Caustics are medicines which destroy the tissues to which they are applied. They excite inflammation and vascular dilatation of the surrounding area. The eschar produced by these drugs is separated from the living tissues by the inflammation and suppuration produced.

The action of caustics is typically a chemical or physicochemical one. The layer of cells that may be destroyed varies in depth with the caustic employed, and the caustic used may cause a hard, smear-like, or fluid eschar, which, by demarcating inflammatory processes, is sooner or later thrown off. The caustics may be divided roughly into caustic acids, caustic alkalies, caustic metallic salts, and some few organic caustic compounds not readily classified. In many of the acids and salts the action of the acid ion of the caustic causes oxidation and reducing processes. By many of the acids an acid albumin or syntonin is formed; by the alkalies, an alkali albuminate; and by the salts, metal albuminates are produced. Many of these metal albuminates are permanent, in which case a characteristic hard and scaly eschar results.

The character of the eschar is determined largely by the tissue involved: thus, by alkaline caustics, the keratin of the skin is dissolved; fatty substances are converted into soaps or soapy masses. Many caustics have the power of being absorbed and of causing systemic poisoning. The salts of chromium, osmium, and arsenic are of importance in this connection.

Many of the caustics cause extreme pain, and their use is gradually being abandoned for the more accurate methods of surgery.

Caustics are employed for the following general purposes:

1. In specific acute poisonous injuries, as from the bite of poisonous insects, reptiles, or other animals. The caustic is applied to the wound directly, in order to destroy the poison.

Permanganate of potash, alkalies, or the direct cautery are the most reliable.

2. To destroy new growths of microbial origin. In lupus, sarcoma, carcinoma, chancres, charbon, etc., nitric acid, alkalies, glacial acetic acid, etc. are most employed.

3. To remove small tumors, warts, polypi (nasal or genital), hypertrophied mucous membranes. Here nitric and chromic acids, silver nitrate, etc., are of service.

4. For depilatory purposes - removing hair, etc.

5. To reduce and destroy inoperable tumors. Here the more caustic alkalies, arsenic, zinc, etc., have been widely employed in the past.

6. To reduce flabby and exuberant granulations in wounds. Nitrate of silver is one of the best drugs.

7. At times to influence deeper parts, in neuralgias and inflammatory action in an internal organ, it is beneficial to employ a superficial escharotic.

Those escharotics which have not been discussed elsewhere will here be considered in detail: