M. brachioradialis or supinator longus ([Fig. 75], k; [Fig. 77], n).—This is a ribbon-like muscle lying on the lateral side of the brachium and the ventrolateral border of the antibrachium just beneath the superficial fascia. It connects the humerus and the radius.
Origin.—By a thin tendon from about the middle fifth of the dorsal border of the humerus, or sometimes proximad of the middle. The muscle curves over the outer surface of the brachialis ([Fig. 75], i), to which it is closely applied, and runs along the lateral border of the antibrachium to its insertion.
Insertion by a short tendon into the ridge which forms the ventral limit of the groove on the outer surface of the styloid process of the radius and upon the adjacent ligaments.
Relations.—Outer surface with the integument, and at the proximal end with the caput laterale of the triceps ([Fig. 75], h). Inner surface with the brachialis (i), the extensor carpi radialis longus (l) and brevis, and the extensor communis digitorum (m).
Action.—Supinator of the hand.
M. extensor carpi radialis longus ([Fig. 75], l; [Fig. 77], o).—This lies along the radial side of the forearm, and connects the humerus and the second metacarpal.
Origin ([Fig. 81], k) from the middle portion of the lateral supracondyloid ridge between the origin of the brachialis (j) and the anconeus. About the middle of the forearm the muscle ends in a slender tendon ([Fig. 84], e) which passes through the groove on the radial side of the dorsal surface of the distal end of the radius.
Insertion ([Fig. 84], c).—The tendon passes across the carpus and is inserted onto the dorsal surface of the base of the second metacarpal on the radial side just distad of the groove for the radial artery.
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).