These are substances which act by causing the death of tissue They may destroy by consuming the tissue, as in the case of sulphuric acid, or by precipitating protoplasm, as by phenol, or by causing an inflammation which results in a slough, as in the case of arsenic. The caustics are:

1. Acids

Sulphuric, nitric, glacial acetic, trichloracetic.

2. Alkalies

The hydroxides of potassium, sodium, and calcium (lime).

3. Metallic Salts

Silver nitrate (lunar caustic), copper sulphate (bluestone), zinc chloride, burnt alum, chromium trioxide (chromic acid), arsenic trioxide (arsenous acid).

4. Carbon dioxide, liquid or solid.

5. Phenol.

Sulphuric acid chars; nitric acid changes the part to yellow, and all acids act by abstracting water and neutralizing the alkalinity of the tissues. They are direct irritants, even when diluted. The alkalies abstract water and saponify the fatty substances of protoplasm; they are very penetrating, and make ulcers which are slow to heal. Chromium trioxide comes in the form of deliquescent, dark reddish crystals, which decompose or explode on the addition of glycerin, alcohol, or other organic substances. Among chromate workers perforation of the nasal septum is the rule, and deep ulcers of the hands known as "chrome holes" may make their appearance. They may be avoided by protection from the dust. There are also a number of caustic substances, such as mercuric bichloride, which are not used as such in therapeutics.

Toxicology

When caustic acids or alkalies are swallowed, they burn and denude the tissues of mouth, esophagus, and stomach, and produce shock. To neutralize acids, mild, non-carbonated alkalies may be used, such as diluted lime or magnesia; the carbonated alkalies set free too much gas. To neutralize alkalies, vinegar and lemon-juice are good. For the burns, demulcents, such as olive oil, lard, white of egg, milk, etc., are indicated. (For poisoning by metallic salts and phenol, see later.)

Therapeutics

To remove exuberant granulations, small polypi, warts, and hypertrophied soft tissues, as in the nose. Caustics are now very little employed except for application to small and superficial areas. Carbon dioxide, in liquid form or in sticks, has been used to remove nevi, and in the treatment of lupus, sluggish ulcers, epitheliomata, and leprosy.

To cauterize is to sear the tissues. It may be done with the thermocautery or electric cautery, or by nitric acid, phenol (carbolic acid), or lunar caustic. Phenol is adapted for infected cavities or sinuses, the area being afterward washed with alcohol to check further penetration of the phenol. For dog-bites, Bartholow, of the New York Department of Health (1911),. recommends the following in the order of their merit, viz.: (1) Fuming nitric acid; (2) silver nitrate; (3) the actual cautery. The employment of the thermo- or electric cautery for the removal of tissue is quite different from its counterirritant use, in which the skin should not be seared.