[Numbering of the dorsal spinal nerve roots in man from the 12th thoracic nerve to the coccygeal nerve].

A bilateral study for locating the dorsal root filaments of specific nerves was accomplished from the twelfth thoracic to the coccygeal level and this using 26 fetal and 8 adult specimens. After laminectomy from the thoracic to coccygeal levels, incision of the dural sac permitted visualisation of the dorsal root filaments, the successive rootlets corresponding to each segmental level (T12 to Co1 ) were counted with the aid of a binocular microscope where they penetrated the spinal medulla at the posterior sulcus. Statistical evaluation shows that for each dorsal nerve the average number of root filaments is greater on the right compared with the left. Numbering the dorsal nerve root filaments in the cephalo -caudal direction allows to assert with 99% probability up to the L5 level and with 95% probability up to the S1 level that one is at a given spinal level of the respective segmental range; caudal-cephalic direction numbering permits to affirm with 99% probability up to the S1 level and 95% probability up to the L4 level that one is at a given spinal level of the respective segmental range.