The superficial branch of the radial nerve arises in the cubital fossa by the division there of the radial nerve into deep and superficial branches (see Plates 3-13 and 3-16). The superficial branch, which is entirely cutaneous, courses through the forearm under cover of the brachioradialis muscle and is accompanied by the radial artery. At the distal third of the forearm, the superficial branch of the radial nerve perforates the antebrachial fascia along the lateral border of the forearm and divides into two branches.
The smaller lateral branch supplies the skin of the radial side and eminence of the thumb and communicates with the lateral antebrachial cutaneous nerve. The larger medial branch divides into four dorsal digital nerves. The first dorsal digital nerve supplies the ulnar side of the thumb; the second supplies the radial side of the index finger; the third distributes to the adjoining sides of the index and middle fingers; and the fourth supplies the adjacent sides of the middle and ring fingers.
There is usually an anastomosis on the back of the hand between the superficial branch of the radial nerve and the dorsal branch of the ulnar nerve, and there is some variability in the apparent source of the last (more median) branch of either nerve. In some such cases, the adjacent sides of the middle and ring fingers are in the territory of the ulnar nerve. Dorsal digital nerves fail to reach the extremities of the digits. They reach to the base of the nail of the thumb, to the distal interphalangeal joint of the second digit, and not quite as far as the proximal interphalangeal joints of the third and fourth digits. The distal areas of the dorsum of the digits not supplied by the radial nerve receive branches from the stout palmar digital branches of the median nerve.
The dorsal branch of the ulnar nerve completes the cutaneous supply of the dorsum of the hand and digits (see Plate 3-16). It arises about 5 cm above the wrist, passes dorsalward from beneath the flexor carpi ulnaris tendon, and then pierces the forearm fascia. At the ulnar border of the wrist, the nerve divides into three dorsal digital branches.
The first branch courses along the ulnar side of the dorsum of the hand and supplies the ulnar side of the little finger as far as the root of the nail. The second branch divides at the cleft between the fourth and fifth digits and supplies their adjacent sides. The third branch may divide similarly; it may supply the adjacent sides of the third and fourth digits, or it may simply anastomose with the fourth dorsal digital branch of the superficial branch of the radial nerve. The dorsal branches to the fourth digit usually extend only as far as the base of the second phalanx, with the more distal parts of the fourth and fifth digits supplied by palmar digital branches of the ulnar nerve.
The palmar branch of the ulnar nerve arises about the middle of the forearm, descending under the antebrachial fascia in front of the ulnar artery (see Plates 3-16 and 3-17). It perforates the fascia just above the wrist and supplies the skin of the hypothenar eminence and the medial part of the palm.
The palmar branch of the median nerve arises just above the wrist (see Plates 3-16 and 3-17). It perforates the palmar carpal ligament between the tendons of the palmaris longus and flexor carpi radialis muscles and distributes to the skin of the central depressed area of the palm and the medial part of the thenar eminence.