of irritation, parasitic or other, than any other organ of the body. Every stage and variety of disease is open to view; minute differences, minor or important, are at once noted; and thus it is that the recognized distinct maladies of the skin are so numerous. In no other organ, with the partial exception of the eye, can the changes be watched from day to day; in none can so many stages of the same disease be simultaneously observed; and in no other is it so simple a matter to remove and instantly fix for microscopic examination the living tissue.
The multitude of its affections renders the difficulties of arrang- ing the diseases of the skin very great, and the absence of any generally accepted scheme of classification has always been and still remains one of the main obstacles to their intelligent study. The older systems, constructed before the days of bacteriology, were commonly based on the form which the eruption assumed (scaly, moist, purulent), but they usually contained in addition a certain number of diseases under the heading of Parasitic. Though obviously illogical, such systems served well enough while the recognized parasitic diseases were few, such as those caused by such gross parasites as the Acarus scabiei (the itch mite), the pediculi (lice), and the hyphomycetic fungi such as the Achorion Schoenleinii. The discoveries of bacteriology have enormously enlarged this class, but the difficulty is that one and the same disease is regarded as parasitic by one authority, as dependent on nerve influence by another, while a third assumes an agnostic position.
The following is a useful working classification.
1. The Dermatoneuroses. — (a) Sensory: anaesthesia, hyper- aesthesia, pruritus; (b) vaso-motor: urticaria, erythema multiforme, angio-neurotic oedema, pellagra, purpura, certain forms of eczema, erythema pernio (chilblains), erythema nodosum, herpes, cheiropom- pholyx, alterations of pigment; (c) trophic: sclerodermia, perforating ulcer, Charcot's bed-sore, the lesions of certain forms of leprosy, Raynaud's disease, Morvan's disease, pemphigus, lupus erythematosus, the skin lesions of syringomyelia ; (d) glandular, according to the gland affected, — as the sweat-glands, hyperidrosis, haematidrosis, bromi- drosis, miliaria papulosa, or prickly heat; the sebaceous glands, rosacea, seborrhoea ; the hair follicles, alopecia, greyness.
2. Local Inoculable Diseases. — The agents producing these are parasitic in origin and may be divided into those caused by animal parasites, vegetable parasites and various micro-organisms, (a) Animal parasites: scabies, due to the Acarus scabiei or itch mite; pediculosis, guinea-worm disease, due to the Dracunculus medinensis; trichinosis, due to the cysticercus cellulosae; elephantiasis, due to the filaria sanguinis hominis; various eruptions produced by accidental parasites such as the harvest bug (leptus autumnalis) , the jigger or sand flea {Dermatophilus penetrans), met with in the tropics. (b) Vegetable parasit.es: ringworm, caused by the Trichophyton tonsurans; favus, caused by the Achorion Schonleinii; tinea versicolor, caused by the Microsporon Furfur; erythrasma, due to the Microsporon minutissimum; actinomycosis, due to the Acti- nomyces or ray fungi; mycetoma or Madura foot, due to Dyscomyces; aspergillosis and pinto, caused by an unknown fungus; streplothrix infections other than from the ray fungus, sporotrichosis; blastomy- cetic dermatitis, due to a fungus of the yeast family. ' (c) Micro- organisms: impetigo contagiosa, caused by inoculation with strepto- cocci; furunculosis or boils, due to the staphylococcus pyogenes aureus and albus; carbuncle, a deeper infection also caused by staphylococci; anthrax, caused by the bacillus anthracis; sycosis, due to a staphylococcic infection of the hairy parts; acne, due to a bacillus called by Gilchrist the bacillus acnes, thought to be identical with the micro-organism of Sabouraud and Unna; furunculosis orientalis (Delhi boil, Aleppo boil, Biskra button), a tropical disease in which the parasite is not yet identified; certain forms of eczema, notably the pustular forms.
3. General Inoculable Diseases. — Tuberculosis, manifesting itself as lupus vulgaris, verruca necrogenica, erythema induratum or as tuberculous ulcerations. In all these Koch's bacillus has been identified. Syphilis, caused by the Spirochaeta pallida of Scbaudinn and Hoffmann, in which there are primary, secondary and tertiary skin lesions. Leprosy due to the bacillus lepra. Yaws (framboesia), caused by a specific parasite, the Spirochaeta pertenius. Glanders, due to inoculation with the bacillus mallei. Added to these are erysipelas and the various exanthematous fevers.
4. Diseases of uncertain Aetiology. — Psoriasis, pityriasis rubra, pityriasis rosea.
5. Eruptions due to Drugs. — These may follow on the internal administration of chloral belladonna, copaiba, phenazone, mercury, quinine, tar, stramonium, sulphonal, salicylic acid and the salicylates and bromides.
