But it is evident that the coincidence or interference of nervous stimuli travelling along definite nerve-paths, will vary according to the rate at which they travel, so that when stimuli which ordinarily interfere with one another, are made to travel more slowly, one may be thrown a whole wave-length, instead of half a wave-length, behind the other : and thus we get coincidence and stimulation, instead of interference and inhibition. When stimuli, whose waves ordinarily coincide and strengthen each other's action, are made to travel more slowly, one may be thrown half a wave-length behind the other, and thus we shall have interference and inhibition instead of stimulation.

On the other hand, when the stimuli travel more quickly, the one which was half a wave-length behind the other, and interfered with it, may be thrown only a small fraction of a wave-length behind it. It will thus, to a great extent, coincide and cause stimulation, while the one which normally coincides with and helps another may, by travelling with increased rapidity, get half a wave-length in front of the other, and cause inhibition.

Opposite Conditions Produce Similar Effects

We see then that results, apparently exactly the same, may be produced by two opposite conditions, increased rapidity or greater slowness of transmission of stimuli.

The Same Conditions May Cause Opposite Effects

We see also that the same conditions may produce entirely opposite effects, by acting more or less intensely. Thus, the application of cold, or of any agent which will render the transmission of stimuli along nervous channels slower than usual, may throw one which ordinarily coincided with another a small fraction of a wave-length behind it, then half a wave-length, then three-quarters, next a whole wave-length, and then in addition to the whole wave-length it will throw it, as at first, a small fraction or a half wave-length behind, and so on.

We shall thus have the normal stimulation passing into partial, then into complete inhibition, which will gradually pass off as the crests of the waves come more nearly together, until they coincide, when we shall again have stimulation as at first. As the action proceeds, this second stimulation will again pass into inhibition. In the same way a gradual retardation of transmission will cause impulses, which normally interfere, gradually to coincide until inhibition gives place to complete stimulation, and this again passes into inhibition. By quickening the transmission and throwing one wave more or less in advance of another, various degrees of heat will likewise produce opposite effects.