Tactile dermatomes LI through COJ were determined in the dog by electrophysiological recording. All dermatomes were continuous skin fields which extended from the dorsal to the ventral midlines and displayed considerable craniocaudal overlap. Most cutaneous regions were innervated by three dorsal roots, although some loci were supplied by two or four segments.
The dermatomes boundaries, which were determined in relation to palpable skeltal landmarks, displayed an inherent variability from subject to subject, around a most common distribution. This variability was limited to the distance of one vertebral spine, cranial or caudal to the mode boundaries.
Dermatomes exhibited central regions of maximal innervation where the response per rootlet appeared greater than at the periphery. The central-peripheral sensitivity distinction was more remarkable in the limb dermatomes where proprioceptive afferents decreased tactile resolution at the periphery of the field. The maximal innervation zones appeared to approximate the locations of the major cutaneous nerves. The experimental findings suggest the hypothesis that the afferent fibers comprising a canine dorsal root are intermixed in the spinal nerve and are distributed at random within the dermatome.