_Origin_ (Fig. 86, _f_) by short, fleshy fibres from the lateral surface of the ulna between the semilunar notch and the junction of the third and fourth (distal) fourths. The muscle-fibres pass obliquely distad and toward the radial side and join the tendon (Fig. 85, _c′_), which runs nearly the whole length of the radial surface. _Insertion_ (Fig. 84, _c_).--The tendon passes through the groove on the ulnar side of the dorsal surface of the base of the radius and divides into two. Both of these usually go to the base of the second phalanx of the second digit, but one may pass to the pollex, or there may be three divisions going to the first three digits. [Illustration: FIG. 85.--DEEP MUSCLES ON THE EXTENSOR SURFACE OF THE RADIUS AND ULNA. _a_, M. extensor brevis pollicis (_a′_, its tendon); _b_, M. supinator; _c_, M. extensor indicis (_c′_, its tendon); _d_, tendon of M. extensor carpi radialis brevis; _e_, tendon of M. extensor carpi radialis longus. 1, humerus; 2, olecranon; 3, styloid process of ulna; 4, distal end of radius; 5, head of radius.] _Relations._--Outer surface with the integument, the extensor carpi ulnaris (Fig. 75, _o_), and the extensor digitorum lateralis (Fig. 75, _n_). Dorsal border with the flexor profundus digitorum (Fig. 75, _q_). Inner surface with the extensor brevis pollicis (Fig. 85, _a_). _Action_ as indicated by the name. =M. supinator= (Fig. 85, _b_).--A flat muscle wrapped spirally about the proximal end of the radius. _Origin_ by a short, strong tendon from the lateral side of the annular ligament of the radius and by tendinous fibres from the radial collateral ligament passing from the humerus to the radius. From the tendon of origin the fibres diverge, passing toward the radial side and distad. _Insertion_ (Fig. 86, _g_).--The dorsal and part of the ventral surface of the proximal two-fifths of the radius to within five millimeters of the articular face of the head. _Relations._--Outer surface with the extensor carpi radialis brevis, extensor communis digitorum (Fig. 75, _m_), and extensor lateralis digitorum (Fig. 75, _n_). Radial border with the pronator teres (Fig. 77, _q_). Inner surface with the radius. _Action._--Supinator of the hand. [Illustration: FIG. 86.--LATERAL OR EXTENSOR SURFACE OF RADIUS AND ULNA, WITH THE AREAS OF ATTACHMENT OF MUSCLES. _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).