These include a very great variety of diseases, and the nomenclature is exceedingly complicated. In the skin, as elsewhere, inflammation is produced by the action of irritants, and the form of the inflammation will depend greatly on the nature and source of the irritant.
In some cases the origin and mode of action of the irritant is perfectly obvious, while in others it is very obscure. We have, for instance, septic inflammations, as in erysipelas, clue to a definite microbe. We have also the various inflammations in connection with the acute fevers in which a specific morbid poison attacks the skin. But there are other cases in which the origin is very obscure, and in which the peculiarities of the individual constitution play an important part. Just as there are persons especially liable to inflammations of the bronchi, of the. kidneys, or of the intestine, so are there* persons whose skin is prone to inflammation. Even in persons not predisposed temporary states of the nervous system apparently lead at times to a special tendency. At such times a very slight external irritation will lead to an inflammatory manifestation. There are indeed cases in which the nervous condition is the only obvious cause, as seen in a mild form in Urticaria, and in a more pronounced form in the so-called Trophoneuroses.
In considering the various forms of inflammation of the skin we shall divide them into four groups on the basis of their causation. These are (1) Inflammatory skin eruptions; (2) Symptomatic cutaneous inflammations; (3) Inflammations from heat, cold, and injury; (4) Septic and infective inflammations.