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.
It is at present no longer doubted that the medicines belonging to this subdivision enter the circulation, and thus come into contact with all the tissues. They operate primarily on the alimentary mucous membrane, in a manner closely analogous to that of the mineral acids: that is, they moderately stimulate or excite the function of the membrane, or, in other words, act as tonics. But they are all, moreover, somewhat astringent, causing a contraction of the tissues, like alum or acetate of lead, though in a less degree; for a very energetic contraction would interfere with the functions of the membrane, and thus prevent the proper tonic effect It is highly probable that they operate, in a manner precisely analogous, upon the various tissues of the body which they reach through the blood. Producing a slight increase of the vital cohesion of the molecules of the tissues, they give them a greater power, while, by a gentle excitation, they call this power into a somewhat higher exercise. But, while thus generally tonic and astringent, they are disposed to act more especially upon the nervous centres; or, to speak more precisely, the organized nervous substance of these centres seems to be peculiarly susceptible to be impressed by them, and to take on, under their influence, a condition of greater firmness or compactness, which enables them, if previously weakened, to perform their proper function more efficiently. Here we have the secret of one of the most important therapeutic effects of this set of tonics; that, namely, of controlling the irritability of the nervous centres, and thus obviating various nervous disorders, and especially muscular spasm. From their influence in this way, they have not unfre-quently been considered as antispasmodic, and been so denominated. Spasm, as well as most other irregular nervous phenomena, depends upon a disturbance in the nervous centres, produced generally from some source extraneous to themselves. The more movable or excitable these centres are, the more liable will they be to give rise to these irregular phenomena under irritating influence. Now the tonics here referred to, by increasing the vital cohesion of their molecules, and rendering their structure firmer, enable them better to resist disturbing causes; at the same time supporting due action in them by a gentle stimulation. The disordered phenomena cease; and, by a sufficient persistence in the use of the remedy, the centres may acquire a permanent capacity of resistance, which may lead to a permanent cure. Hence, it is in the various nervous diseases, such as hysteria, chorea, and epilepsy, that these tonics have acquired the highest reputation, and are most extensively used.
Some suppose that they act by entering into chemical combination with the constituents of the tissue, and thus forming a part of the organization itself; and that their effects are to be explained by the new qualities which the tissues acquire through this change of structure. They support this opinion by the fact, that the metallic ingredient of these medicines is found in the very substance of the organs, from which it can be separated after death, supposing the patient to have died while under their influence. But this is no proof whatever, and scarcely even in any degree confirmatory of the notion of a chemical union. The metal may be in the capillaries of the organ; it may be deposited in the very substance of the tissue; it may even be there combined with some organic principle derived from the blood or from the tissue; but even admitting all this, it does not follow that it is chemically combined with the constituents of the tissue in their organized state; and it is not easy to conceive how a foreign body, thus thrust into the constitution of the nucleus or cell, which performs the office of the tissue, would enable it to perform that office more effectively. We may conceive that a metallic substance may possibly enter into such a union; but it would be to impair, not to improve the function; it would be to produce the effects of a poison, and not those of a remedy. Indeed, it is not improbable that some metallic poisons operate in this way; but more positive proof must be adduced, before we can admit that medicines produce their curative effect through such a combination, and especially medicines whose immediate operation is to improve and to invigorate.