3. General depression or debility may result from the torpidity of a particular function or organ, upon which, in turn, the general deficiency may react, so as to sustain and even increase its inertness. Thus, the stomach, or the function of digestion, may have been depressed by some cause acting upon it alone. The food, of course, is not properly digested, the quality of the blood is impaired, the general function of nutrition suffers, and consequently the stomach with other organs. The disease thus runs on in a vicious circle, at each round increasing the local condition in which the whole originated, and deepening the general debility. By moderately stimulating the digestion, we restore the due qualities to the blood and the due energy to nutrition, the stomach recovers its powers when properly nourished, and the whole evil is corrected. A like train of consequences, and a similar mode of repair, may occur in reference to any one of the great organs or functions. Tonics often operate in this method in the cure of disease.

It is seen, therefore, that numerous cases of depression and debility occur in which tonics may be useful. But they are not applicable to all affections of this kind indiscriminately.

1. That tonics may be serviceable in debility, there must be a certain amount of excitability remaining. Otherwise they may be useless, or even worse than useless. Depression and debility, resulting from exhausted excitability, cannot be repaired by tonics. In the debility of drunkards, for example, which is the result of over-stimulation, tonics, if felt at all, could produce only a slight excitement, to be followed by still greater depression: and their habitual use would only hasten the fatal issue. The only hope in these cases is in the cessation of the cause. The habit of stimulation must be abandoned, or there can be no remedy. With future abstinence, if the excitability of the organs has not been fatally impaired, and no destructive disorganization has taken place, a gradual amendment and ultimate restoration may be hoped for, under the recuperative powers of the system. The only principle upon which tonics, under such circumstances, could be used with propriety, rests on the occasional necessity of not too hastily withdrawing all support from the system, lest fatal prostration should ensue. They may sometimes be resorted to, as a feebler stimulus than the accustomed one, in order that the transition may not be too abrupt,

2. Tonics cannot be relied on when the debility results from a constantly operating and irremovable cause. They would, in such cases, generally prove injurious by their secondarily debilitating influence, and would probably lead to a more rapid exhaustion of the patient. The debility, for example, resulting from cancer cannot be repaired by tonics. They may occasionally be useful in counteracting accidental debility of some one organ, or association of organs, which may be interfering with the proper exercise of the others; but the general rule holds true.

3. Depression, amounting even to apparent prostration of the system, sometimes depends upon an active and overwhelming congestion, or extensive inflammation of some important organ or tissue, as the brain, heart, lungs, stomach, peritoneum, etc., which either concentrates so much of the blood and nervous force in one part that there is insufficient elsewhere to support the systemic actions generally, or immediately cramps the organ affected so as to arrest its function, and thus prostrate all dependent functions. In either of these cases, tonic medicines could be of no service; the indication being by depletory measures to unload the congested organ.

4. In cases of great or sudden and transient depression, tonics are of little or no service; their action being too slow and moderate to meet the indication presented. When stimulation is required, under such circumstances, it is necessary to have recourse to the diffusible stimulants for internal use, and the rubefacients externally.

The general rule, then, in relation to the use of tonics, is that they are indicated in all cases of depression and debility, in which the excitability has not been exhausted by previous stimulation, which do not proceed from a permanent and irremovable cause, and in which the depression is neither the result of active congestion or irritation, nor so sudden and transient as to call for stimulation more prompt and fugitive than that which characterizes this class of medicines.

Of the particular diseases in which tonics may be used all that can be advantageously said will come better under the heading of the several individual medicines; as the peculiar character of each medicine very much influences its application. It remains here only to treat of their precise mode of action, so far as that is known.

Mode of Operation. Most of the tonics probably act directly on the mucous coat of the stomach and bowels, thus stimulating immediately the digestive function; and there is reason to believe that some act chiefly, if not exclusively, in this way. It was formerly thought, and some still think, that the impression made on this surface is propagated, to a greater or less extent, over the system by means of sympathy, or the intervention of the nervous centres; and it is not possible to prove that the tonic impression is never diffused in this way. But. when it is considered how slowly the tonics act, and that many of them, particularly those of mineral origin, have been detected by chemical reagents in the blood or secretions, it seems most reasonable to suppose that those, the direct influence of which extends beyond the digestive organs, operate through the route of the circulation. The different modes of action of the different tonics may be included under the following heads. 1. I have stated that some appear to act chiefly, if not exclusively, so far as their immediate tonic influence is concerned, on the digestive organs. By promoting the appetite and invigorating digestion, they cause more food to be taken, and that which is taken to be more thoroughly prepared for absorption and assimilation. They thus enrich the blood, rendering it at once more stimulating to the functions, and nutritive to the tissues, and produce indirectly the general tonic effects upon the system at large. Such are the mineral acids, and to a certain extent the simple or pure bitters, which, though they may possibly operate directly on the system through absorption, display their effects much more obviously upon the stomach and bowels.