Reference is not had, in this place, to the chemical changes which medicines undergo in the alimentary canal preparatory to absorption, nor even to those which may take place in the course of the circulation or elimination of the medicines, so far as relates exclusively to the medicines themselves. But, when these chemical changes of the medicine in the circulation result from reactions between them and the constituents of the blood or the tissues, it cannot but happen that physiological alterations must take place, which may possibly be more or less remedial; and the consideration of such changes belongs strictly to this section.

Some medicines have so strong an affinity for the constituents of the tissues, that, when in contact with them in a concentrated state, they overcome their vital resistance, and cause the decomposition and consequently the death of the part. In this way many caustic substance act. In a dilute state, this affinity is more than counterbalanced by the vital forces, and no chemical change ensues. As medicines never extend, in this state of concentration, far beyond their original surface of contact, it is obvious that similar decomposition of the tissues cannot result from their operation on the system, either by the circulation, or through the medium of the nerves.

It is highly probable, however, that many medicines produce chemical changes in the blood, which enter into their remedial action. The blood consists of certain vitalized, and certain unvitalized constituents, the latter of which are probably merely held in solution, and are Subject to the same chemical reactions as if dissolved in a fluid out of the body. Thus, an acid medicine, absorbed into the blood, may prove remedial by neutralizing a morbid excess of alkalinity; and, conversely, an alkaline medicine, by correcting any morbid acidity of that fluid. It is highly probable that many analogous changes may take place; and to some of these there will hereafter be occasion to allude in the account of particular medicines, or classes of medicines. It is even possible that the normal condition of the blood may be so far modified by such a chemical agency, exerted for example upon its albumen, its fibrin, its salts. etc., as to alter materially the influence of that fluid upon the tissues and functions, and thus to give rise to systemic actions or conditions, which, though in fact only the secondary effects of the medicine, are usually regarded as the results of its direct and characteristic operation. But these changes can be traced only conjecturally; and there are very few, if any, which can be truly said to have been experimentally demonstrated.

Another mode in which a medicine may act chemically, after absorption, is at the time of elimination by the secretory functions; when, coming in contact with the products of secretion, it, or one of its constituents, or some compound which may have been formed by chemical reactions with it in the course of the circulation, may materially modify these products. Thus, alkaline medicines, eliminated by the kidneys, render soluble the uric acid, or insoluble urates, which may have been deposited from the urine after its secretion.

It is not impossible that the peculiar effects of medicines on the tern, may sometimes result from chemical reaction between the medicines and the tissues with which they are brought into contact while in the blood, or into which they may have been diffused from the capillaries. Reactions between the constituents of the tissues may take place through the agency of the medicine, acting either as a ferment, or by what has been called disposing affinity, as when the presence of an alkali disposes to oxidation, so as to form an acid with which it may unite. Combinations may be formed between the medicine, or one of its constituents, and some one or more of the constituents of the tissues, at the moment of their physiological disintegration; and these compounds, retained in the organs, perhaps within the organic cells, perhaps in the interstices of the tissue, may modify the functions in a manner that may prove remedial. These results, I repeat, are possible; and the theory has been advocated that medicines generally act by affecting the normal oxidation of the tissues; some favouring oxidation, and thus producing one set of effects, others retarding or suspending it, and thus causing opposite results; but it has not been proved positively that medicines do really produce their peculiar effects in this way in any one instance; and it is altogether premature to explain the whole agency of medicines upon the grounds of their chemical influence. The only well-ascertained fact which can be adduced in support of the hypothesis is, that certain medicines, particularly the metallic, may be detected in the substance of the organs, often for a considerable time after they have been taken.