Medicines either increase, lessen, or alter the healthy functions; and, in reference to these several effects, are called stimulants, sedatives, and alteratives; the effects themselves being distinguished by the names of stimulation, sedation, and alteration. It has been maintained that, from the very nature of the vital functions, medicines can affect them in no other way than either by increasing or diminishing them, and that they differ from each other only in the degree in which they produce these effects respectively, or in the seat of their action. But this doctrine is purely hypothetical, and is opposed, apparently at least, by numerous facts. Nothing is more common than to witness peculiar effects from different medicines, without any observable increase or diminution of the vital functions; and this difference may often be observed in medicines acting on the same part. Besides, in relation to those which are essentially either stimulant or sedative, we observe characteristic peculiarities which cannot be ascribed either to the degree or direction of their action. Thus, in relation to medicines the effects of which are visible, we have intense redness with comparatively little tendency to vesication from mustard, a less degree of redness with large vesication from cantharides, a copious vesicular eruption from croton oil, a peculiar pustular eruption from tartar emetic, and the production of urticarious wheals from the nettle, all acting on the same portion of the surface, and all excitant in their operation. Now it is possible that, in these and similar cases, the result may be owing to the direction of the action of the irritants severally to some distinct constituent of the skin; but this has not been proved; and, in reference to a great number of the peculiar effects of medicines, such a direction to distinct constituents of the structure is altogether insusceptible of proof. In the present state of knowledge, therefore, it is best to admit, as the result of observation, that medicines do materially differ in the nature of their effects, independently of degree and position; and to leave to further investigation the determination of the precise nature of the causes which occasion such difference.

Another opinion denies the existence of directly sedative or alterative medicines, maintaining that all substances which act on the system are essentially stimulant, and that whatever sedative effects may be produced are purely secondary. According to one view, medicines operate on the vital excitability, producing primarily an elevation of action, which is followed by secondary depression in consequence of the exhaustion of the excitability; and whenever a direct depression takes place, it is in consequence of the diminution of the ordinary healthy excitants, as through the influence of cold, abstinence, etc. According to a second view, medicines operate as foreign bodies, offensive to the economy, in which, consequently, an excess of action is induced in order to rid it of the offending cause; in other words, all medicines operate by calling forth vital reaction, as inflammation is induced in a part by the presence of a foreign body, in order either to isolate it by a coating of coagulable lymph, or to throw it off through the agency of suppuration and ulceration. But in opposition to all such purely hypothetical views is the simple fact, that certain medicines, when brought into contact with certain parts or organs of the body, are immediately followed by a depression of function in those parts or organs, without any other discoverable intervening derangement; just as, under similar circumstances, certain other medicines are followed by immediate increase of function; and the obvious explanation is, that the susceptibilities of the parts are such that the presence of a body constituted in one mode occasions depression, that of another, differently constituted, excess of excitement.

But, admitting that some medicines are stimulant, others sedative, and others again alterative, we are not called on to believe that any one medicine is essentially one or the other under all circumstances. Much and very unnecessary discussion has taken place in relation to particular medicines, whether they were stimulant or sedative. It might all have been spared by the admission of the simple truth, that the same medicine may be stimulant or sedative according to the part upon which it acts, or the state of the system, or parts of the System, at the time of its action. Thus, tartar emetic is stimulant to the skin or mucous coat of the stomach, but sedative to the heart; digitalis, which depresses directly the circulation and nervous system, excites the kidneys; and opium, at the same time that it stimulates the heart and brain, diminishes secretion. These different primary effects of the same medicine are dependent on the different susceptibilities of the parts affected, rendering them liable to opposite impressions from the same cause; and, as these susceptibility are often different in disease and health, the same medicine may produce opposite effects in these two states. Thus, Cayenne pepper, which produces in the healthy fauces redness and burning pain, acts as a sedative in the sore-throat of scarlet fever. A mere difference in the mode in which a medicine is employed may cause it to be either stimulant or sedative. A concentrated solution of acetate of lead applied to the skin denuded of its epidermis, or to a mucous membrane, acta as an irritant; while the same solution, very much diluted, will operate as a sedative through the peculiar powers of the medicine. This principle is of great importance in therapeutics, as will be hereafter more particularly shown.

The consideration of the special phenomena, whether of stimulation, depression, or alteration, produced by particular medicines or classes of medicines in the several functions, belongs to the department of special therapeutics. It is obvious that they must be extremely diversified, from the difference in the degree and nature of the action of the medicine, from its direction to one or to several functions at the same time, and from the great diversity in the character of the functions affected.