Axiom. The level at which motor (and sensory) branches leave a nerve is very variable; but the side from which they leave is constant—naturally, they leave from the side nearest the muscles to which they are distributed. Certain nerves have sides of safety and sides of danger; sides on which it is safe to dissect and sides on which it is dangerous. The median and radial nerves at the elbow exemplify this rule. The median nerve supplies flexors, therefore, its branches pass medially; the radial nerve supplies extensors, therefore, its branches pass laterally. It is, therefore, safe, or relatively safe, to dissect on the lateral side of the median nerve and on the medial side of the radial nerve.
Surgical Anatomy. Arch Surg. 1996;131(3):271. doi:10.1001/archsurg.1996.01430150049009