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a reaction of the cutaneous blood vessels manifested by a red (dermographia rubra) or white (dermographia alba) line where the skin has been scratched.
Dermographia occurs in everyone. Dermographia alba appears during spasm; pronounced dermographia rubra, upon dilation of the capillaries in the irritated area. The appearance of dermographia rubra or alba also depends upon the amount of pressure with which the irritation is applied: dermographia alba appears five to 20 seconds after a light scratch; dermographia rubra, when greater pressure is applied. Dermographia is used to study the state of the autonomic nervous system, which innervates the blood vessels: the appearance of dermographia alba after either light or strong pressure indicates that sympathetic nervous tone is predominant; the appearance of dermographia rubra is indicative of the predominance of parasympathetic tone. In some cases (for example, in exhaustion) dermographia may not appear at all. Determination of the nature of the dermographia is important in the diagnosis of a number of diseases of the nervous and endocrine systems.