7. Habit

This is a powerful agency. Its effect is almost uniformly to lessen the susceptibility to the influence of medicines, and thus to require an increase of their dose for the production of a certain amount of impression. In relation to medicines which are purely functional in their operation, this augmentation of the dose, under the influence of habit, may be carried on almost indefinitely. The quantities of alcohol and of opium to which the system may become accustomed, with present impunity, are enormous. It is not exactly the same with medicines of powerful chemical action, corrosive mineral substances for example; for, though the quantity may, through the agency of habit, be very considerably increased beyond what could be borne without its aid, yet, at a certain point, the chemical forces necessarily overcome the vital re-sistance of the tissues, and decomposition must take place. The mineral acids, therefore, the caustic alkalies, and the corrosive metallic salts, such as nitrate of silver, corrosive sublimate, and sulphate of copper, cannot be indefinitely increased without the danger of great organic mischief. But it must be remembered that, even with those acting functionally, the ultimate effects are in the highest degree injurious, either through chronic inflammation, induced at length by the constant irritation sustained, or by the failure of susceptibility to the ordinary vital stimuli, and the consequent loss of all power of action.

The rule in relation to the effect of habit in diminishing susceptibility is probably universal. There are, it is true, some apparent exceptions; as in the case of emetic substances, which often operate, on successive occasions, in successively diminishing doses; but the exception is only apparent; for the result in this case is ascribable not directly to the medicine, but to a diseased state of irritation produced by it. which itself is sufficient to induce vomiting. If an emetic substance is given at first in a small dose, and afterwards gradually increased, the stomach becomes accustomed to it, and very large quantities may be given without provoking vomiting.

The practical inferences from this effect of habit are, 1. that when it is desirable to maintain for a long time a given medicinal impression, the dose should be gradually, but at the same time cautiously increased. so as not too rapidly to wear out the susceptibility; 2. that when one medicine has been given so long as materially to impair its powers, another of analogous mode of action, but exerting its influence on a different tissue or part, should be substituted, until the susceptibility to the firs: returns; and 3. that, in omitting a medicine which has been long given. in gradually increasing quantities, it should be withdrawn gradually, or its place should be supplied for a time with another of similar but feebler powers, lest the system or part should suffer from the want of an influence to which it had become habituated, and which might be essentia] to the performance of its proper functions.

8. Modes of Living

The occupation and mode of life of an individual modify the action of medicines, in so far as they affect the condition of his system. But it would be quite impossible, in the present place, to follow out this influence into all its results. Little more can be done here than to call attention to its existence. One consideration, however, is worthy of notice, as it has a general bearing, and can be brought forward nowhere else so appropriately. Firm and vigorous health affords the strongest resistance to all disturbing influences, and consequently to the action of medicines, which must, therefore, be given more freely, to produce a certain effect, than in conditions of the system either above or below that standard. This is true in relation not only to medicines which stimulate, but to those also which depress or alter the vital functions. Thus, persons in full health will bear both the stimulant influence of alcohol, and the sedative operation of digitalis, better than the plethoric or the feeble. But it must be remembered that full health does not consist in that richness and abundance of blood, and that high activity of the functions, which lire sometimes mistaken for it. This is indeed a condition, if not itself morbid, at least closely bordering on disease, and capable of being excited into positive disease by slight causes. The system is most healthy when all its parts and all its functions are in due relation; when the quantity and quality of the blood are in exact accordance with the offices it has to perform in the economy; when the nervous system has no higher nor lower activity than is sufficient to maintain every function in its just vigour and subordination; and when no one organ or apparatus is excessively or deficiently developed. It is in this condition of system that medicines are best borne, and that, upon the occurrence of disease, vigorous treatment may be most safely adopted. Modes of life, therefore, which tend to produce an over-elevated condition of system on the one hand, or a debilitated condition on the other, render peculiar caution advisable as to the quantity of medicines employed, and the energy of the treatment in general.

9. Mental Action

The influence of the mind over the operation of medicines is often very considerable. As a general rule, they will act with greater certainty when their legitimate effects are known and expected. An emetic will be more likely to vomit, if the patient anticipate this effect from it. The co-operation of faith with the medicine will often favour its action. This is more especially true when the nervous system is prominently concerned. The full belief in the efficacy of quinia in intermittent diseases aids considerably in the prevention of the paroxysm. But mental causes sometimes interfere with the regular operation of a medicine. When this is given to procure sleep, especially in divided doses, at certain intervals, if the patient is made acquainted with the object, his anxiety for the result may tend to prevent it. I have noticed, when I have directed, with the knowledge of the patient, a certain dose of opium to be given at bedtime, and repeated, at intervals of an hour, until sleep is produced, or a certain number of doses have been exhibited, that the whole quantity prescribed is generally taken. The nervous disturbance occasioned by the expectation of the next dose, and the watchfulness for the appointed time, tend to keep the patient awake. Hence, in prescribing an anodyne in this way, particular care should be taken that the patient shall not know that the dose is to be repeated if requisite. It is to mental influence that empiricism is partly indebted for its seeming triumphs, especially in nervous diseases; and regular practitioners sometimes employ bread pills, with the happiest effect, in accomplishing certain results which the patient has been previously taught to expect.