The Divisional Organization of Afferent Fibres of the Trigeminal Nerve.

DURING evoked-potential studies of the projection of the three divisions. of the trigeminal nerve on the corresponding spinal tract and nucleus (to be reported elsewhere), the writer was disconcerted by certain observations, which appeared to be at variance with the classical conception of the organization of the trigeminal system. The early observation of Harrison and Corbin (1942) and of McKinley and Magoun (1942), that evoked responses of the third division of the trigeminal were usually not recorded much below the obex, was confirmed. In addition, it was found that the first and second divisions could be traced by these electrophysiologic techniques as far as the midpoint of the second cervical segment and that the evoked response from second-division' stimulation was almost invariably of greater amplitude and often obtained at a slightly more caudal level than that from activation of the first division. Since it is usually stated that the third (mandibular) division terminatesfirst, followed by the second (maxillary) division, and that the first (ophthalmic) division reaches the most caudal level in the spinal tract, it appeared difficult to reconcile the experimental results with the accepted notions. Most anatomical work on divisional representation in the spinal tract of V has been carried out with the use of the Marchi or the Weigert technique; these provide excellent demonstrations of myelinated fibres of large and medium size, but are less satisfactory when applied to finer myelinated fibres and are of no value in tracing unmyelinated fibres. Sjoqvist (1938) indicated that the mean diameter of trigeminal fibres at caudal levels decreases markedly; Darian-Smith and Mayday (1960) suggested that this