_Origin._—Immediately below the tubercle of the external condyle of the humerus. _Insertion._—By a small tendon to the outer side of the base of the pollex, and at a mid-point upon the anterior rim of the proximal phalanx of the middle finger. _Shape._—Small spindle-shaped, becoming tendinous at about the middle of the forearm. _Relations._—Occupies the middle of the group of muscles on the outer side of the forearm. _Action._—An extensor of the digit. =Supinator Brevis= (Fig. 14, No. 7). _Location._—The superior part of the radius. _Origin._—From the external condyle of the humerus below the origin of the tendon of the extensor digitorum communis. _Insertion._—To the outer side of the shaft of the radius, for nearly one-third of its length. _Shape._—Thin, slender. _Relations._—Inferiorly, with the bone. _Action._—A supinator of the radial region, and antagonistic to the pronators. =Flexor Metacarpi Radialis= (Fig. 15, No. 10). _Location._—The lowest of the group of three muscles on the outer part of the forearm. _Origin._—By two tendinous heads, the longer from the external condyle of the humerus, and the other just beyond the base of the olecranon. _Insertion._—Becomes tendinous at about the middle of the shaft of the ulna and inserts to a prominent process on the proximal third of the posterior part of the shaft of the mid-metacarpal. _Shape._—Fusiform. _Relations._—Inferiorly, the anconeus and superiorly, the extensor digitorum communis. _Action._—A powerful flexor of the hand. =Pronator Brevis= (Fig. 13, No. 18). _Location._—Supero-lateral side of the forearm. _Origin._—From just above the internal condyle of the humerus. _Insertion._—The tendon passing obliquely across the interosseous space and is inserted to the ulnar side of the shaft of the radius, just beyond the juncture of the proximal and the middle thirds. _Shape._—Fusiform. _Relations._—Superior to the pronator longus. _Action._—Pronates the forearm, and flexes the forearm upon the arm. =Pronator Longus= (Fig. 14, No. 6). _Location._—Lateral side of the radio-ulnar region. _Origin._—From the middle of the internal condyle of the humerus. _Insertion._—To the shaft of the radius just beneath the pronator brevis. _Shape._—Massive, ellipsoidal. _Relations._—Between the pronator brevis and flexor digitorum profundus. _Action._—A pronator. =Extensor Ossis Metacarpi Pollicis= (Fig. 13, No. 23). _Location._—Slightly interposed between the ulna and radius. _Origin._—Immediately in front of the greater sigmoid cavity of the ulna. _Insertion._—To the palmar side of the base of the first metacarpal, in common with the extensor metacarpi radialis longior. _Shape._—Delicate, straight. _Relations._—Superiorly, the extensor indicis longus, and inferiorly, the anconeus and the flexor digitorum profundus. _Action._—Extends the hand upon the forearm. =Anconeus= (Fig. 15, No. 4). _Location._—Between the ulna and the radius.