These are medicines which, without essentially elevating or depressing the vital actions, nevertheless produce changes in the organization or functions, which render them available for remedial purposes. The stimulant or sedative power is not incompatible with the alterative. The latter may operate at the same time with one of the two former, either conjointly upon the same, or separately on a different part or function. But, so far as the medicine is an alterative, its curative influence depends, not on the exaltation or depression, general or local, which it may produce, but on changes wholly independent of these effects, and which are usually unappreciable, or at least not obvious in health. in fact, the name is generally applied to medicines which, so far as regards their relations to the system as alteratives, produce no readily discernible change in the healthy state, and are only known to possess remedial powers by the result of their use in disease.

The nature of the change effected by the alteratives may be a subject of conjecture, speculation, or investigation; but has not been determined; for, if satisfactorily known, it would serve as a ground of distinct classification, and the medicine would be removed from the class of alteratives, into another founded on a definite basis. This class, therefore, may be considered as a temporary and convenient receptacle of those otherwise unclassifiable medicines, which experience has proved to be useful in disease, but the modus operandi of which is undetermined.

I have said that we may speculate in relation to the action of alterative medicines; and the conclusions to which we may be conducted by our reasoning on the subject may possibly be correct; but, in the present state of our knowledge, they cannot be demonstrated to be so; and the best that can be said of them is that they are highly probable.

There are various modes in which alteratives may be supposed to act. Thus, they may change the condition of the blood, and may do so either chemically or dynamically; that is, in the former case, by taking something from, or adding something to that fluid through the influence of affinity, or by causing new reactions among its ingredients through the mere influence of presence, as emulsin, added to a watery solution of amygdalin, causes a reaction resulting in the generation of hydrocyanic acid; or, in the latter case, by operating on the vital susceptibilities of the living constituents of the blood, and changing it through modifications in the actions of these constituents. The chemical result might be produced equally in the blood removed from the body, and destitute of life; the dynamic results could happen only in the fluid while still living. Thus, alkalies may be supposed to render the fibrin of the blood more soluble, and in this way diminish its coagulability; while the presence of mercury in the circulation may bring about the same result by poisoning the fibrin, as it were, and affecting it through its vital properties.

Another mode in which the alteratives may be supposed to act, and probably do act, is by modifying the state of the solid tissues; and here too they may either exert a chemical agency upon the structure, combining with it, or in some measure decomposing it, or may simply affect its functions through its vital susceptibilities, without undergoing any change themselves, or producing any chemical change in the tissue. A highly probable operation of some, if not most of them, is to stimulate the disintegrating and renovating processes constantly going on in nutrition, and thereby to remove diseased structure, the place of which is supplied with new structure, either healthy, or disposed to become so.

A third method, in which the alterative may be imagined to act, is by neutralizing, decomposing, or eliminating some noxious agent that may exist in the system, either the result of some pathological process, or introduced from without. Thus, it has been conjectured that mercury cures syphilis by destroying the peculiar contagious virus, the presence of which in the body causes and sustains that disease; colchicum has been supposed to cure gout by eliminating uric acid and urea through the kidneys; and there is good reason to believe that iodide of potassium acts favourably in lead-poisoning, by dislodging the lead from its seat in the tissues, and forming with it a compound soluble in the blood, and thus capable of elimination.

Still another mode of operation, to which fancy has ascribed the efficiency of alteratives in certain diseases, the cutaneous eruptions for example, is by a poisonous action on the sporules of microscopic fungi, or on the fungi themselves, which may be imagined, by circulating in the blood, to take root in various tissues, and produce disease by their growth and propagation.

Each one or all of the above methods of alterative action may possibly be true; but no one of them can be said to have been positively demonstrated; and, in the mean time, the medicines may continue to rank with the alteratives, until experimental investigation shall have satisfactorily settled their claim to another and better defined position.

The alterative property is by no means identical in the different individuals of the class. Each probably has a mode of action more or less peculiar to itself, adapting it specially to certain curative purposes. Thus, mercury has extraordinary curative powers over inflammation, arsenic over chronic cutaneous diseases, iodine over scrofula, and colchicum over gout.

It is obvious, from the foregoing remarks, that the applicability of the alteratives to the several diseases in which they have been found useful or curative, is a point to be determined only by experience. it is on this ground alone, and not that of a priori reasoning, based upon their known physiological action, that their special application in practice must rest. There is little, therefore, to be said of them in common, in relation either to their effects on the system, or therapeutic application. Each one of them must be considered separately in all its relations.

Most of the medicines here treated of as alteratives have also other properties, which might attach them to other classes of medicines; and, again, there are many medicines, which, though usually employed in reference to certain well-known physiological methods of operating, and therefore considered in other classes, have also alterative properties, which render them occasionally useful in diseases, in which the class with which they are associated are not indicated. Thus, mercury, which is the most efficient alterative, often does good by increasing the various secretions, and, in one of its combinations, is an excellent cathartic, and in another a tolerably certain emetic; on the other hand, tartar emetic, which is usually employed for its sedative or emetic property, or for its stimulant influence over the secretions, sometimes does good in disease without any discoverable modification of the functions, or, in other words, by an alterative operation. These accessory qualities are either considered incidentally to the main property upon which the classification of the medicine depends, or serve as the basis of a double or triple position in different classes, as their relative importance may seem to demand.