the muscle spreads outwards and downwards, 
radius, and is inserted into the anterior and 
forwards as the bicipital 
tubercle, as far upwards as 
the neck, and as far down- 
wards as the oblique line 
and the insertion of the 
pronator radii teres. 
The supinator brevis is 
deeply placed, and is un- 
covered only anteriorly in 
the hollow of the elbow. 
It is covered by all the 
superficial muscles on the 
back of the forearm. It 
conceals the back of the 
elbow joint and the upper 
part of the radius. At its 
lower border is the extensor 
ossis metacarpi pollicis, 
separated from it by the 
posterior interosseous 
artery. The muscle is 
divisible into superficial 
anddeep parts with humeral 
and ulnar regions, between 
which the posterior inter- 
osseous nerve passes to 
reach the back of the fore- 
arm. 
The extensor  ossis 
metacarpi pollicis (m. 
abductor pollicis longus) 
arises below the supinator 
brevis from the posterior 
or extensor surfaces of the 
radius and ulna, and from 
the intervening portion of 
the interosseous mem- 
brane. Becoming super- 
ficial in the lower part of 
the jforearm along with 
the extensor brevis pollicis, 
it passes beneath the 
posterior annular hgament, 
to be inserted into the 
outer side of the base of 
the first metacarpal bone. 
At its origin the muscle 
is deeply placed beneath 
the superficial extensor 
muscles and the posterior 
interosseous vessels and 
nerve. Above is the sup- 
inator brevis; below and 
internally, the long and 
short extensors of the thumb. 
MUSCLES ON THE BACK OF THE FOREARM. 
333 
enveloping the upper part of the 
outer surfaces of the bone, as far 
— TrRIcEPs (long head) 
TRICEPS (Outer head) 
BRACHIO-RADIALIS 
BRACHIALIS ANTICUS 
BIcErsS 
External condyle x 
of humerus ™ 
~~ TRicEps (inner head) 
INSERTION OF 
TRICEPS EXTENSOR CARPI RADIALIS 
LONGIOR (origin) 
EXTENSOR MUSCLES 
(origin) _ EXTENSOR CARPI RADIALIS 
BREVIOR (origin) 
Musculo-spiral nerve 
SUPINATOR RADII ff 
BREVIS ff | 
iit adial nerve 
FLEXOR CARPI ij 
ULNARIS hee: 
Posterior interosseous nerve 
EXTENSOR CARPI 
ULNARIS 
EXTENSOR OSSIS METACARPI 
POLLICIS 
EXTENSOR CARPI RADIALIS 
LONGIOR 
EXTENSOR LONGUS 
POLLICIS 
EXTENSOR CARPI RADIALIS 
BREVIOR 
EXTENSOR BREVIS POLLICIS 
EXTENSOR INDICIS 
Radial nerve 
Dorsal branch of ulnar 
nerve 
EXTENSOR MINIMI 
pIGITI (tendon) 
EXTENSOR COMMUNIS 
DIGITORUM (tendons) 
EXTENSOR CARPI RADIALIS 
_-LONGIOR 
' EXTENSOR OSSIS METACARPI 
POLLICIS 
EXTENSOR CARPI RADIALIS 
\{ BREVIOR 
“EXTENSOR BREVIS POLLICIS 
EXTENSOR INDICIS EXTENSOR LONGUS 
POLLICIS 
FIRST DORSAL 
INTEROSSEOUS 
MUSCLE 
Fie. 247.—THE MuscLes OF THE BACK OF THE FOREARM (the 
superficial muscles being reflected). 
In the lower third of the forearm it becomes 
superficial along with the extensor brevis pollicis, between the extensor communis 
fo) 
digitorum and the radial extensors of the carpus. 
It covers the last-named 
