crossed by an oblique 
line, less distinct than the anterior. This serves to define the superior limit of the 
origin of the extensor ossis metacarpi pollicis muscle. Above this, the back of the 
neck and upper part of the shaft is overlain by the fibres of the supinator brevis, 
which become attached to this surface of the bone in its outer half. Below the 
posterior oblique line the posterior surface in the upper part of its inner half gives 
origin to the extensor ossis metacarpi pollicis, and the extensor brevis pollicis 
muscles in order from above downwards. 
| The lower extremity, which tends to be turned slightly forward when viewed 
from below, has a somewhat triangular form. Its inferior carpal articular surface 
concave from before backwards, and slightly so from side to side, is divided into two 
facets by a shght antero-posterior ridge, best marked at its extremities where the 
panterior and posterior margins are notched; the external of these areas, of tri- 
angular shape, is for articulation with the scaphoid, whilst the inner, quadrilateral in 
| form, is for the semilunar bone. The anterior border, prominent and turned forward, 
; 
p) 
is rough at its edge, where it serves for the attachment of the anterior part of the 
capsule of the wrist joint. The posterior border is rough, rounded, and tubercular, 
and is grooved by many tendons ; of these grooves the best marked is one which passes 
obliquely across its posterior surface. This is for the tendon of the extensor longus 
Epollicis muscle. The outer lip of this groove is often very prominent, and forms 
an outstanding tubercle. To the ulnar side of this oblique groove there is a broad 
shallow furrow in which the tendons of the extensor communis digitorum and 
extensor indicis muscles are lodged, whilst to its outer side and between it and the 
styloid process, there is another broad groove, subdivided by a faint ridge into two, 
for the passage of the tendons of the extensor carpi radialis brevior and the extensor 
carpi radialis longior in that order from within outwards. The styloid process (pro- 
cessus styloideus) lies to the outer side of the inferior extremity ; broad at its base, 
it becomes narrow and pointed below where by its inner cartilage-covered surface 
it forms the summit of the inferior triangular articular area. The outer surface of 
this process is crossed obliquely from above downwards and forwards by a shallow 
groove, the anterior lip of which is sharp and well marked, and serves to separate it 
from the anterior surface of the bone, whilst the posterior lip is often emphasised 
by a small tubercle above. The tendon of the brachio-radialis muscle is inserted 
into the upper part of either lip, and also spreads out on to the floor of the groove, 
