Plasters are in themselves essentially protective; and, though very often purposely medicated in such a way as to irritate or inflame, or with a view to a general impression on the system through absorption, they nevertheless exclude the influence of the air, and thus far act anti-phlogistically upon underlying inflammations. Some of them probably operate usefully in the latter method without having been intended to do so. Thus, the plasters made with ammoniac (i. 604), galbanum (i. 602), Burgundy pitch (II. 768), and Canada pitch (II. 769), which are employed as revulsives in chronic rheumatism in the loins and elsewhere, chronic swellings of the joints, and various subcutaneous inflammatory tumefactions, and which certainly often produce very happy effects in those complaints, probably operate as much by excluding oxygen from the subcutaneous tissues, as by a revulsive influence. in this way, too, we can understand the useful effects of the iron plaster or strengthening plaster (i. 448), which, applied to the small of the back, and the larger joints, has seemed to impart strength; whereas, it has simply removed rheumatic or other inflammatory conditions, partly at least through the method here suggested. So also with the mercurial plaster (II. 298), which, while it is slightly affecting the system, is operating favourably on the protective principle. All the above preparations have been noticed under the proper heads. There are a few which, from their peculiar applicability to protective purposes, deserve to be more particularly noticed here.

1. Lead Plaster. - Emplastrum Plumbi. U. S. - Emplastrum Lithargyri. Br. - Litharge Plaster. - Diachylon

This has been already sufficiently described (i. 111). it is here noticed only to call attention to its peculiar usefulness, not only in relieving excoriation and other superficial inflammation, which it does partly by the sedative influence of the lead, but also as a protection against these conditions. To prevent bed-sores, or injury to the skin from other modes of pressure and friction, as from the pad of a truss, and various surgical dressings, this answers often an excellent purpose; the only objection being its want of sufficient tenacity. This deficiency is supplied in the following preparation.

2. Resin Plaster. - Emplastrum Resina. U. S., Br. - Emplastrum Adhaesivum. - Adhesive Plaster

This consists of the lead plaster with the addition of a little resin, which gives it adhesiveness. it is very much used in the dressing of wounds and ulcers, in the former to keep their edges together, in the latter to approximate and give support to the granulations. But, while answering these purposes, it probably aids the cure, especially in the ulcers, by excluding the air. There is, on the whole, no plan of treating ulcers in the legs more effectively than by the adhesive plaster and bandaging.

3. Soap Plaster. - Emplastrum Saponis. U. S., Br

This is the lead plaster incorporated with a little soap. it has long been employed as a discutient application to tumours and swollen joints. if the principles above inculcated are founded in truth, we can easily understand how this preparation may act antiphlogistically, and thus perhaps be disposed to give greater weight to the testimony of those who have found it a very useful remedy.