STIMULATION.
176. Passing now from these general considerations to their special application, we may formulate the law of stimulation:
Law III. A faint or moderate stimulation increases the activity of the organ; but beyond a certain limit, increase of stimulation diminishes, and finally arrests, the activity. Duration of stimulation is equivalent to increase.
A muscle stimulated contracts; if the stimulation be repeated, the muscle becomes tetanized, and in this state has reached its limit; a fresh stimulation then relaxes the muscle. A very faint stimulation of the vagus quickens the pulsation of the heart, but a slight increase, or duration of the stimulation, slackens and arrests the heart.[193] Every one knows how a moderate feeling of surprise, pleasure, or pain quickens the heart and the respiration; and how a shock of surprise, joy, grief, or great physical pain depresses, and even arrests them. Excess of light is blinding; excess of sound deafening.
177. The nervous system is incessantly stimulated, and variably. Hence a great variation in the excitability of different parts. While the regular and moderate activity of one part is accompanied by a regular flow of blood to it, so that there is a tolerably constant rhythm of nutrition and discharge, any irregular or excessive activity exhausts it, until there has been a nutritive restoration. We can thus understand how one centre may be temporarily exhausted while a neighboring centre is vigorous. Cayrade decapitated a frog, and suspended light weights to each of its hind legs; when either leg was stimulated, the weight attached to it was raised. After each repetition the weight was raised less and less, until finally the weight ceased to be raised: the centre had been exhausted. But now when the other leg, which had been in repose, was stimulated, it energetically contracted, and raised its attached weight; showing that its centre was not exhausted by the action of the other.[194]
178. This seems in contradiction with the principle that the excitation of one centre is an excitation of all. It also seems in contradiction with the principle urged by Herzen, that irritation of one sciatic nerve diminishes the excitability of the opposite leg; and this again seems contradicted by the principle urged by Setschenow, that although moderate excitation of one sciatic nerve will diminish the excitability of the other, a powerful excitation will increase it.
179. All three principles are, I believe, exact expressions of experimental evidence; and their seeming contradictions may be reconciled on a wider survey of the laws of neural activity, interpreted according to the special conditions of each case. These laws may be conveniently classified as laws of Discharge, and Laws of Arrest; the second being only a particular aspect of the first.