AbstractScanning along human acoustovestibular nerves from cross‐sections closely proximal to the brain to locations distinctly peripheral thereto, by means of small‐angle X‐ray diffraction, has disclosed transitional junctions at which the myelin structure typical of central nervous system (CNS) axons gives way to one characteristic of peripheral (PNS) fibers. The junctions correspond to regions along the nerves, previously recognized histologically, at which the satellite cells responsible for axon myelination change character, from the oligodendrocytes of the CNS to the Schwann cells of the PNS. Thus the structural discontinuity between the CNS and PNS myelins can be ascribed to differences in the biosynthetic processes of the respective satellite cells. Junctions of this kind are to be expected in all cranial and spinal roots near the locations where they leave th