_Insertion_ into the radial side of the base of the first metacarpal. The radial sesamoid bone of the wrist is imbedded in the tendon at its insertion.
_Relations._--Outer surface with the extensor carpi radialis brevis, the extensor communis digitorum (Fig. 75, _m_), extensor lateralis digitorum (Fig. 75, _n_), and extensor indicis (Fig. 85, _c_). Inner surface with the radius and ulna, and distad with the tendons of the two extensores carpi radiales (Fig. 84, _e_ and _f_).
_Action._--Extends and abducts the pollex.
B. MUSCLES ON THE RADIAL AND VENTRAL SIDE OF THE FOREARM (flexors and pronators).
_Origin_ (Fig. 82, _l_) by a short strong tendon from the extremity of the medial epicondyle of the humerus.
_Insertion_ (Fig. 86, _i_) by fleshy fibres and short tendinous fibres along the medial border of the radius, at its middle. The radial edge of the muscle is continuous with the strong deep layer of the antibrachial fascia.
_Relations._--Outer surface with the superficial fascia. Radial border with the tendon of the biceps (Fig. 79, _l′_), with the extensor carpi radialis brevis (Fig. 77, _p_), and the supinator (Fig. 85, _b_). Ulnar border with the flexor carpi radialis (Fig. 77, _r_) and part of the flexor profundus digitorum (Fig. 77, _u_).
_Action._--Pronates the hand by rotating the radius.
=M. flexor carpi radialis= (Fig. 77, _r_).--A slender fusiform muscle.
_Origin_ (Fig. 82, _m_) from the tip of the medial epicondyle of the humerus.
_Insertion._--The slender tendon passes through a deep groove between the os magnum and the first metacarpal. The groove is converted into a canal by the overlying tendons and muscles. The tendon is finally inserted into the bases of the second and third metacarpals.
_Relations._--Outer surface with the pronator teres (Fig. 77, _q_), the superficial fascia, and the third head of the flexor profundus (_u_). Radial border with the pronator teres (_q_). Ulnar border with the third and fourth heads of the flexor profundus. Inner surface with the ulna proximad and the fifth head of the flexor profundus (_u′_) distad.
=M. palmaris longus= (Fig. 77, _s_).--A flat fusiform muscle beneath the fascia on the medial border of the forearm.
_Origin_ (Fig. 82, _p_) by a short flat tendon from the distal part of the medial surface of the medial epicondyle of the humerus.