Dermatome- anatomy and publication

According to the clinical, surgical, and experimental findings of Keegan and Garrett, the dermatomes of the limbs from the 5th cervical to the 1st thoracic and from the 3rd lumbar to the 2nd sacral extend as a series of bands from the mid-dorsal line of the trunk into the limbs, as illustrated. 

When the function of even a single dorsal nerve root is interrupted, a faint but definite diminution of sensitivity can be demonstrated in the dermatome. The best method to detect and plot the area of diminished sensitivity is by the use of a light pin scratch for pain sensation, although it can be found for temperature and for tactile sensation also. 

Note: Pearson, A. A., and Bass, J. J. find that in the human embryo, it is unusual for the dorsal rami of C. 6, 1, and 8 to possess cutaneous branches, for only about 10% do; and that C.5 and T.I possess them in only 67% and 82% respectively. In contrast, they do find that in rabbit embryos, each of these dorsal rami (C.5—T.I) possesses a cutaneous branch.

Grant, John Charles Boileau - An atlas of anatomy, / by regions 1962

A dermatome is an area of skin supplied by the dorsal (sensory) root of a spinal nerve. 

The dermatomes were determined by plotting (a) the areas of vasodilation that resulted in stimulation of individual dorsal nerve roots and (b) the areas of "remaining sensibility" after cutting three roots above and three roots below a given nerve root. 

The areas plotted represent the average finding for each dorsal root based on pain sensation. Areas determined for temperature sensation have similar limits, but those for touch are more extensive. 

Note that there is considerable overlapping of contiguous dermatomes; that is to say, each segmental nerve overlaps the territories of its neighbors. As a result, no anesthesia results unless two or more consecutive dorsal roots have lost their functions. The 7th cervical dermatome (not depicted) begins lower on the upper arm than the 6th and includes most or all of the hand.

Grant, John Charles Boileau - An atlas of anatomy, / by regions 1962