Part 42

=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.

Relations.--Outer surface with the integument and at the origin with the caput laterale (Fig. 75, h) of the triceps. Radial surface with the extensor longus (l) and extensor brevis. Ulnar surface with the extensor digitorum lateralis (n). Inner surface with the supinator (Fig. 85, b) and the extensor pollicis brevis (Fig. 85, a).

Action.--Extensor of the four ulnar digits.

=M. extensor digitorum lateralis= (Fig. 75, n). (This corresponds in position to the extensor digiti quinti proprius of man.)

Origin (Fig. 81, n) from the lateral supracondyloid ridge of the humerus distad of the origin of the extensor communis.

The muscle passes along the ulnar border and dorsal surface of the forearm between the extensor communis (Fig. 75, m) and the extensor carpi ulnaris (o). Near the carpus it ends in a tendon (n′), which divides immediately into three or four (Fig. 84, b). These pass through the groove on the ulnar side of the dorsal surface of the radius and diverge to the digits lying beneath the extensor communis tendon.

Insertion (Fig. 84, b).--The three tendons on the ulnar side join the ulnar sides of the tendons of the extensor communis (a) at their insertions. The radial of the four tendons (not always present) joins the tendon of the extensor indicis (c) and may also give a branch to the radial side of the base of the first phalanx of the third digit.

Relations.--Outer surface with the integument. Radial border with the extensor communis digitorum (m); ulnar border with the extensor carpi ulnaris (o). Inner surface with the supinator (Fig. 85, b) and the extensor brevis pollicis (Fig. 85, a); distad with the extensor indicis (Fig. 85, c).

Action.--Extensor of the four ulnar digits.

=M. extensor carpi ulnaris= (Fig. 75, o).

Origin.--(1) By a short, broad tendon from the distal portion of the lateral epicondyle of the humerus (Fig. 81, o) distad of the origin of the extensor lateralis, and (2) by a smaller tendon from the ulna at the dorsal tip of the semilunar notch (Fig. 86, e).

The muscle passes along the ulnar side of the forearm dorsad of the extensor digitorum lateralis (n). It ends near the junction of the middle and distal thirds of the forearm in a large, flat tendon (o′) which is closely adherent to the ligaments of the wrist and which passes over the distal end of the ulna to its

Insertion into the tubercle on the ulnar side of the base of the fifth metacarpal (Fig. 84, g).

Relations.--Outer surface with the integument. Radial (or ventral) border with the extensor digitorum lateralis (Fig. 75, n); ulnar (or dorsal) border with the flexor profundus digitorum (q). Inner surface with the radius, the extensor indicis (Fig. 85, c), and the extensor pollicis brevis (Fig. 85, a).

Action.--Indicated by its name.

=M. extensor indicis (proprius)= (Fig. 85, c; Fig. 75, p) (includes extensor pollicis longus also).

Origin (Fig. 86, f) by short, fleshy fibres from the lateral surface of the ulna between the semilunar notch and the junction of the third and fourth (distal) fourths.

The muscle-fibres pass obliquely distad and toward the radial side and join the tendon (Fig. 85, c′), which runs nearly the whole length of the radial surface.

Insertion (Fig. 84, c).--The tendon passes through the groove on the ulnar side of the dorsal surface of the base of the radius and divides into two. Both of these usually go to the base of the second phalanx of the second digit, but one may pass to the pollex, or there may be three divisions going to the first three digits.

a, M. extensor brevis pollicis (a′, its tendon); b, M. supinator; c, M. extensor indicis (c′, its tendon); d, tendon of M. extensor carpi radialis brevis; e, tendon of M. extensor carpi radialis longus. 1, humerus; 2, olecranon; 3, styloid process of ulna; 4, distal end of radius; 5, head of radius.]

Relations.--Outer surface with the integument, the extensor carpi ulnaris (Fig. 75, o), and the extensor digitorum lateralis (Fig. 75, n). Dorsal border with the flexor profundus digitorum (Fig. 75, q). Inner surface with the extensor brevis pollicis (Fig. 85, a).

Action as indicated by the name.

=M. supinator= (Fig. 85, b).--A flat muscle wrapped spirally about the proximal end of the radius.

Origin by a short, strong tendon from the lateral side of the annular ligament of the radius and by tendinous fibres from the radial collateral ligament passing from the humerus to the radius. From the tendon of origin the fibres diverge, passing toward the radial side and distad.

Insertion (Fig. 86, g).--The dorsal and part of the ventral surface of the proximal two-fifths of the radius to within five millimeters of the articular face of the head.