Action.—Supinator of the hand.
[Fig. 86].—Lateral or Extensor Surface of Radius and Ulna, with the Areas of Attachment of Muscles.
![[Fig. 86]](#3418956513767812439_illo178lg.jpg.id-4370966763111853135.wrap-0.html.html){kind=link}
a, long portion of caput mediale of M. triceps; b, caput longum of M. triceps; c, caput laterale of M. triceps; d, M. anconeus; e, M. extensor carpi ulnaris; f, M. extensor indicis; g, M. supinator; h, M. extensor brevis pollicis; i, M. pronator teres; j, M. flexor profundus digitorum; k, M. biceps.
M. extensor brevis pollicis or extensor ossis metacarpi pollicis ([Fig. 85], a) (includes M. abductor longus pollicis of man).
Origin ([Fig. 86], h) by fleshy fibres from the ventral half of the lateral surface of the shaft of the ulna from the semilunar notch to the styloid process; from the ulnar half of the dorsal surface of the radius from the bicipital tubercle to a short distance distad of the middle of the bone; and from the interosseous membrane between these two areas. The fibres converge to form a strong, flat tendon ([Fig. 84], d) which passes obliquely over the tendons of the extensor carpi radialis longus (e) and brevis (f) to its
Insertion into the radial side of the base of the first metacarpal. The radial sesamoid bone of the wrist is imbedded in the tendon at its insertion.
Relations.—Outer surface with the extensor carpi radialis brevis, the extensor communis digitorum ([Fig. 75], m), extensor lateralis digitorum ([Fig. 75], n), and extensor indicis ([Fig. 85], c). Inner surface with the radius and ulna, and distad with the tendons of the two extensores carpi radiales ([Fig. 84], e and f).
Action.—Extends and abducts the pollex.