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, 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 light pin scrateh 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.