Definition. Crusts are dried effete masses of exudation, more or less changed by desiccation.
Crusts vary greatly in constitution, color, size, form and other features. They usually contain serum, pus, blood and extraneous matter such as fungus growths and epithelial debris. They are secondary to vesicular, pustular and ulcerative inflammation of some sort. When composed exclusively of serum they are yellowish as the crusts of eczema vesiculosum, when of serum and pus they are dark or greenish yellow as in impetigo, or if the blood is present in quantity, the crusts may appear dark red or black in color. They may, however, consist of fat and epithelium as in seborrhea; or of fungus growths as in favus. If the exudation is free and thin, they must soon be thrown off; if thick, they may be formed in layers and raised above the level of the skin. They may be small or large; firm or friable; thick or thin; adherent or loose; may cover a slightly changed skin or a superficial or deep ulcer. In outline, they generally follow the lesions which produce them; but may be disposed among other lesions in a way to obscure the shape of the original lesion or hide the real consecutive relation.