Part 43

Relations.--Outer surface with the extensor carpi radialis brevis, extensor communis digitorum (Fig. 75, m), and extensor lateralis digitorum (Fig. 75, n). Radial border with the pronator teres (Fig. 77, q). Inner surface with the radius.

Action.--Supinator of the hand.

a, long portion of caput mediale of M. triceps; b, caput longum of M. triceps; c, caput laterale of M. triceps; d, M. anconeus; e, M. extensor carpi ulnaris; f, M. extensor indicis; g, M. supinator; h, M. extensor brevis pollicis; i, M. pronator teres; j, M. flexor profundus digitorum; k, M. biceps.]

=M. extensor brevis pollicis= or extensor ossis metacarpi pollicis (Fig. 85, a) (includes M. abductor longus pollicis of man).

Origin (Fig. 86, h) by fleshy fibres from the ventral half of the lateral surface of the shaft of the ulna from the semilunar notch to the styloid process; from the ulnar half of the dorsal surface of the radius from the bicipital tubercle to a short distance distad of the middle of the bone; and from the interosseous membrane between these two areas. The fibres converge to form a strong, flat tendon (Fig. 84, d) which passes obliquely over the tendons of the extensor carpi radialis longus (e) and brevis (f) to its

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).

=M. pronator teres= (Fig. 77, q).

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.

Action indicated by the name.

=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.

About one centimeter from the wrist the muscle ends in a flat tendon which passes through the transverse ligament and divides on the hand into four or five (or sometimes only three) tendons (Fig. 77, s′) which diverge to the insertions. The ulnar portion of the flexor sublimis digitorum (Fig. 77, x) takes origin from the common tendon before its division.

Insertion.--Each tendon except the first gives off a branch which spreads out in the trilobed pad in the palm. The middle two of these may be traced to the integument covering the pad. The tendon is then inserted (Fig. 88, a) near the base of the first phalanx onto the outer surface of the perforated portion of the flexor sublimis tendon (b, b′) at its side, except that to the thumb, which divides near its distal end into two which are inserted into the sesamoid bones at the base of the first phalanx. The tendons of insertion are closely united to the fibrous pulley-ring at the base of the first phalanx, and each may send a slip to the base of the phalanx at either side of the ring.

Relations.--Outer surface with the superficial fascia and one head of the flexor sublimis (Fig. 77, x). Radial border with the flexor carpi radialis. Ulnar border with the flexor carpi ulnaris and a part of the flexor profundus digitorum. Inner surface with the flexor carpi radialis (r), the flexor carpi ulnaris (t), the flexor profundus digitorum (u), and the radial head of the flexor sublimis digitorum.

Action.--Flexor of the first phalanx of each of the digits.

=M. flexor carpi ulnaris= (Fig. 77, t, t′).

Origin.--There are two heads. The first or humeral head (t′) takes origin in common with the second part of the flexor profundus from the median surface of the distal end of the humerus just distad of the medial epicondyle (Fig. 82, q). The second or ulnar head (t) takes origin by fleshy fibres from the lateral surface of the olecranon and from the dorsal border of the ulna from the olecranon to a point distad of the semilunar notch (Fig. 87, f). The two heads join proximad of the middle of the forearm. The muscle passes obliquely across the ventral surface of the forearm to its

Insertion by fleshy and tendinous fibres into the proximal surface of the pisiform bone.