During the course of a general study of the structure of medullated nerve fibers, a Weigert stained longitudinal section of the sensory root of the trigeminal nerve from a wild rat (Mus norvegicus) was examined. It presented, about 1 mfn. from the brain stem, a sharply defined line of transition between the structure of the peripheral nerve trunk and the central nervous tissue, which change was revealed particularly by a difference in the intensity of staining. This change occurred along a regularly curved transverse line which was slightly convex peripheralward. A cursory examination of similarly prepared sections from the same and other animals showed that this condition was normal and not an artifact. This picture has been described by a number of observers, more from the standpoint of microscopic relations than from the detailed histologic standpoint. A brief review of the most important communications will be both valuable and interesting. R. Thomsen ('87) observed, in cross-sections of the human abducens and oculomotor nerves from a case of multiple alcoholic neuritis, that there were small, round, glistening placques or 'Herde' lying in the normal nerve tissue of the trunk and sharply delimited from it. They consisted of a horny substance staining readily in carmine. Oppenheim ('87) found similar placques in cross-sections of the human facial and hypoglossal nerves aqd designated them as the 'Herde' of Thomsen. The latter, having recognized that these 'Herde' were not specific 1 In partial fulfilment of the requirements for the degree of Master in Science, Major in Anatomy, University of Michigan. 165 'Aufhellung, ' which was bow-shaped with its convexity outwards. Obersteiner ('95) and E. Redlich ('97) defended and added details to their observations, urging especially the importance of the change from extramedullary to intramedullary