Chemical and Mechanical Factors. Chemical substances, applied externally, may excite irritation, inflammation or destroy the superficial and deep layers of the skin. The degree of injury will depend upon the nature of the irritant, the duration of the application and the sensitiveness of the part involved. Agents of this class are very numerous; they include many plants which contain an active principle deleterious to the skin such as arnica, ivy, sumac, mustard, etc.; most of the etheral oils and resins; the poisons introduced into the skin by the stings and bites of insects and reptiles such as bees, mosquitoes and snakes; the various antiseptic preparations, when carelessly or freely used, such as iodoform, mercuric chloride, phenol and creolin; substances used in the process of manufacturing dyes as the aniline preparations; strong acids as nitric, muriatic, sulphuric, acetic and lactic; the stronger alkalies as caustic potash; the effects of heat from unusual exposure to sun-rays, overexposure to X-rays, radiation from heaters or flames, or actual contact, and also the extremes of cold. To these might be added the effects of chemicals used in various trades, frequently unavoidable, and especially the many medicated applications, often unnecessary. It should always be remembered that chemical irritants may not be limited in their effects to the area of the skin directly acted upon; but, through their influence on innervation, may cause disturbances at distant points or, by lessening the resistance of the skin, permit other agents to act more readily. Regarding this relation of medicinal irritants, Kaposi has well said that “these relations are altogether too little known for, if they were, physicians would not use cutaneous irritants so indiscriminately.”
Mechanically acting agents are often chemical in their action at the same time. They are incidental to nearly every active employment of mankind and may act to produce changes in the epidermis or deeper portions of the skin. In fact, the effects of occupation trade eruptions, professional or occupations dermatitis are quite common. Plain water may cause a surface irritation if used for long periods or often. Active exercise which commonly promotes health, if too long continued without protection of the surface, may cause chafing and consequent dermatitis. Fissured eczema is common in plasterers, masons and wash-women, whose hands are frequently brought in contact with alkalies. Those who handle animals or animal substances, as herdsmen, tanners and butchers, are most liable to anthrax and ecthyma. Occupations necessitating exposure to heat, as with cooks, blacksmiths and firemen, are favorable to attacks of erythema, eczema and dermatitis; while workers at oil refining, tar distillation and in aniline color making, etc., are peculiarly subject to dermatitis. Intermittent pressure, incident to the trade of shoemakers, blacksmiths, etc., produces thickening of the corneous layer of the epidermis, known as callous; while prolonged pressure over a point, as from a tight shoe, may produce a corn. Tight bands and garters may obstruct the local circulation, causing swelling of the skin or enlargement of the veins. Among the many other unmentioned causes of mechanical irritation, one group needs special notice excoriations from scratching with the finger- nails and all sorts of accidental abrasions, bruises and lacerations of the skin.
Parasites. It is hardly necessary to mention the large number of diseases that might be classified as parasitic from an etiological standpoint. Parasites are animal or vegetable and the diseases caused by these factors may be so grouped. Affections, such as pediculosis, scabies, ringworm and favus, caused by the grosser animal and vegetable parasites need no explanation. A large group of diseases, including impetigo contagiosa, leprosy, tuberculosis cutis, etc., are caused by lower microorganisms, but conclusive proof of the exact relations existing between dermatoses supposedly due to bacteria and the various cocci, bacilli, etc., supposedly causative, is lacking at the present time. The subject just mentioned offers a large field of possibility in the way of future investigation. The subject of contagion naturally presents itself along with the consideration of parasites. Even admitting the probable parasitic origin of many diseases and the inferential belief as to their communicability, it is a fact that in many instances contagiousness does not exist with any regularity or certainty. Probably the normal and healthy skin does not afford the proper soil for the habitat of most vegetable or animal parasites; hence, some predisposition may be the first element in the etiology of these diseases. Conversely, some peculiarity of the skin, such as the odor of its secretions, may serve to protect an individual from parasitic attacks.