194 OSTEOLOGY. 
below supports the carpus. It consists of a head, a neck, a tubercle, a shaft, 
and an expanded lower extremity. 
OLECRANON 
Triceps ‘ills 
SUBCUTANEOU 
SURFACE 
| 
HI BAD 
Aneoneus 
TUBEROSITY 
Supinator radii 
brevis 
POSTERIOR 
BORDER | 
Flexor 
profundus 
digitorum 
Flexor carpi 
ulnaris 
Extensor carpi — 
ulnaris 
Post. OBLIQUE 
INTEROSSEOUS LINE 
BORDER 
Pronator radii 
teres 
Ext. ossis 
inetacarp. poll. 
HEAD 
nat 
STYLOID 
Al 
LL: 
Ext. carpi rad. 
longior 
~Ext. carpi rad. 
brevior 
GROOVE FOR EXT. 
CARPI ULNARIS 
Fie. 137. 
Ext. commun. Ext. long. poll. STYLOID PROC. 
dig. and ind, 
—THE Rapius AND ULNA AS SEEN FROM BEHIND. 
Ext. brevis poll. 
The shaft 1s narrow above, but increases in all 
its diameters below. 
Upper Extremity.—The 
head (capitulum) is disc- 
shaped and provided with 
a shallow concave surface 
(fovea capituli radii) superi- 
orly for articulation with 
the capitellum of the 
humerus. The circum- 
ference of the head (cir- 
cumferentia articularis) is 
smooth and is embraced by 
the orbicular igament. On 
the inner side it 1s usually 
much broader, and dis- 
plays an articular surface, 
plane from above down- 
wards, which rolls within 
the small sigmoid cavity 
of the ulna in the movye- 
ments of pronation and 
supination. The character 
of the outer half of the 
circumference differs from 
the inner in being nar- 
rower, and rounded from 
above downwards. 
The neck (collum radi) 
is the narrow part of the 
shaft which supports the 
head, the overhang of the 
latter being greatest to- 
wards the outer and pos- 
terior side. Below the 
neck, on the inner side, 
there is an outstanding 
oval prominence, the bi- _ 
cipital tuberosity (tuber- 
ositas radii). The posterior 
half of this is rough for 
the insertion of the biceps 
tendon, whilst the anterior 
half is smooth and covered 
by a bursa which intervenes 
between it and the tendon. 
The shaft (corpus radi1), 
which has an outward curve 
and is narrow above and 
broad below, is wedge- 
shaped on section. The 
edge of the wedge corre- 
sponds to the sharp imner 
interosseous margin of the 
bone (crista interossea), 
whilst its base corresponds 
to the thick and rounded outer border around which the anterior or flexor surface 
becomes confluent with the posterior or extensor surface. 
