_Relations._--Outer surface with the integument. Radial border with the extensor communis digitorum (_m_); ulnar border with the extensor carpi ulnaris (_o_). Inner surface with the supinator (Fig. 85, _b_) and the extensor brevis pollicis (Fig. 85, _a_); distad with the extensor indicis (Fig. 85, _c_).
_Action._--Extensor of the four ulnar digits.
=M. extensor carpi ulnaris= (Fig. 75, _o_).
_Origin._--(1) By a short, broad tendon from the distal portion of the lateral epicondyle of the humerus (Fig. 81, _o_) distad of the origin of the extensor lateralis, and (2) by a smaller tendon from the ulna at the dorsal tip of the semilunar notch (Fig. 86, _e_).
The muscle passes along the ulnar side of the forearm dorsad of the extensor digitorum lateralis (_n_). It ends near the junction of the middle and distal thirds of the forearm in a large, flat tendon (_o′_) which is closely adherent to the ligaments of the wrist and which passes over the distal end of the ulna to its
_Insertion_ into the tubercle on the ulnar side of the base of the fifth metacarpal (Fig. 84, _g_).
_Relations._--Outer surface with the integument. Radial (or ventral) border with the extensor digitorum lateralis (Fig. 75, _n_); ulnar (or dorsal) border with the flexor profundus digitorum (_q_). Inner surface with the radius, the extensor indicis (Fig. 85, _c_), and the extensor pollicis brevis (Fig. 85, _a_).
=M. extensor indicis (proprius)= (Fig. 85, _c_; Fig. 75, _p_) (includes extensor pollicis longus also).
_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.