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Myelinated and unmyelinated fibresFigure 4. Sciatic nerve of a living mouse in situ (incident light, dark ground, x 1000). (By kindpermission of Professor Susan Standring). Note the presence of narrow Schmidt-Lantermanincisures and intra-axonal inclusions (Wilson and Hall, 1970).Myelinated fibres are easily recognised ( Ochs, 1982; Landon,1986). On lightmicroscopy, a single sciatic nerve fibre appears as a tube about 15-20 µm in diameter, the axon,containing a fluid axoplasm (Figure 4). This tube is surrounded by another tube, the myelinsheath, which, at intervals, is narrowed by nodes of Ranvier. It has faults, called Schmidt-Lanterman clefts and a network, the Golgi-Rezzonico apparatus; no one knows what the lattertwo structures do, but it has been suggested that they enable the fibres to elongate and shortenwithout breaking. The possibility that they are artefacts resulting from teasing can not bedismissed. High power magnification of living myelinated fibres reveals Brownian movement ofparticles, both in the axons and in the myelin sheath. Occasionally, spherical inclusions appearin the axoplasm.19