_Relations._--Outer surface at the proximal end with the caput laterale of the triceps (Fig. 75, _h_), farther distad with the integument and the brachioradialis (_k_); the distal tendon with the extensor pollicis brevis (Fig. 84, _d_). Radial surface with the brachialis (Fig. 75, _i_) and the integument. Ulnar surface with the extensor carpi radialis brevis (Fig. 77, _p_), and near the proximal end with the extensor communis digitorum (Fig. 75, _m_). _Action._--Extensor of the hand. =M. extensor carpi radialis brevis= (Fig. 77, _p_).--A slender muscle from the humerus to the third metacarpal. _Origin_ (Fig. 81, _l_) from the distal part of the lateral supracondyloid ridge distad of the extensor longus. Near the carpus the muscle ends in a slender tendon which passes through the groove with the extensor longus tendon. The tendon then diverges from the extensor longus and has its _Insertion_ (Fig. 84, _f_) into the radial side of the dorsal surface of the base of the third metacarpal. _Relations._--Outer surface with the extensor longus (Fig. 77, _o_), the integument, and distad with the extensor pollicis brevis (Fig. 84, _d_). Radial surface with the extensor longus. Ulnar surface with the extensor communis digitorum (Fig. 75, _m_) and the extensor brevis pollicis. Inner surface with the pronator teres (Fig. 77, _q_), supinator (Fig. 85, _b_), and extensor brevis pollicis (Fig. 85, _a_). _Action._--Extensor of the hand. [Illustration: FIG. 84.--ARRANGEMENT OF THE TENDONS ON THE BACK OF THE HAND. _a_, M. extensor communis digitorum; _b_, M. extensor lateralis digitorum; _c_, M. extensor indicis; _d_, M. extensor brevis pollicis; _e_, M. extensor carpi radialis longus; _f_, M. extensor carpi radialis brevis; _g_, M. extensor carpi ulnaris; _h_, tendons of the interossei. 1, transverse ligament of the wrist.] =M. extensor digitorum communis= (Fig. 75, _m_).--A slender muscle from the lateral supracondyloid ridge of the humerus to the phalanges of the second, third, fourth, and fifth digits. _Origin_ (Fig. 81, _m_).--The distal surface of the lateral supracondyloid ridge dorsad of the origin of the extensor carpi radialis brevis. At the junction of the middle and distal thirds of the radius the muscle passes into a large, flat tendon (Fig. 75, _m′_) which passes through the groove on the middle of the dorsal surface of the distal end of the radius. The groove is converted into a canal by a strong transverse ligament. The tendon divides near the groove into four portions (Fig. 84, _a_) which diverge and pass to the four ulnar digits. _Insertions._--Each division of the tendon as it passes over the dorsal surface of the first phalanx has its border connected by strong fascia to the dorsal surface of the phalanx. It finally passes through the groove at the distal end of the first phalanx and is inserted mostly into the base of the second phalanx; a portion continues distad to be inserted into the third phalanx.