_Relations._--Outer surface with the integument and at the origin with the caput laterale (Fig. 75, _h_) of the triceps. Radial surface with the extensor longus (_l_) and extensor brevis. Ulnar surface with the extensor digitorum lateralis (_n_). Inner surface with the supinator (Fig. 85, _b_) and the extensor pollicis brevis (Fig. 85, _a_). _Action._--Extensor of the four ulnar digits. =M. extensor digitorum lateralis= (Fig. 75, _n_). (This corresponds in position to the extensor digiti quinti proprius of man.) _Origin_ (Fig. 81, _n_) from the lateral supracondyloid ridge of the humerus distad of the origin of the extensor communis. The muscle passes along the ulnar border and dorsal surface of the forearm between the extensor communis (Fig. 75, _m_) and the extensor carpi ulnaris (_o_). Near the carpus it ends in a tendon (_n′_), which divides immediately into three or four (Fig. 84, _b_). These pass through the groove on the ulnar side of the dorsal surface of the radius and diverge to the digits lying beneath the extensor communis tendon. _Insertion_ (Fig. 84, _b_).--The three tendons on the ulnar side join the ulnar sides of the tendons of the extensor communis (_a_) at their insertions. The radial of the four tendons (not always present) joins the tendon of the extensor indicis (_c_) and may also give a branch to the radial side of the base of the first phalanx of the third digit. _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_). _Action._--Indicated by its name. =M. extensor indicis (proprius)= (Fig. 85, _c_; Fig. 75, _p_) (includes extensor pollicis longus also).