Recent investigators suggest that dermatomes extend as consecutive bands from the dorsal median line and question the existence of dorsal axial lines. Our observations were made on serial sections of human embryos and fetuses prepared with neurofibrillar stains.
Cervical nerves 1, 6, 7 and 8 failed to have cutaneous branches in most cases, the remainder usually had cutaneous branches.
With a few exceptions in T 1, all thoracic dorsal rami had cutaneous branches. Usually T 1, 2 and 3 became cutaneous through medial branches, while T 9 through 12 did so through lateral branches. However T 4 through 8 constitute a transition zone where many of these nerves became cutaneous through both medial and lateral branches. Thoracic 4, 5 and 6 tended to have cutaneous distribution through medial branches, but T 7 and 8 through lateral branches.
All lumbar dorsal rami having cutaneous distribution did so through lateral branches, but independent branches became progressively less frequent below L 1. Lumbar 4 lacked direct cutaneous branches in most cases and succeeding nerves in all cases. These nerves form the dorsal sacral plexus.
The deficit in cutaneous distribution of lower lumbar rami was not as pronounced as in the lower cervical region. A deficit is significant in relation to dorsal axial lines.