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Each nerve fibre consists of a number of nerve fibrils collected into a central bundle the axis cylinder which is surrounded by an envelope, the neurolemma or sheath of Schwann. Between the neurolemma and the axis cylinder is the medullated sheath, composed of a fatty substance known as myelin.
The mere interruption of the continuity of a nerve results in degeneration of its fibres, the myelin being broken up into droplets and absorbed, while the axis cylinders swell up, disintegrate, and finally disappear. Both the conducting and the insulating elements are thus lost.
The Wallerian doctrine is that the axis cylinders in the central end grow downwards, and enter the nerve sheaths of the distal portion, and continue growing until they reach the peripheral terminations in muscle and skin, and in course of time acquire a myelin sheath; the cells of the neurolemma multiply and form long chains in both ends of the nerve, and are believed to provide for the nourishment and support of the actively lengthening axis cylinders.
This medullated sheath is interrupted at the nodes of Ranvier, and in each internode is a nucleus lying between the myelin and the neurolemma. The axis cylinder is the essential conducting structure of the nerve, while the neurolemma and the myelin act as insulating agents.
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