Blistering a surface with cantharides may be effected in two ways; one, by the application of the ordinary blister plaster, the other by painting with blistering liquid.
When the plaster is used it is usual to leave it on the skin of an adult for eight or ten hours; when, if it has raised a blister, this is to be cut, and the fluid having run out, the surface is then to be covered with a piece of fine dry wadding or carded wool. This dressing being, left on for two or three days, the skin will be found healed underneath. This plan is simpler and less painful than dressing with lard or spermaceti ointment.
If desirable to "keep the blister open" - i.e., its surface discharging - it may be dressed with savine ointment spread on lint or linen.
In the cases of young children the blister plaster should not be allowed to remain longer than two hours, after which period a muslin bagful of warm bread-and-water poultice should be laid on, and the blister will form under that. After the blister has been cut, the surface can either be dressed with continuation of the poultice or with dry wool.
A warm poultice is a most suitable dressing for blisters, when applied for quinsy or other sore throat.
As this is intended to be swift in its action it should be of the strongest kind that can be purchased. After it has been painted on for a few minutes the skin will be seen to turn white; that is a sign that enough has been painted on. In the course of half an hour blisters will begin to form. These can be dressed as above directed.
This mode of raising a blister has many advantages over the plastering. It is speedy in its operation, it is cleaner, and it is more manageable for children and persons in a state of delirium.
For cases of apoplexy or paralysis, where a speedy impression upon the nervous centres is desirable, the blistering liquid possesses great advantage, as it does also in acute rheumatism, in which affection the pain is often quickly relieved by having a strip of the liquid painted round the limb near to the swollen joint.
Counter-Irritation acts by derivation or diversion of a morbid action from one part by setting up another equally or more powerful influence on the nerves of another part. It places in our hands a very powerful means of acting upon diseases of internal organs that are not absolutely close to the part acted upon, as well as when applied near to the seat of the malady. An example of the latter is afforded by the influence of belladonna or aconite on rheumatic or neuralgic pains; of the former, in the beneficial effects produced on the brain by a blister plaster applied to the nape of the neck.