The sheath components of the giant nerve fibres of the squid

Abstract

Recent analysis of the ultrastructure of the sheaths of vertebrate nerve fibres has revealed that from the largest, heavily myelinated to the smallest, so-called unmyelinated fibres the molecular architecture is essentially similar, the differences in optical and chemical properties being referable to the relative proportion of oriented lipoid and protein molecules (Schmitt and Bear 1937). Moreover, the birefringence varies in a continuous manner from the largest to the smallest fibres. This essential continuity of ultrastructure of the sheath reveals the distinction between myelinated and unmyelinated fibres to be entirely arbitrary. The analysis has been extended to the sheaths of crustacean nerves, and it has been demonstrated that the metatropic reaction of Göthlin (1913) is not, as he supposed, an artifact due to dehydration tensions, but is evidence that the ultrastructure of these sheaths likewise conforms with that of vertebrate fibres, the amount of oriented lipoid being so small that special methods are required for its detection (Bear and Schmitt 1937; Chinn and Schmitt 1937).