VESICANTS, or BLISTERING AGENTS, substanc, s which, if kept in contact for some time with the surface of the body, excite such irritatiGn as to cause effusion of scrum from the true skin, leading to separation and elevation of the cuticle, and formation of a Nesicic or blister. Vesicants are employed: (1) For relieving or removing the diseased condition of some internal part, by producing a determination of blood from the interior to the surface over the seat of the affection; (2) As general stimulants to the system, often beneficial in advanced stages of low continued fever; (3) for the direct purpose of withdrawing serum from the vascular system, for which they are prescribed with advantage in cases of sudden effusion into the pericardium or the pleura; blisters for this purpose should be of large size, and should be kept in contact with the skin sufficiently long to produce their full effect (24 hours being necessary in the case of some persons) ; (4) For removing the cuticle, to per mit the direct application of various medical agents, especially mercury and morphine, to the absorbing surface of the true skin. In infancy and childhood, these agents must be used with extreme caution.
To produce vesication, Cantharidine, the active principle of Cantharides, or Spanish Flies, in one of its various forms, is generally em ployed, though other substances also are used. Cantharidine is a white crystalline substance, extracted from the powdered insects by rectified spirit, and whose composition is represented by the formula Cali°. It is a very active poison, and produces imtnediate inflammation of the skin wherever it comes in contact with it, is very volatile even -at ordinary temperatures, and is soluble not only in alcohol, but also in chloroform, ether, strong acetic acid and many oils. In preparing a blister, it is expedient to sketch the desired shape and size, and before applying it the skin should be well washed with warm water. If the patient's skin is not easily acted upon, the part should be sponged with vinegar; but if it is very susceptible a piece of tissue-paper should be placed between the skin and the plastrr. To insure close contact with the skin, the blister should be gently warmed, carefully applied, avoiding creases, and kept in its place by a bandage; and to produce its full action, it should be kept on for 10 to 12 hours. If, on removal after that time, full
vesication has not been produced, a hot poultice will often secure the desired effect The raised cuticle should be punctured, to allow the escape of the serum, except in the case of children and persons of very irritable skin, when the vesications should be left unopened, and a dressing of simple ointment or spermaceti ointment on soft rag applied and repeated in 24 hours; or the part may be at once covered with cotton-wool. The troublesome itching which often follows application of a blister is best relieved by a bread-and-water poultice, moistened with the dilute solution of acetate of lead. Collodium Vesicans is prepared by mixing equal parts of collodium and cantharidal ether. It possesses the advantage that its strength can be readily increased or diminished.
When a blistering agent with very rapid action is required, as in the state of collapse in cholera, recourse may be had to the application of boiling or nearly boiling water to a portion of the abdomen, the surrounding surface being protected by a wall of damp cloths. Cold water may be used as efficiently as boiling water: a piece of bibulous paper, such as common blotting paper, should be soalced in cold water, applied to the part to be vesicated, and covered with three or four folds of dry paper. A common smoothing-iron heated to 212° F. is then pressed three or four times over all; and on removing the paper, the part will be found vesicated. In less urgent cases an almost immediate blister may be produced by saturating. a piece of lint in a strong isnolution of ammonia, and applying it to the de with moderate pressure. By the time the ammonia has evaporated, the required result is usually obtained. When it is desired to keep up a discharge from a blistered surface, instead of healing it as is commonly required, or to. produce a perpetual blister, the raw sur face is dressed with irritants of various kinds, such as savine ointment At each fresh dressing (in summer twice a day), the part should be cleansed in warm water.