The median nerve gives off no branches during its course along the arm, with the exception of an occasional communicating branch to the musculo-cutaneous below the level of the insertion of the coraco-brachialis ; and a branch which is usually found coming off from the anterior part of the trunk a little above the elbow. This is directed along the brachialis anticus to the pronator teres, which it supplies, and sends a few filaments back wards to enter the articulation.
The branches given off in the fore-arm are muscular, interosseous, and cutaneous.
77ze muscular branches for the lower part of the pronator teres, flexor carpi radialis, pal marls longus, and flexor sublimis, are generally derived from a primary branch, which arises behind the pronator teres a little below the elbow-joint ; the lower part of the flexor sublimis, however, receiving several smaller branches from the main trunk. The branches for the flexor longus pollicis and flexor digi torum profundus are given off lower down, there being generally one for the former and two for the latter, the outer part of which only is supplied ; the inner part of the muscle being supplied by the ulnar nerve.
The anterior interosseous nerve is the most deeply seated branch of the median, coming off at an acute angle from the trunk, between the origin of the deep-seated muscular branches. It runs vertically downwards in company vt ith, but to the radial side of, the corresponding artery, in front of the interosseous membrane between the flexor digitorum profundus and flexor longus pollicis, giving on either side small filaments to them. Flaying reached the upper edge of the pronator quadratus, it passes behind that muscle, and terminates either by sending numerous filaments into its posterior surface, or, after having supplied it, perforates the lower aperture of the interos seous membrane, and reaches the back of the carpus.
The palmar cutaneous branch is given off at the lower fourth of the fore-arm, passes for wards from beneath the tendons of the flexor sublimis, and behind the fascia, which it per forates a little above the wrist, and divides into an external filament, which, having com municated with the radial, terminates in the skin of the vola major, and an internal de scending over the annular ligament to be lost in the skin of the upper part of the palm.
The terminal digital branches of the median are derived from two primary branches, into which the flattened and expanded nerve di vides, after having passed from beneath the annular ligament. These are external and
internal, the former supplying the muscles of the thumb, and sending off three digital branches for the thumb and radial side of the index finger, and rather smaller than the latter, which gives off two digital branches for the opposed sides of the index and middle, and the middle and ring finger. The muscular branch passes in a slightly curved manner out wards and upwards, and terminates in fila ments for the supply of the abductor, opponens and flexor brevis pollicis.
The first digital nerve is directed obliquely downwards and outwards in front of the tendon of the flexor !onus pollicis, and near the head of the metacarpal bone, crosses it to its outer side, and continues its course to the extremity of the outer side of the anterior aspect of the first phalanx, where it terminates in dorsal and palmar branches. The dorsal branch winds on to the back of the last pha lanx, communicates with the radial, and sup plies the skin at the root of the nail ; the palmar continues in the original course of the nerve to the skin at the extremity of the thumb.
The second digital nerve, not so oblique in its direction as the first, crosses over the adductor pollicis, gives a filament to it, and is conducted along the inner side of the flexor longus pollicis tendon to the ulnar side of the thumb, sending in its course some fila ments backwards to communicate with the dorsal branches of the radial, and terminating in a similar manner to the preceding branch.
The third digital nerve is directed in front, and to the outside of the first lumbrical muscle, gives a filament to it, and reaches to about the middle of the outer side of the proximal phalanx of the index finger, where it divides into dorsal and palmar branches. The dorsal branch passes on to the back of the phalanx, communicates with one of the dorsal cutaneous nerves, to form a nerve which ends in the integuments of the back part of the last phalanx : the palmar branch passes in the original direction of the nerve, and terminates on the outer side of the distal pha lanx by again dividing into palmar and dorsal branches, having a similar distribution to the two first nerves.