Blister, a topical application, which, applied to the skin, produces an irritation, and raises the cuticle in the form of a vesicle filled with serous fluid. The powder of the dried cantharis, or Spanish fly, operates rapidly, with certainty, and is now invariably used for this purpose. (See Cantharides.) Morbid action in one part of the organism may often be relieved or removed by counter-irritation in another and a neighboring part, and on this principle the blister is applied. When the immediate effect of a blister is required, the vinegar of cantharides is a very prompt and effectual application. A piece of blotting paper moistened with this fluid raises a blister almost immediately. It is, sometimes thus applied behind the ears in toothache, or over the stomach in cases of sudden cramp. The raw surface produced in this manner affords a ready means of introducing certain medicinal substances into the system by absorption; morphine, for instance, sprinkled on this raw surface, is quickly absorbed, and patients may be thus relieved where remedies could not be otherwise employed, as in colic and cholera.