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.
In treating of the effects of medicines, the first point which offers itself for consideration is the method by which they can be ascertained. Is it possible to determine, before trying a medicine upon the human system, what will be its effects? Do its sensible or chemical properties, its botanical relations, or its action upon inferior animals, afford us any facilities in this respect?
Much importance has been attached by some to the sensible properties of colour, taste, and smell. In relation to the first, it is probably altogether useless. The times are long past, when a correspondence between the colour of a medicine, and that of the fluids or solids of the body, was considered as indicative of a therapeutic relation; when, for example, dragon's blood was thought useful in hemorrhage because it was red, and turmeric in jaundice because it was yellow. The smell and taste, however, are more significative; substances resembling each other in these respects, having in many instances a similarity of medicinal effect. Thus, aromatics are usually excitant, carminative, and anti-emetic; fetid substances, often antispasmodic; sweet substances, demulcent; those having an astringent taste, styptic; and bitters, tonic: but, in such general statements, allowance must be made for so many exceptions as to deprive them, in a great measure, of practical value.
Analogy in chemical constitution is also not unfrequently attended with similarity in medical virtues. The preparations of any one of the ordinary metals have a remarkable correspondence in their effects upon the system; and there are several metals which greatly resemble one another. The mineral acids, the vegetable acids, the inorganic alkalies, and the neutral salts of the alkalies, constitute groups, of which one individual may often be substituted for another without disadvantage. But in this respect, as well as in relation to sensible properties, there is so much uncertainty, that no practical conclusion in reference to the properties of any particular medicine should be relied on without careful trial.
The same may be said of botanical affinities; though more importance has perhaps been attached to these than to either of the preceding grounds of judgment, It might indeed be inferred, that the similarity in internal constitution, which gives to plants those resemblances in obvious structure which serve as the basis of their arrangement into natural families, would also give them a certain identity in other respects, and among the rest, in their operation upon the system; and observation has, to a considerable extent, confirmed the truth of the inference Plants belonging to the same genus yield very frequently not only similar, but identical medical products. Thus, the oaks yield tannic acid, the pines oil of turpentine, the cinchonas quinia and cinchonia, the different species of strychnos strychnia and brucia, the gentians gentianin, the poppies morphia, and the garlics a characteristic volatile oil. This resemblance extends also very frequently to much larger groups; and many of the natural orders of plants have great similarity in medical virtue. Examples of this we have in the Malvaceae, which are demulcent, the Gentianaceae tonic, the Convolvulaceae purgative, the Solanaceae narcotic, the Euphorbiaceae emeto-cathartic and acrid, the Pinacese stimulant, the Branxicaceae stimulant, pungent, and acrid. Yet in almost every family there are instances, and, in some, very striking instances, in which not only are the characteristic medical properties wanting, but others wholly different, and even in some measure opposite are possessed; while similar and even identical medical virtues belong to plants having no botanical affinity whatever. Thus, in the Convolvulaceae above mentioned, there are some species wholly destitute of purgative properties, as the esculent sweet potato; and among the Solanaceae is capsicum, which is simply stimulant, without being in the least narcotic. Among the Ranunculaceae is Hepatica, which is slightly astringent, tonic, and demulcent; Helleborus, powerfully purgative; Zanthorrhiza and Goptis, simply tonic; Aconitum, acrid, sedative, and narcotic; and Cimicifuga, chiefly nervine. Of medicines having analogous properties, yet derived from different families, we have examples in the volatile oil of Pimpinella anisum, belonging to the Umbelliferae, and that of Illicium anisatum, belonging to the Anonaceae; in the oil of turpentine proceeding from the Pinaceae, and from Pistacea Terebinlhus of the Anacardiaceae; in the aromatic products of the Umbelliferse, Myristicaceae, Myrlaceae, and Zingiberaceae; in the astringent roots and juices of plants belonging to the Fabaceae, Polygonaceae, Cin-chonaceae, Geraniaceae, and many other families; and in the simple bitter tonics obtained from the Gentianaceae, Simarubaceae, Ranumnluceae, and Menixpermaceae.
But in reference to each of the analogies above alluded to, the sensible, the chemical, and the botanical, though none should be relied on in estimating the virtues of a medicine, yet each may be frequently suggestive, and, in relation to any new subject of inquiry, may be valuable by giving a proper direction to experimental investigation.
The effects produced on the inferior animals are more to be relied on, and will generally be a safe guide to the employment of medicine in man; but even this rule is not without exceptions. It is well known, for example, that sheep, goats, and cows eat with impunity the leaves of Hyoscyamus niger, which are highly narcotic, and in large quantities poisonous to the human subject.
The only certain means, therefore, of judging of the effects of medicines, is to observe carefully their operation in man; and, even in this mode, multiplied observation under diversified circumstances, and a most cautious comparison of results, are necessary in order to arrive at the truth. From the want of these precautions, many errors in relation to the action of medicines have originated, and been handed down from writer to writer for many years; and, even at the present time, there are medicines which have been long in use, upon the precise virtues of which opinion is yet unsettled.