keratin /ker″ətin/ [Gk, keras, horn] , a fibrous sulfur-containing protein that is the primary component of the epidermis, hair, nails, enamel of the teeth, and horny tissue of animals. The protein is insoluble in most solvents, including gastric juice. For this reason, it is often used as a coating for pills that must pass through the stomach unchanged to be dissolved in the intestines.
