Cantharides, or Spanish flies

Cantharides, or Spanish flies, when used internally, are diuretic and stimulant; and epispastic and rubefacient when applied externally.

Mode of application.—A portion of the blistering plaster is spread with the thumb upon brown paper, linen, or leather, to the size required; its surface then slightly  moistened with olive oil and sprinkled with camphor, and the plaster applied by a light  bandage: or it is spread on adhesive plaster, and attached to the skin by the adhesive margin of the plaster.

Caution.—If a blister is to be applied to the head, shave it at least ten hours before it is put on; it is better to place a thin piece of gauze, wetted with vinegar, between the skin and the blister. If a distressing feeling be experienced about the bladder, give warm and copious draughts of linseed tea, milk, or decoction of quince seeds, and apply warm fomentations of milk and water to the blistered surface. The period required  for a blister  to remain on varies from eight to ten hours for adults, and from twenty minutes to two hours for children: as soon as it is removed, if the blister is not raised, apply a "spongio-piline" poultice, and it will then rise properly. When it is required to act as a rubefacient, the blister should remain on from one to three hours for adults, and from fifteen to forty minutes for children.

To dress a blister.—Cut the bag of cuticle containing the scrum at the lowest part, by snipping it with the scissors, so as to form an opening like this—V ; and then apply a piece of calico, spread with spermaceti or some other dressing. Such is the ordinary method; but a much better and more expeditious plan, and one that prevents all pain and inconvenience in the healing, is, after cutting the blister as directed above, to immediately cover it with a warm bread and water poultice for about an hour and a half, and on the removal of the poultice to dust the raw surface with violet powder; apply a handkerchief to retain the powder, and lastly dust the part every two hours. It will be healed in twelve hours.

Caution.—Never attempt to take cantharides internally, except under the advice of a medical man, as it is a poison, and requires extreme caution in its use.