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Rh but something sent on from point to point. It travels slowly compared with the velocity of light or of electricity. In the nerves of the frog the velocity is about eighty-seven feet per second, and in higher animals of constant temperature, such as in man, it only reaches a speed not exceeding three hundred feet per second.

The real nature of the change in a nerve-fibre during the transmission of the "current" is unknown. A nerve is both a receiver and a conductor of impressions. It can be stimulated at any part of its course, and from the stimulated point something is propagated along the nerve. Many explanations have been offered, but none is satisfactory. Naturally one thinks of the passage of electricity along a conductor, but, as I have said, the current is incomparably slower than the passage of electricity even along a nerve. The appearance of a nerve-fibre with its axis-cylinder is not unlike a wire insulated by some substance like silk or gutta-percha. Wires which direct the electrical change are insulated for the purpose of preventing the electricity from passing from one wire to another. We have no evidence