SKIN DISEASES. The diseases of the skin do not essentially differ from those of the other organs of the body. But skin has certain differences from other organs, some dependent on its structure and some on its exposed position. There are no depths to be attacked, and any diseases, if they spread, must do so super ficially ; spreading as they often do equally in all directions, the diseases of the skin have a tendency to assume a circular form, independently of any parasitic cause, though when such cause is present the patches are of a more perfectly circular shape. Fur ther, from the extent of its superficial area and its exposed posi tion, the skin is liable to be attacked by more forms of irritation, parasitic or other, than any other organ of the body.
The classification of skin diseases, at first based on naked-eye appearances, and, later, on the underlying pathological changes, is becoming simplified by the adoption of an etiological basis, and, as our knowledge of their causation becomes more complete, such an ideal becomes possible, and is being attempted in the more recent text-books. At the present time, however, the cause of certain skin diseases remains obscure, but, as the gaps in our knowledge become gradually filled in, the group of the unclassified affections becomes correspondingly diminished.
Of the new forms of skin disease which have been recognized in recent years, the most numerous have been those occurring in tropical countries, and of them a considerable number are due to fungi, and constitute the group of the tropical dermato-mycoses. In Great Britain the cutaneous lesions associated with diseases of the blood have been the subject of careful study, especially those which occur in connection with the leukaemias and lymphadenoma.
It is now established that the disease, popularly known as ringworm, is due to a number of mould-fungi, which vary in different climates and countries, like other members of the vegetable kingdom, and have a wide distribution in the animal kingdom, occasionally even in birds, from which they are capable of being transmitted readily to man. Certain resistant forms of dermatitis, affecting various parts of the body, but more especially the groins and the extremities, which may closely resemble eczema, are not variants of that protean disease, but the result of the presence in the skin of certain ringworm fungi. This type
of ringworm has increased since the World War, as it was common among the troops, especially those who went through the Gallipoli campaign, or served in the East, and was brought home and spread by them.
Eczema, the commonest of skin affections, is no longer regarded as a disease due to a single cause, but as a form of cutaneous reaction, due to a variety of irritants, acting either locally, from without, or circulating in the blood, possibly in an individual predisposed by some lowered resistance. In the group of the eczemas it is customary now to include the various forms of occupational dermatitis. These are attracting attention, as they are responsible for considerable disablement. The list of irritants, chemical and physical, which may cause them, is con tinually being extended, and the problem of their prevention is receiving the close attention of the Public Health Authorities (see INDUSTRIAL WELFARE AND MEDICINE).
The actinic rays of light have been shown to be an important factor in causing inflammation of the skin. For years it has been known that they are responsible for the peculiar eruption of the face and hands which recurs in summer in individ uals sensitive to light (Hydros aestivate), and for the more serious freckled affection which goes on to cutaneous cancer (Xeroderma pigmentosum). It is only recently, however, that the peculiar, dry, atrophic, freckled condition of the skin, dotted over with dilated capillaries, and horny thickenings which may become malignant, has been shown to be due to the sun's rays. This is met with on the unprotected parts, namely the arms, hands and face, of people long exposed to the sun, in countries where its rays are intense, such as India, South Africa, Australia, etc. ; it is now designated as chronic solar dermatitis, and corresponds closely to a chronic burn from the X-rays. In Australia the ears of sheep sometimes develop cancer as the result of exposure to the sun's rays on the plains.
The role of micro-organisms and parasites in causing skin disease is highly important. Bacteria (see BAC