The unbruised seeds of Mustard were commended by Dr. Mead,[[284]] in Ascites, and by Bergius, in Intermittents; Dr. Cullen, however, has very properly observed that the seeds given in the above manner are never broken down or dissolved in the stomach, but pass away entire by stool, and cannot therefore occasion any beneficial result.
It is unnecessary to multiply examples in proof of the numerous errors into which a physician must unavoidably fall, who presumes to compose prescriptions without a knowledge of the chemical habitudes of the different substances which he combines. The file of every apothecary would furnish a volume of instances, where the ingredients of the prescription are fighting together in the dark, or at least are so adverse to each other, as to constitute a most incongruous and chaotic mass.
“Obstabat aliis aliud: quia corpore in uno
Frigida pugnabant calidis, humentia siccis,
Mollia cum duris, sine pondere, habentia pondus.”
Ovid. Metamorph. lib. 1. 19.
The Doses of Medicinal Substances are specific with respect to each, and can therefore be only learnt from experience; the young and eager practitioner, however, is too often betrayed into the error of supposing that the powers of a remedy always increase in an equal ratio with its dose, whereas The dose alone very often determines its specific action. “Medicines,” says Linnæus, “differ from poisons, not in their nature, but in their dose,” which is but a paraphrase of the well known aphorism of Pliny, “Ubi virus, ibi virtus.”[[285]]—So that food, remedies, and poisons, may be said to branch into each other by indefinable gradations;—Five grains of Camphor act as a mild sedative and slight diaphoretic, but twenty grains induce nausea, and act as a stimulant; so again, Opium, in too large doses, instead of promoting, prevents sleep, and rather stimulates the bowels than acts as a narcotic. Two ounces of any neutral salt are apt to be emetic, one ounce even of Alum to be cathartic, and two drachms to be refrigerant; in like manner the preparations of Antimony either vomit, purge, or sweat, according to the quantity exhibited.
Would it not appear that powerful doses rather produce a local than a general effect? Experience seems to prove in this respect, that the effect of an internal application is similar to that of an external impression; if violent, it affects the part only to which it is applied, as pinching does that of the skin, whereas titillation, which may be said to differ only from the former in degree, acts upon the whole system, and occasions itching and laughter, and if long continued, weakness, sickness, vomiting, and convulsions; in like manner Digitalis, if given in large doses, acts immediately upon the stomach or bowels, becoming emetic and cathartic, but in smaller proportions it produces a General effect, increasing all the excretions, especially that of urine; so, again, large doses of the Mercurial Pill act upon the bowels, and are eliminated from the body, whereas the same remedy in small doses affects the system generally, and excites a universal influence. I am well satisfied that the regulation of the dose of a medicine is even more important than it is usually supposed to be. Substances perfectly inert and useless in one dose may prove in another active and valuable. Hence may be explained the great efficacy of many mineral waters, whilst the ingredients which impart activity to them are found comparatively inert, when they become the elements of an artificial combination; and hence probably the failure of many alterative medicines, when no other rational cause can be assigned for it. We need not seek far for an example of the very different and opposite effects which the same substance can produce in different doses; the operation of Common Salt is familiar to us all; Sir John Pringle has shewn that in quantities such as we usually take with our food, its action is highly septic, softening and resolving all meat to which it is applied, whereas in large quantities it usually preserves such substances from putrefaction, and therefore, when so taken, instead of promoting, destroys digestion.
It is moreover probable that medicinal, like nutritive substances, are more readily absorbed into the circulating system when presented in small quantities, than when applied in more considerable proportions. It is upon this principle that a large quantity of food, taken seldom, does not fatten so much as smaller quantities at shorter intervals, as is exemplified in the universal good condition of cooks and their attendants. It is not pressing the principle of analogy too far to suppose that the action of alteratives, which require to be absorbed, may be more effectually answered by similar management; that is, by exhibiting small doses at short intervals.
The operation of medicines is influenced by certain general circumstances, which should be also kept in mind when we apportion their dose; e.g. Age—Sex—Temperament—Strength of the Patient—Habit—Diet—Profession—Climate—Duration of the Disease—State of the Stomach—Idiosyncrasy—and The variable Activity of the Medicinal Substance.