VESICANTS, or BLISTERING AGENTS, are substances which, if kept in contact for some time with the surface of the body, excite such irritation as to cause the effusion of serum from the true skin, leading to the separation and elevation of the cuticle, and the formation of a vesicle or blister. They are employed in the practice of medicine for the purpose of relieving or removing the diseased condition of some internal part, by pro ducing a determination of blood from the interior to the surface over the seat of the affection, They likewise are of great value from their action as general stimulants to the system, and as such arc often used with great benefit in the advanced stages of low continued fever. Moreover, they are not untrequently employed for the direct purpose of withdrawing serum from the vascular system, and with this view they are prescribed with advantage in cases of sudden effusion into the pericardium or the pleura. Blisters used with this object should be of large size, and should be kept in contact with the skin sufficiently long to produce their lull effect (24 hours being in some persons neces sary for that purpose). Lastly, vesicauts are occasionally applied to the surface of the body, for the purpose of removing the cuticle, so as to permit the direct application of various medical agents (especially mercury and morphia) to the absorbing surface of the true skin. It must be recollected that in infancy and childhood, owing to the ex treme readiness with which inflammation of the skin is then set up, 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 employed, although other substances, afterward to be noticed, are occassionally used. Cantharidine is' a white crystalline sub stance, which is extracted from the powdered insects by rectified spirit, and whose position is represented by the formula 0,011.04. It is every active poison, and produces immediate inflammation of the skin whenever it comes in contact with it, is very vola tile, even at ordinary temperatures, and is soluble not only in alcohol, but in chloroform, ether, strong acetic acid, and many oils. This substance is employed in the form of plaster (emplastrum cantharidis of the Pluirm. Brit.), blistering fluid (of which there are several excellent forms, such as acetzein canth., ether canth., and collodion calla., none of which are in the Pharna. Brit.), and blistering tissue fof which there are several forms, known as tela xesicatoria, charts sec., blistering cloili,* etc., none of which are officinal). Although the fluids and tissues are the cleaner and neater preparations, the old-fashioned cantharides plaster is far the most commonly employed in general practice, and is, by many of the authorities in the profession (among whom we may name the late prof. Syme of Edinburgh, and prof. Lister, now of London), considered as the most efficacious (its superiority being due to its slower and more prolonged action). In pre scribing a blister, it is expedient to sketch the size and shape desired. 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; while if it is very susceptible, and he is liable to strangury from the application of blisters, a piece of tissue-paper should be placed between the skin and the piaster. (In speaking of the plaster, which is a
solid mass, we assume that it is spread on some fitting material, as wash-leather, soft brown paper, etc., the popular idea of a plaster always including the material on which it is spread.) In order to insure close contact with the skin, the blister shhuld be gently warmed, carefully applied, so as to avoid creases, and kept in its place by a bandage. To produce their full action, blisters remain from ten to twelve hours, on their removal after that time full vesication has not been produced, a hot bread-and-water poultice will often produce the desired effect. The raised cuticle should be punctured, to allow of 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 twenty-four hours afterward; or the part may be at once covered with cotton-wool, which until it gives off a bad smell, can remain till the skin is healed. The troublesome itching which often follows the application of a is best relieved by the application of a bread-and water poultice, moistened with the dilute solution of acetate of lead, formerly known as Goulard's tegeto-mineral water. Dr. Neligan, in his highly practical work On Medicines, speaks so strongly of collodium vesicans as a bliStering agent, that although we have no personal experience of it, we shall, on his authority, briefly notice it. It is prepared, when required, by mixing together equal parts of collodium and cantharidal ether (obtained by digesting for three days onP part of coarsely powdered cantharides in two parts of sulphuric ether, and expressing). It possesses the advantage that its strength can be readily increased or diminished. "It is now much used for blistering," he observes "owing to its cleanliness, its certainty, and the facility with which it may be applied in the neighborhood of joints, or to other parts which are difficult to blister by the ordinary method. It is applied with a camel-hair pencil; two scruples are sufficient lister a surface as large as the palm of the hand. It is preferable to apply the quantity to be used twice, instead of at one time, on the place to be blistered.' 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 t to a portion of the abdomen, the surrounding surface being protected by a wall of damp cloths; or iu less urgent cases, as retrocedeut gout showing itself internally, an almost immediate blister may be produced by saturating a piece of lint of the size of the desired blister in the strong solution of ammonia, and applying it to the skin with moderate pressure. By the time that 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 most commonly required), or to produce a perpetual blider, we dress the raw surface with irritants of various kinds, such as savine ointment, papier d' Alba peyred, etc. At each fresh dressiug, which in summer should take place twice a day, the part should be cleansed with warm water.