This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
The special application of this class of medicines is to the relief of nervous disorder. They are used in all affections of this kind, whether the result of over-excitement, or of depression of the nervous centres, provided only they are purely functional; that is, unconnected with active congestion or inflammation, or any other organic disease in those centres. This may at first sight seem singular; that the same remedy should prove useful in morbid excess and morbid deficiency of action; but the apparent anomaly is not insusceptible of explanation.
The characteristic effect of these medicines is to stimulate the nervous centres. It will, therefore, be readily conceded that they may prove serviceable in disease, consisting in a depressed state of the nervous functions. But how can they, by their stimulating power, relieve a disease, consisting essentially in an already morbidly excited condition of the parts upon which they act?
To answer this question, we must admit, as a starting point, that there is only a limited amount of nervous excitability in the system; in other word; that, taking the whole nervous system together, it is insusceptible of unlimited exaltation, and that there is a point beyond which its actions cannot be elevated. Again, it must be admitted that the nervous energy is transferable, like the blood, from one part to another: that an over-excitement in one or more parts will call it off from others; and that in health there is a general tendency to an equilibrium of distribution. By supposing the existence of a nervous fluid, this reasoning might, perhaps, be rendered somewhat more intelligible; but 1 avoid this advantage, as the existence of such a fluid has not been proved; and the argument is equally cogent without it. Admitting the above propositions, which I believe are nothing more than statements of facts susceptible of satisfactory demonstration, we have only further to recollect, that the nervous stimulants are characterized by the universality and equability of their action on the nervous centres. Suppose now that one of the cerebral centres is irritated into diseased action, which exhibits itself in spasms of the muscle directly connected with and dependent on that centre. A nervous stimulant is administered. It of course excites all the centres, operating on the one diseased in the same degree as on the others. Each becomes the seat of an attractive effort calling to itself as much of the nervous power as may correspond with the degree of excitation applied. All, therefore, draw with a united force upon the surplus in that one centre, in which there is supposed to be a morbid accumulation. To this united force it can oppose only its own attractive force, under the irritation to which it is exposed. If, therefore, the combined excitation applied to the nervous centres generally, is not less than that existing in the one diseased, under the morbid irritation, united to the excitation of the remedy, which it shares equally with the others, it must part with its surplus, and be reduced to the general level, or near it. If the causes of irritation shall have ceased, and the disordered centre be continuing to act morbidly simply from having begun to do so, the equilibrium can be entirely restored, and the disease cured. If not, the equilibrium is but partial or temporary; and the disease, though relieved, will be liable to return. The nervous stimulants, there-fore, though they may afford much relief, even during the continuance of the cause, cannot be expected to effect a cure until this shall have ceased to act.
In former times, these stimulants were said to prove useful by equalizing excitement. This term conveys succinctly the idea which I have endeavoured to demonstrate in the above paragraph. Our predecessors could not but notice the effect, though I am not aware that they have attempted a precise explanation. Their notions of the influence of the nervous centres were less definite, probably, than those now prevailing.
Functional nervous disease may be, in the first place, idiopathic, or self-existent, and alone; or, secondly, it may be idiopathic, and associated with other diseases; or, thirdly, it may depend upon other diseases; and each of these conditions has a bearing upon the therapeutic application of this class of medicines.
1. It appears to me beyond dispute that the nervous centres may become originally the special seat of functional disease, as well as any other part of the body, the cause operating on them directly through the same avenues by which they receive impressions in health; and the disease may exist without any complication whatever, other than such as may be induced in the functions under the control of the centre affected. It is to disorders of this kind that the nervous stimulants are peculiarly applicable. Such are, in many instances, the morbid phenomena denominated hysterical. There is here no other disease than that directly produced by causes external or internal, operating strongly upon the healthy centres, or moderately upon centres abnormally excitable. Even during the continuance of the cause, the nervous stimulants will often, by sustaining an equable tension of the nervous force, keep the disorder at bay; but, for a permanent cure, measures must be taken to obviate the cause, when the nervous centres are not in fault, or to give these a healthful power of resistance when unduly excitable. When the cause has been removed, and the disorder continues, as it often does through a sort of law of continuity in the actions of the system, the nervous stimulants will often remove it like a charm. Thus a female has been thrown into violent hysterical disorder by some exterior influence, slight or severe, as the case may be, which, however, no longer acts; but the disorder continues with little abatement for hours, perhaps for days. In such eases, one of the nervous stimulants, a few doses of assafetida for example, perhaps even a single dose, may put a speedy end to the phenomena.