334 THE MUSCULAR SYSTEM. 
muscles in its further course in the forearm, and at the wrist it crosses the 
radial artery. 
The extensor brevis pollicis, an essentially human muscle, is a specialised 
portion of the previous muscle. It arises from the back of the radius and the 
interosseous membrane below the extensor ossis metacarpi pollicis. It is closely 
adherent to that muscle, and accompanies it beneath the posterior annular ligament 
and over the radial artery to the thumb. Its tendon is then continued along the 
back of the first metacarpal bone, and the metacarpo-phalangeal articulation, to be 
inserted into the base of the first phalanx of the thumb. Before reaching its 
insertion the tendon helps to form the capsule of the metacarpo-phalangeal joint. 
In the forearm the muscle is deeply placed beneath the superficial extensors, and 
is separated from the extensor longus pollicis by the posterior interosseous nerve. 
The extensor longus pollicis arises from the posterior or extensor surface of 
the ulna, and from the interosseous membrane, below the extensor ossis metacarpi 
pollicis. Its tendon grooves the back of the radius, and occupies a special compart- 
ment beneath the posterior annular ligament. Extending obliquely across the 
back of the hand, it is inserted into the base of the second phalanx of the thumb. 
The muscle is deeply placed beneath the superficial extensors of the forearm, 
and lies between the extensor brevis pollicis and the extensor indicis. It separates 
the posterior interosseous artery from the nerve, the latter passing beneath it. On 
the back of the hand the tendon crosses the radial artery, and helps to form the 
capsule of the first metacarpo-phalangeal articulation. 
The extensor indicis (m. extensor indicis proprius) arises below the extensor 
longus pollicis from the back of the ulna, and sometimes also from the interosseous 
membrane. Its tendon passes through a compartment of the posterior annular 
ligament along with the tendons of the extensor communis digitorum, and is 
inserted into the forefinger, joining the membranous expansion of the tendon of 
the extensor communis digitorum on the dorsum of the first phalanx. 
Lying deeply in the forearm, the muscle is placed internal to the extensor 
longus pollicis, and covers the posterior interosseous nerve. On the back of the 
hand its tendon lies on the inner side of the tendon of the common extensor 
destined for the forefinger. 
NERVE SUPPLY. 
Four nerves are engaged in supplying the muscles of the forearm and hand—the median and 
ulnar on the front, the musculo-spiral and posterior interosseous nerves on the back of the 
limb. 
Muscles. Nerves. Origin. 
A. Pronators and Flexors. 
Pronator radii teres 
Flexor carpi radialis i 
Palmaris longus | 
Flexor AMER digitorum ! 
Flexor carpi ulnaris : : : : Ulnar 
Flexor profundus digitorum : ra f Ulnar and anterior In- | 
\ terosseous (median) 
Median . F . ’ @ 
> 
~I OO © 
Flexor longus pollicus) 5 one 
Pronator quadratus f - F sa Anterior interosseous . C! 
B. Muscles of the Hand. | 
Abduetor pollicis | | 
Opponens pollicis - | 
Flexor brevis pollicis (superfici¢ al) | 
Flexor brevis pollicis (deep) | | 
Adductor obliquus pollicis 
Adductor transversus pollic is | 
Lumbricales Ist and 2nd_ . R : Median . : : 5 CSGiit: 
Lumbricales 3rd and 4th | 
Median . , : : Onis 7h 
Ulnar 
(00) 
a 
a 
_ 
WwW 
Interossei 
Flexor brevis minimi | : ; Ulnar . ; ; : C. 
es) 
— 
ra 
— 
= 
Opponens minimi digiti 
Abduetor minimi digiti 
