The body can be divided into regions that are mainly supplied by a single spinal nerve. There are eight cervical (one for the head, and one for each cervical vertebra), twelve thoracic, five lumbar and five sacral spinal nerves.
This innervates the body in a patterned form. Along the thorax and abdomen it is simply like a stack of discs forming a human, each supplied by a different spinal nerve.
Along the arms and the legs, the pattern is different: the dermatomes run longitudinally along the limbs.
Dermatomes are useful for finding the site of damage to the spine.