1. In the alkaline medium, the structure of the nerve fibre began to change at first in Schmidt-Lantermann's incisures and at last structural changes spread to all the parts of the myelin sheath.
2. Alkali gave rise to change of incisures easily and enlarged its widening, accompanied with the increase in its depth.
3. The structural change was the most promi n ent in the incisures near the node and not so remarkable in the incisures at the middle of the internodal segment.
4. The change of the myelin sheath caused with alkali consisted of the swelling and the liquefication of myelin. In strong alkali, solid myelin became oily and started to flow.
5. The oily myelin flowed acros s the node, but did not flow out from the fibre, including incisures. From the cut end, myelin flowed away and dispersed in the medium.
6. The most plausible interpretation of the phenomenon may be an assumption of the existence of a sheath which packs the whole fibre, including the incisures and the nodes. It has been known that the nerve fibre has only two sheaths. And neurilemma can act as such a sheath. The observation of myelin flow across the node enabled us to assume that neurilemma does not adhere closely to the axon at the node.
7. Acid caused similar changes in the structure of incisure. A characteristic change induced with acid was the stiffening of the axon.
8. The relation between the electric leakage from the no r mal myelin sheath and Schmidt-Lantermann's incisure was assumed from our microscopic observations and electrical measurements.
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