- Emplastrum Cantharidis. Br. - Emplastrum Epispasticum. - Blistering Plaster.

Preparation

Though commonly designated as a plaster, and so designated in the British Pharmacopoeia, this preparation is not entitled to the name, which is now applied to substances requiring to be heated when spread, and adhesive at the temperature of the body, neither of which characters belongs to that here considered. it is in fact a cerate, as it is called in the U. S. Pharmacopoeia. in its preparation, the finely powdered flies are thoroughly incorporated with a melted mixture of wax, resin, and lard. Suet or olive oil is sometimes substituted for lard. For the proportions, and necessary manipulations, the reader is referred to the U. S. Dispensatory.

The cerate thus made is of a consistence which allows it to be readily spread by a knife or spatula, at common temperatures.

When used, it is generally spread on leather; but a piece of thick linen or canvas, or of stout paper, may be substituted if more convenient.

When the officinal cerate is not to be had. its place may be supplied by an extemporaneous preparation, made by thoroughly incorporating two parts of the powdered flies with four parts of resin cerate, or simple cerate, previously softened by heat; and, to increase the efficacy of the preparation, finely powdered cantharides may be sprinkled over its surface when spread, and made to adhere by gentle pressure with a roller or otherwise.

Application. As" the officinal cerate is not adhesive, or but very slightly so, it is necessary to employ some method of retaining it in its place. When it is applied to the extremities, or to the head, this may be done by a neatly fitted muslin roller; but, when on the trunk, neck, or face, some other method must be employed; and none is more convenient than to fix it by means of narrow strips of adhesive plaster, one-half of which is attached to the back of the leather, the other half to the skin. Sometimes the cerate is spread upon leather previously covered with adhesive plaster, and a margin of the latter left uncovered, so as to come in contact with the skin.

The shape of the prepared leather or cloth must be accommodated to the surface to which it is to be applied. Thus, for the forearms and legs, it should be rectangular, and nearly twice as long as broad; for the chest, back, or abdomen, nearly square, but usually somewhat longer than broad; for the back of the neck, triangular, with the upper angle truncated, so that it may be narrow at top, and spread out as the neck widens; for the space behind the ears, in the shape of a new moon, with the lower limb circularly expanded; for the whole scalp, oval; for the female breast, circular or semicircular, with a round opening in the centre; and for the epigastrium in a woman, triangular, with one of the angles extending upward over the sternum, and the base below. When a pair of blisters are made for corresponding parts on the two sides of the body, if the shape is not perfectly symmetrical, the caution must be observed to spread the two upon opposite surfaces, as in blisters behind the ears.

The size of the blister is of considerable importance. As a general rule, it should be large; as the pain and inconvenience are little increased, while the remedial impression is proportioned to the extent of the blistered surface. Care, however, should be taken not to make it so large as to endanger an oppressive effect on the system, or too extensive a disturbance of the cutaneous functions. For the forearms and legs, a pair may be used, each, for an adult man, about six inches by three or four; for the chest or abdomen, a single blister from eight to twelve inches in length, and six to ten in breadth; for the head, it should usually be made large enough to cover the whole scalp. it is a great therapeutic error to apply small blisters in serious diseases. The patient is made to suffer almost as much as from large ones, and receives little or no compensating benefit. For children, of course, the size must be pro-portionably diminished.

Before applying a blister to a hairy part, the hairs should be shaved off, and, if delay be admissible, ten or twelve hours should be allowed to pass before the application is made, in order that the abrasions may heal. But no sacrifice of important time should, be made for this object.

When the surface of application is convex, as the head, the shoulder, the neck, the mamma, etc., slits should be made deep into the blister from the circumference, so that it may be fitted accurately to the part.

The surface of the skin should be moistened with water or vinegar before the application, to facilitate the solution of the active principle, and thus cause it to act more quickly. When the epidermis is perfectly dry, it is sometimes difficult, without this precaution, to produce a blister.

In selecting the place of application, the practitioner will be guided by the effects to be produced. For general impression on the system, and for revulsion in cases of simple sanguineous or morbid nervous determination, without fixed local disease, the inside of the forearms or of the legs should be preferred, and the blister should be applied lengthwise along the limb; care being taken, when the application is made to the forearm, to protect a space of two or three inches at the wrist, by wrapping closely around it a linen roller, so as to prevent the blistering plaster from slipping down, and thus interfering with the pulse. When the object is to act revulsively, or through depletion, upon a local inflammation, the blister should be applied to the surface, as nearly as possible immediately over the seat of the disease. When the base of the brain is affected, the back of the neck is the nearest part to which the application can be conveniently made.

It is advisable to cover the surface of the cerate, when spread, with a piece of fine gauze, or very thin unsized paper, which scarcely interferes with the action of the cantharides, while it prevents the adhesion of portions of the cerate to the skin when it is removed. This is specially important when the blister is applied directly over the eyes, as it sometimes is, with great advantage, in obstinate ophthalmia.