Dermatome: area of skin supplied by one spinal nerve, through both its rami.
General Rules of Dermatomal Distribution
With the exception of the first cervical nerve (C1), every spinal nerve is associated with a specific dermatome.
A dermatome typically extends from the anterior median line to the posterior median line: its upper half is supplemented by the nerve above and its lower half by the nerve below.
No area of skin is supplied by a single spinal nerve alone because adjacent dermatomes overlap.
Dermatomes of the Trunk and Limbs
In the trunk, dermatomes are arranged simply, each extending around the body wall from the posterior median line to the anterior median line.
In the limbs, the dermatomal arrangement is complicated.
Limb dermatomes are complicated because spinal nerves supply the limb through plexuses before innervating the skin, and because the limbs rotate during embryonic life.