Tonics arc medicines which moderately and somewhat durably exalt the vital actions. They promote the appetite, invigorate digestion, render the pulse fuller, stronger, and sometimes more frequent, raise the temperature of the body, augment in some degree the various secretions, give increased firmness to the muscles, and probably operate also on the nervous centres, especially those of organic life, somewhat elevating, though not always observably, the various functions over which they preside. It is not pretended that all these effects are produced by every tonic medicine, or by any one of them at all times, and under all circumstances. It will, however, I think, be generally admitted that, as a class, they operate as above stated upon the system in health.

They differ from astringents in the universality of their action. The latter affect the single vital property of organic contractility; and whatever other effects they may produce result from their influence upon this. Tonics operate not only on the vital contractility, but upon all the other vital properties, and may be said to be universal excitants to the functions. But this very diffusiveness of action prevents a concentration of their influence on anyone function; and consequently their power of producing contraction of the tissues is much less obvious than that of the astringents.

A notion formerly prevailed that strength depended on a certain rigidity, tension, or lone, as it was called, of the living fibres, and especially of the muscular; and medicines calculated to increase the strength were supposed to do so by increasing this tension or tone of the fibres, and hence were denominated tonics. But views so mechanical as this are now no longer tenable. There may be a certain physical tension of the muscles, tendons, and ligaments, resulting from mere position; but this has nothing to do with vital force, and an increase of it will not increase the general strength. The arteries have an elasticity which, under the pressure of the heart's action, permits a tensive expansion of their coats; and a diminution of this property might lead to a defective condition of the circulation. We may even conceive that, in case of diminished elasticity, tonics might have some effect in restoring it by improving the nutrition of the tissue; but the remedy would not in this case act immediately upon the physical property, but only secondarily through the vital functions.

It is true that there is a certain vital cohesion of the living molecules, in every highly organized tissue, which is essential to the due performance of its office; and a moderate augmentation of this vital cohesion may give increased energy to the function; but this is very different from the physical property of tension. The muscles possess this kind of cohesion in common with the living tissues generally; but the strength of the system is not more dependent upon its due state in these than in other structures, and probably much less than in some others, as the nervous centres, and the mucous coat of the stomach. If, therefore, we admit the existence of tension or tone in this modified sense, and that tonics may act by increasing it, still, it does not follow that this class of medicines operates exclusively, or even mainly, on the muscles.

In the present state of our knowledge, it is best to throw out of view the origin of the terms tone and tonic, and to consider that, by the former, is expressed simply the vital power by which the several constituents of the body arc enabled, under the influence of the proper excitants, to perform their function duly; and, by the latter, the means calculated moderately to elevate the several functions, by causing an increase of this power, or an increased exertion of it.

In order to estimate properly the operation of this class of medicines, it is necessary to discriminate between strength and action. The former is obviously the capacity to act, the latter the exercise of that capacity. By increasing the latter, we do not necessarily increase the former. Tonics do not, therefore, essentially augment strength, and the name of roboranlia or corroborants, by which it has been proposed to designate them, is not appropriate. In a state of perfect health, they cannot be said, in any degree, to increase the vital force or strength. I conceive that the greatest strength of system is that which enables it to perform all its functions in the best manner, and to offer the firmest resistance to all disturbing agencies, of whatever kind, whether excitant, depressing, or perverting. It is in perfect health that this condition is presented. If any function, or any number of functions are exalted, either from a peculiar state of the power, or a peculiar application of excitant agency, beyond the healthy medium, the system generally, instead of being stronger, that is, better able to perform all its offices justly, and to resist noxious influences, is, in fact, upon the verge of disease, and may be brought into that state by causes, which, in its healthy condition, would not affect it injuriously. In health, therefore, tonics are not strengthening.

They may be, indeed, and not unfrequently are indirectly debilitating. They are ranked among the permanent stimulants; but this epithet is only relative. No stimulant is or can be permanent. The excitability of living parts, in other words, their susceptibility to excitant impressions, is limited. If called into excessive action, it is proportionally exhausted; and, in this state of exhaustion, the ordinary healthful excitants have less than their normal effect. Depression, therefore, necessarily follows stimulation. This is obvious in the use of powerful stimulants; but it is no less true of tonics; though, in the use of these, as the excitation is more moderate and more slowly induced, the subsequent depression is less in degree and longer delayed. By a constant repetition of the stimulant, we may sustain the excess of action longer; but the result is obtained at the expense of the excitability, which is sooner or later still more exhausted, and may at length be so much reduced that the stimulus ceases to be felt, and depression occurs even under its continued use. This depression can be counteracted only by increasing the amount of the stimulus; but the same penalty is inevitably exacted; and in the end excitability is worn out altogether, and function ceases. It may be said, however, that excitability is not strength, and, consequently, that the latter does not necessarily diminish with the former. This is to a certain extent true. But the strength of an organ, or its power to act, depends on its due nutrition, on the steady repair of its losses by the assimilation of new material, on the maintenance in fine of its normal state of organization. Now, if its excitability is permanently impaired, it cannot feel duly the influence of the materials essential to its repair, and they cannot, therefore, be suitably appropriated. Its nourishment fails, its structure is impaired, and consequently its ability to perform its function is diminished. This is debility. It follows that a constant use of tonics not only exhausts excitability, and secondarily depresses function, but positively debilitates the system.