6. New Growths. — Benign: cheloid and fibroma, naevus pigmen- tosa, vascular naevi, telangiectasis, lymphangioma, myoma, mycosis fungoides, papilloma, adenoma, moluscum contageosum, rhinoscleroma,
cysts and warts (including corns and horny growths). Malignant: sarcoma, carcinoma, rodent ulcer, Paget' s disease.
The skin is liable to the same pathological conditions as other structures of the body, such as changes in vascularity, inflammations, invasion by parasites and new growths together with changes due to the special structure of the skin such as hypertrophy and atrophy, disorders of the sweat glands and sebaceous glands and alterations of pigment. Some of the groups of diseases classed as the der- matoneuroses are manifestations of widely different diseases; thus anaesthesia and hyperaesthesia occur in hysteria; while the acute bed-sore of Charcot (a form of local gangrene) and perforating ulcer are generally due to an inflammatory condition of the nerve trunks. In the group of diseases known as purpura, where haemorrhages of varying size make their appearance on different parts of the skin, the lesion is considered to be due to a toxin or autotoxin acting directly on the vascular walls. In some cases we know it to be inorganic, such as phosphorus or mercury, in others organic as smallpox, measles, typhus or tuberculosis; or the haemorrhages may occur in connexion with new growths such as sarcoma and lymphadenoma. Why these very different causes should combine to produce the phenomenon of haemorrhage is not clear.
The disease known as urticaria or nettle-rash is probably due to some irritant poison circulating in the blood, but the causes pro- ducing it vary from constitutional diseases such as gout and malaria to certain articles of diet which act as gastro-intestinal irritants such as pork and shell-fish. It has been known also to follow on mental emotion and is said to be frequent in the neurotic diathesis, but an attack may be set up by any local irritant such as stings or bites. The pathology of the lesions in this disease is as follows: reacting to some irritant, the blood-vessels dilate, serum is poured out from them into the tissues around, and compressing the vessels from without empties them of blood. This explains the white centre of the urticarial weal, the red margin of which is the clinical expression of the dilated and uncompressed vessels at the border. In those diseases grouped together under the name of erythema, although the majority of authors place them under the heading of inflammation, there is a good deal suggestive of a close relation to urticaria. Some cases are caused by the ingestion of certain drugs, a good many are directly associated with the rheumatic poison, while others are apparently connected with fermentative changes in the gastro- intestinal tract. Thus all those examples of the disease with the cause of which we are approximately acquainted are readily enough attributed to some circulating irritant. This disease differs histologi- cally from urticaria in the persistent dilatation of the vessels. Although serum is poured out from them as freely as in urticaria, the dilatation of the vessels is so active that they are not compressed as in that disease, while the presence of numerous cells around the vessels seems to suggest a more severe irritant, and the fact that the lesions are clinically more persistent further confirms that suggestion.
When certain irritants are applied to the skin we know before- hand what effects they will produce. Thus croton oil produces a vesicular and pustular eruption, that of cantharides is vesicular or bulbous, while other drugs are followed by results dependent on their concentration, ranging from a mere redness produced by dilute applications to actual death of the skin from concentrated ones. With the milder irritants which produce the results clinically known as eczema we have invariably more or less pronounced certain definite phenomena. The blood-vessels dilate; serum is exuded from them — it may be merely into the deeper layers of the skin, or it may reach into and among the epidermic cells, or burst its way through these and appear in drops on the surface. The epithelial cells are, immediately if the irritation be slight, later if it be more severe, stimulated to increased activity of growth and production; and this activity, often misdirected, is so great that the normal process of hardening in the cells is interfered with, and we have what is known as parakeratosis (irregular cornification) and the conse- quent production of scales. Should this be the prominent pathologi- cal change, the exudation spends itself among the cells of the scales, and a condition pathologically moist appears to the clinical observer as a dry eruption. Thus according to the reaction — which is pre- sumably largely dependent on the irritant to which it is due — we have various degrees and forms of inflammation of the skin, all of them covered clinically by the term eczema. When such a dermatitis is produced experimentally by the application of such an irritant as croton oil we can more or less accurately predict the duration of the inflammation, which gradually becomes less and less and usually terminates in dry scaling. So in eczema, as long as the irritant con- tinues to act, so long will its results be evident on the skin. Un- fortunately the irritant which is the cause of eczema is still a matter of dispute.
In studying other inflammations we have the advantage of de- finitely knowing their cause. Thus in impetigo contagiosa we know, mainly owing to the work of Saboraud, that the cause of the disease is the streptococcus pyogenes. The first result of inoculation is a minute red spot (dilatation of the vessels) , which is rapidly followed by the appearance on the surface of a vesicle or bleb (exudation of serum), which is soon converted into a pustule, the whole dries up into a scab, which when thrown off discloses a healthy or slightly reddened skin. Fresh areas may be constantly attacked.
In ringworm, where the cause of the disease is the growth in the
