Sheaths of the Motor Axons of the Crab Carcinus

In crab leg nerves, the largest axons, which are the motor axons usually isolated for physiological experiments, have a sheath structure which is different from that in medium sized and smaller axons of the same nerve or of any other described nerves. Axons with a diameter over 20 µ have (a) an outer sheath, about 5µ thick, of wellspaced layers of alternating glial cell cytoplasm and extracellular fibrous material, formed from fewer cells than there are layers, and (b) an inner sheath of elongated cells which creep along the axon longitudinally and interdigitate where they meet, as seen 2 or 3 times round the outside of the membranes of axons in cross-section. Therefore, possible channels between inner glial cells are elongated and few. On these structural grounds, together with physiological evidence, they seem unlikely to be preferred pathways of diffusion of ions in crab axons. Smaller axons have simple sheaths; some occur in groups within a fibrous sheath; the thinnest axons frequently occur in bundles and have no glial cell membrane in contact with them.