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110 that the nervous change can pass from one nerve-fibre to another. We know also that when an electric current passes along one wire it may produce currents, so-called induction currents, in adjacent wires; but there is nothing analogous to this in nerves. We do not know of induced nerve-currents. Each nerve-fibre appears to conduct its own change or current.

The phenomenon is more like that of a rapid series of chemical changes passing quickly along a tract, as when a train of gunpowder slowly burns, or when a long thin band of cfun cotton, such as we have here, is seen to burn slowly from end to end. But the analogy is not complete. The train of gunpowder and the band of gun cotton disappear and leave nothing behind, but the nerve-fibre remains. It must be said that the evidence we possess of chemical changes in the nerve-fibre is very meagre, no doubt because of their comparative insignificance. Still, small as the change is, it is sufficient to set off the highly unstable material in a muscular fibre and to produce chemical changes attended by the liberation, as we have seen, of mechanical energy.