This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The median nerve may be wounded in any part of its course in the forearm, but it is superficial only in its lower portion for about 5 cm. above the wrist. From this point up it is covered by the flexor sublimis, the flexor carpi radialis and the pronator radii teres.
While these muscles tend to protect it from injury, if the traumatism is extensive enough to divide it they render it all the more difficult to treat. Accompanying the nerve, especially in the middle third of the forearm, is the comes nervi mediani artery, which may cause annoying bleeding. Careless attempts to secure the artery may injure the nerve. Should the nerve be divided, paralysis ensues of all the superficial flexor muscles except the flexor carpi ulnaris, and of the deep muscles, except the inner half of the flexor profundus. This includes the pronator radii teres and pronator quadratus, so that the power of pronating the forearm is impaired as well as the ability to flex the hand. The flexor carpi ulnaris and outer half (that going to the ring and little fingers) of the flexor profundus digitorum are the only flexor muscles not paralyzed.
To find the nerve in the upper third of the forearm an incision may be made at the inner side of the biceps tendon and brachial artery. The median nerve will be found to the inner side of the artery and may be followed down. When the pronator radii teres is reached it must either be drawn to the ulnar side or divided.
The fascial expansion covering the flexor sublimis is next reached; it must be slit up and the muscular fibres parted to reach the nerve lying between it and the flexor profundus, with the volar (anterior) interosseous nerve alongside.
To reach the nerve in the middle third of the forearm the guide should be the palmaris longus tendon. The nerve lies in a line joining the outer edge of the palmaris longus tendon at the wrist and the brachial artery at the inner side of the biceps tendon at the elbow. If an incision is made in the middle of the forearm one comes down on the belly of the flexor carpi radialis muscle and it is necessary to part its fibres as well as those of the flexor sublimis beneath. If one goes a little lower down and places the incision between the palmaris longus and flexor carpi radialis the latter may be drawn outward, but the fascia covering the flexor sublimis will still have to be incised. The comes nervi mediani artery will be found accompanying the nerve.
To reach the nerve in its lower third is the easiest because it becomes superficial about 5 centimetres (2 in.) above the wrist. Here it lies either beneath the tendon of the palmaris longus or between it and the flexor carpi radialis. The incision should be made between the muscles. A layer of deep fascia will be found beneath them, which must be incised. From this point the nerve can be followed up beneath the flexor sublimis or downward beneath the annular ligament. Care is to be taken not to disturb the tendons of the flexor sublimis at the wrist.