Nerve · Upper arm

Posterior brachial cutaneous nerve

Nervus cutaneus brachii posterior

The first skin branch the radial nerve gives off, and it does so early — in the axilla itself, before the nerve has begun to spiral behind the humerus. It passes backwards through the armpit, pierces the deep fascia near the long head of triceps, and supplies a strip of skin down the back of the arm as far as the point of the elbow. Because it leaves so proximally, it is the only radial cutaneous nerve that is already on its way before the nerve enters the spiral groove. Standard descriptions differ over its exact root value, so none is recorded here.