This opposite action of large and small doses seems to be the basis of truth on which the doctrine of homoeopathy has been founded. The irrational practice of giving infinitesimal doses has of course nothing to do with the principle of homoeopathy - similia similibus curantur: the only requisite is that mentioned by Hippocrates, when he recommended mandrake in mania; viz. that the dose be smaller than would be sufficient to produce in a healthy man symptoms similar to those of the disease. Now in the case of some drugs this may be exactly equivalent to giving a drug which produces symptoms opposite to those of the disease; and then we can readily see the possibility of the morbid changes being counteracted by the action of the drug, and benefit resulting from the treatment. For example, large doses of digitalis render the pulse extremely rapid, but moderate ones slow it.1 The moderate administration, when there is a rapid pulse, is sometimes beneficial: this might be called homoeopathic treatment, inasmuch as the dose administered is smaller than that which would make the pulse rapid in a healthy man; but it might also be called antipathic, inasmuch as the same dose administered to a healthy person would also slow the pulse.

Homoeopathy can therefore not be looked upon as a universal rule of practice, and the adoption of any such empirical rule must certainly do harm by leading those who believe in it to rest content in ignorance instead of seeking after a system of rational therapeutics.

Dose

The amount of a drug, which actually comes in contact with and affects the tissues, depends upon several conditions - (1) the quantity actually given; (2) its proportion to the body-weight; (3) the rapidity of its absorption by the blood from the place of introduction; (4) the condition of the circulation in various parts of the body, which determines the quantity of the drug carried to each; (5) the rate of its absorption by the tissues; (6) the rapidity of excretion.

The word dose, as employed in medicine, usually means the quantity given at one time, but sometimes this may be very different from what actually produces any effect. It is the amount of the drug existing in the blood at any given time, or rather the proportion of it that actually comes in contact with or is absorbed by the tissues, which really acts. We must therefore consider more in detail the circumstances which affect this proportion.

Size

As the action which a drug has on the body is not dependent on its absolute amount, but on the proportion it bears to the body on which it is to act, an amount which is a small dose for one person is a very large one for another.2 Thus if a grain of some active substance be injected at the same time into the veins of a full-grown man and into those of a boy of only half his weight, it will be distributed through twice as much blood in the man as in the boy, and each tissue will only receive half as much of it. The dose of a drug must therefore be regulated by the weight of the patient; and thus women, being lighter, require a smaller amount than men, and children less than adults. Though it would be more exact, it is not always convenient, to weigh patients; but in experiments on animals we usually weigh the animal carefully, and describe the dose in terms of the body-weight. For example, in describing the lethal dose of physostigmine we do not say that it is so many grains for an animal, but that it is 0.04 grain per pound weight of a rabbit. This relation, however, is not always an exact one, and other circumstances must be taken into account. Thus the species of the animal must be considered, for the same dose which would kill one kind of animal will not kill another. In animals of the same species the state of nutrition must be taken into account, for two animals of the same species, which would be nearly of the same size when equally nourished, may have very different weights if the one is fat and the other is lean. But the fat is a comparatively inert tissue, and if we give to each animal a dose regulated by its body-weight, the vital organs, brain, heart, and spinal cord of the fat animal will get a larger share in proportion than those of the lean one.

1 Vide Traube, Med. Centr. Ztg. xxx. p. 94, 1861, and Brunton On Digitalis, p. 21. 2 Buchheim, Arzneimittcllehre, 3rd edit. p. 54.

In testing the action of poisons on frogs, also, it must be remembered that a female frog with a quantity of spawn will be very heavy, but the spawn, like the fat, is not to be reckoned as tissue; so that a dose given in proportion to the actual weight would be much larger than the same proportion given to the frog after spawning.