AbstractEarlier small‐angle X‐ray diffraction studies have indicated that central and peripheral nerve myelins may be significantly different structurally, although relatively few examples for each system and for individual species have been examined. In order tounderstand better the intra‐ and inter‐system relationships, this study has developed more extensive information for a single species: six cases centrally and ten peripherally, featuring cranial nerves and a few others of the human nervous system. Peripheral myelin spacings (membrane pair thicknesses) are relatively similar, 184.4 ± 1.40 Å, and the ratios of diffraction peak height intensities of the second to fourth orders are also closelybunched: 1.85 ± 0.216. Central myelin spacings and intensity ratios are distinctly different and more variable: spacings 160.3‐165.8 A and intensity ratios 2.81‐4.46. It appears that within a given species or between closely related (e.g., mammalian) species peripheral myelins possess relatively invariant structures, though significant spacing declines are encountered for both systems as phylogenetic relationships become more distant. The observed greater variability of CNS structures within a singlespecies may correspond to known compositional differences between CNS regionsor result from observational difficulties. In any case there is a marked discontinuity between the myelin structures of CNS