The skin is by no means so simple an organ as it appears, being, in fact, composed of several sets of organs closely combined.

From its complicated structure, and its location upon the exterior of the body, the skin is very liable to injury from the influence of external irritants of various sorts, such as friction from clothing or scratching; the the extremes of heat and cold; the action of acids or alkalies; various vegetable irritants, as vegetable and animal parasites. Persons engaged in certain trades, as brick-layers, masons, millers, bakers, cooks, washerwomen, etc., are liable to particular forms of skin disease, originating from the action of various irritants produced by these occupations.

Next in importance as a cause of skin disease, should be mentioned errors in diet. Indigestion is indeed a very frequent accompaniment of skin disease; and in a large number of cases, it is possible to trace the causative relation. The use of pastry, the excessive use of fats and sugar, fried food, condiments, as mustard, spices, and particularly pepper, excessive use of meat, and the use of tea, coffee, tobacco, and alcoholic liquors, are very frequent causes of certain forms of skin disease. Skin affections are also, in many cases, dependent upon other diseases, as diseases of the kidneys, inactivity of the liver, constipation of the bowels, scrofula, and syphilis. The skin erruptions which occur in eruptive fevers and which are the result of the disturbance of the system by a specific poison, need not be here considered, as the eruption itself is but a minor symptom of the disease. The popular notion of attributing every disease to a "humor," as remarked above, is without scientific foundation.