Revulsion consists in the diversion of disease from one part of the system, by the production of inflammation or irritation in another part. The term derivation is applied to the same process, but may be extended also to cases in which the diversion is effected by a degree of excitement, which may still be within the limits of health. Counter-irritation, strictly defined, applies to the revulsive impression rather than to the revulsion itself. The system has only a certain capacity of nervous action, and a certain amount of blood. When either the former or the latter is strongly directed to a particular part of the body, there is a tendency to its diminution elsewhere. This is absolutely necessary of the blood: and it is true, to a great extent, in relation to nervous action. Such a direction is given by the application of irritants of any kind. Hence, in order to relieve inflammation, any of the forms of vascular irritation, or mere nervous disorder as indicated by pain or spasm, in any particular part of the body, we apply irritants, which under these circumstances are called revulsives, to some other part. This principle is of very extensive applicability to the cure of disease. It often comes into play as an auxiliary force, in cases in which the remedy is used for other purposes. Thus, while emetics are employed to relieve spasmodic affections of the air-passages, in consequence of the relaxation they produce, they are probably also useful by a revulsive influence towards the stomach. Cathartics act very powerfully, upon this principle, in the relief of inflammations and active congestions, though they may be employed chiefly in reference to their depleting power. There is no remedy whatever, allowing it to have the power of producing excitement in any part of the body, which may not act as a revulsive. But the remedies usually employed, in special reference to this principle of action, are external irritants, such as hot water, rubefacients, epispastics, and caustics. Derivation may often be advantageously effected by exercise, calling off undue excitement from internal organs to the exterior, or from one part of the body to another, and thus producing an equilibrium of the vital actions.

The application of the principle of revulsion requires discrimination.

When the local affection consists rather in a determination of blood to the part affected than in inflammation, as in cases of vertigo threatening apoplexy, and of frequently recurring epistaxis or haemoptysis, the indication is to divert the general current of excitement, and of the blood, towards the most distant parts of the body. Hence the use of strongly stimulating pediluvia, and of sinapisms to the legs, in cases of cerebral affection of the character just alluded to. But, when the disease is fixed in a part in the form of inflammation, it is necessary to bring the revulsive impression into nearer contiguity with the diseased part, though it may be proper also to employ remote revulsion as an adjuvant. Hence, in inflammation of the lining membrane, or of the contents of the great cavities, the revulsive remedy is most advantageously applied over the outer surface of the walls of those cavities, as over the abdomen in cases of peritonitis or enteritis, the chest in eases of pleurisy or pneumonia, and the scalp in those of encephalic inflammation.

The revulsive influence of remedies is peculiarly indicated in cases of metastasis, or of diseases which are especially liable to assume the metastatic form. In these cases, the agent should be applied to a portion of the body towards which there is a natural tendency of the morbid action to flow, and in which it would be safe; as, in gouty cases, to the feet, and in retrocedent eruptions, to the part of the surface from which the retrocession has taken place.

Another important principle is not to employ a highly irritative revulsive agent, in inflammatory cases, during the greatest violence of the disease. A strong impression upon the surface may sometimes prove useful in a commencing inflammation, before any febrile action has been excited, and in the declining stages, when the fever has in some measure yielded to depletion, or subsided spontaneously. But, during the existence of high constitutional excitement, the revulsive agent is not sufficient to unseat the inflammation, and, if itself very irritant, as in the instances of the more powerful rubefacients and of blisters, may add to the existing excitement by the sympathy of the system with the superficial inflammation it produces. But, when the revulsive impression is conjoined with copious depletion, as in the case of the saline hydragogue cathartics, which produce a revulsion towards the whole lining membrane of the bowels, while they evacuate the contents of the bloodvessels, it may be resorted to in the greatest height of the inflammation. The copious secretion prevents the production of an intestinal irritation sufficient to bring the constitution into sympathy.

When the local affection to be remedied is a mere nervous irritation. such as spasm or neuralgic pain, it is generally best to produce a quick, powerful, and transient revulsion; when inflammatory, especially when the inflammation is severe, to sustain a more moderate impression for a longer time. Hence, the more active rubefacients, such as mustard and am-monia, are applicable in the former case, and epispastics in the latter.