9. N. medianus ([Fig. 160], e).—The median nerve is formed by the junction of three branches, one each from the seventh and eighth cervical with the brachial artery (g) passing between them, and one from the first thoracic nerve. It accompanies the brachial artery (g) lying on its medial side, and passes with it through the supracondyloid foramen; here it is connected by a branch to the musculocutaneous nerve. It passes into the forearm ([Fig. 161], b), lying at first beneath the pronator teres (5). Here it gives branches to the pronator teres and flexor muscles (flexor carpi radialis, palmaris longus, five heads of the flexor profundus digitorum, but not to the flexor carpi ulnaris). Continuing along the forearm, lying on the flexor carpi radialis, it sends a posterior interosseous nerve to the pronator quadratus muscle and branches to the deep and superficial divisions of the flexor sublimis digitorum. It thus supplies the flexor and pronator muscles (except the flexor carpi ulnaris). It then passes into the palm beneath the transverse ligament and gives rise to three principal branches. The first of these supplies the integument of the thumb, sending a branch on either side of it; and onto the radial side of the second digit. The second passes along the contiguous sides of the second and third digits and supplies their integument. The third passes along the contiguous sides of the third and fourth digits to their integument. Each sends twigs into the trilobed pad in the palm, so that there are thus seven terminal branches in the palm. From these, branches (probably) pass to the three radial Mm. lumbricales.

[Fig. 161].—Nerves and Arteries of the Forearm, seen from the Flexor Side.

Mm. palmaris longus, flexor carpi radialis, flexor carpi ulnaris, and the third and fourth heads of M. flexor profundus digitorum have been removed; also part of the pronator teres. 1, M. biceps; 2, conjoined tendon of M. brachialis and M. clavobrachialis; 3, short portion of caput mediale of M. triceps; 4, intermediate portion of caput mediale of M. triceps brachii; 5, cut ends of M. pronator teres; 6, M. brachioradialis; 7, M. extensor carpi radialis longus and brevis; 8, fifth head of M. flexor profundus digitorum; 9, second head of M. flexor profundus digitorum; 10, radial part of M. flexor sublimis digitorum; 11, common tendon of M. flexor profundus digitorum; 12, first head of M. flexor profundus digitorum; 13, cut origin of M. flexor carpi ulnaris. a, ulnar nerve; b, median nerve; c, dorsal cutaneous branch of the ulnar nerve; d, palmar branch of ulnar nerve; e, deep palmar branch; f, superficial palmar branch; g′, A. brachialis; g, A. radialis; h, A. collateralis ulnaris superior; i, A. radialis recurrens; j, A. ulnaris recurrens; k, A. ulnaris; l, A. interossea anterior; m, large branch of A. radialis, joining A. ulnaris.

10. N. radialis.—The radial (or musculospiral) nerve is formed by the junction of branches from the seventh and eighth cervical and first thoracic nerves. It immediately sends branches to the epitrochlearis, long head of the triceps, and long and intermediate portions of the medial head of the triceps. It then passes between the long and intermediate portions of the medial head of the triceps, following the profunda brachii artery, and curves about the humerus to its cranial side. It supplies there the lateral head of the triceps and the anconeus, and, lying on the brachialis muscle, divides into a superficial and a deep branch. The superficial branch is the superficial radial nerve ([Fig. 130], g); the deep one forms the dorsal (or posterior) interosseous nerve. The former is sensory and the latter motor.

The superficial radial nerve ([Fig. 130], g, [page 319]) becomes cutaneous at the junction of the second and third thirds of the upper arm, where it emerges from between the lateral head of the triceps and the brachialis muscle. It then follows the course of the vena cephalica (c) to the wrist and follows the dorsal tributary of the same vein onto the dorsum of the hand. It supplies the integument of the distal part of the ventral surface of the upper arm and that along the forearm. On the hand it is distributed to the dorsal surface in the same manner as the median nerve on the ventral surface, i.e. to the integument of the thumb on both sides and to that of the medial side of the second digit; to that of the contiguous sides of the second and third digits and of the contiguous sides of the third and fourth digits. There are thus seven terminal branches, one for each of these regions.

The dorsal interosseous nerve follows the brachialis muscle and passes onto the forearm between it and the extensor carpi radialis longus. It supplies the supinator and extensor muscles of the forearm (brachioradialis, supinator, extensor carpi radialis longus, extensor carpi radialis brevis, extensor communis digitorum, extensor digitorum lateralis, extensor indicis, extensor brevis pollicis). The dorsal interosseous and radial nerves thus supply all the extensor muscles of the forearm and upper arm and the supinators (modified extensors).

11. N. ulnaris ([Fig. 160], n).—The ulnar nerve arises from the eighth cervical and first thoracic and accompanies the brachial artery (g) and median nerve (e) through the upper arm. It does not pass through the supracondyloid foramen, but curves over the dorsal end of the medial epicondyle of the humerus within the short division of the medial head of the triceps, and reaches the dorsal border of the forearm. In the forearm ([Fig. 161], a) it lies beneath the flexor carpi ulnaris. It supplies muscular branches to the flexor carpi ulnaris and the first or ulnar head of the flexor profundus digitorum. Near the middle of the forearm it divides into two branches, dorsal cutaneous (c) and palmar (d).

The dorsal cutaneous branch (c) curves about the ulnar side of the wrist to the dorsum of the hand and divides into twigs which supply the dorsal part of the ulnar side of the fifth digit and of the contiguous sides of the fourth and fifth. The palmar branch (d) sends a twig to the integument on the flexor surface of the arm near the wrist, and then divides into deep palmar (e) and superficial palmar (f) branches.

The superficial palmar branch (f) passes into the palm and supplies the ventral portion of the integument on the ulnar side of the fifth digit, and on the contiguous sides of the fifth and fourth.