The causes of skin disease are numerous, and will naturally be a subject of enquiry as soon as a diagnosis is arrived at. First, it will be evident that the causes must be either general or local, but it would appear that the two greatest dermatologists differed widely as to which of these two was in operation.

Hebra contends for the local origin of skin diseases, while Erasmus Wilson adopts the reverse of that doctrine, and advocates the view that skin diseases are, more commonly than not, the expression of some internal derangement. The question is, however, only one of degree, as it is universally admitted that there are skin diseases both of local and constitutional origin.

A good idea of the causes which are generally in operation, as well as example of the diseases they respectively induce, may be gained by reference to the following succinct, and at the same time comprehensive statement, made by Dr. Tilbury Fox in his work on skin diseases: -

1. Poisons of acute specific diseases. Examples, those of small-pox, scarlatina, rubeola, etc.

2. The circulation of special poisons: (a) of animal origin - as syphilitic; (b) medicinal substances, e.g. arsenic, belladonna, copaiba, nitrate of silver, iodides, bromides; (c) dietetic, such as shell-fish, giving rise to urticaria; roseola, erythema.

3. Dietetic errors, as in wine-drinkers, high livers, non-vegetarians, etc. leading to the increase of urea and uric acid in the blood.

4. The tuberculous, scrofulous, and lymphatic dyscrasiae, giving rise to non-specific eruption - as impetigo, acne.

5. The gouty and rheumatic diathesis, as in lichen agrius.

6. Altered and lowered nutrition, from such causes as bad living, poverty, misery.

7. The accumulation of excreta in the blood from suppression of natural discharges, kidney disease, etc.

8. Convalescence from severe and lowering affections, by which the body is rendered much less able to resist disease.

9. Climateric or endemic influences, often malarial in nature, which act by deteriorating the system generally.

10. Disorders of the liver and spleen, leading to pigmentary deposit in various parts, jaundice and pruritus, etc.

This extract has been modified by the omission of certain diseases which are not recognized in the horse. All the morbid conditions above referred to relate to changes produced in the blood generally by the introduction of poisonous substances, or by the accumulation of animal poisons, or from the failure of the excretory organs to carry them out of the system. There are, however, in addition, to be considered various forms of malnutrition which lead to changes in the tissues of the skin. Chancres are also produced by local irritants, as chemical agents, burns, and the ravages of parasites. It must be allowed, also, that there are certain conditions of the skin, due to heredity, which dispose the surface to certain forms of disease, such as psoriasis, cancer, and ichthyosis. Further, there are causes which act specially upon the nerves, as frequent contact with local irritants, changes of temperature. The action of local irritants, it may be observed, is frequently connected with occupation, illustrated in the case of horses which are working where building is being carried on, which exposes them to frequent contact with lime, either dry or moist, or in the case of hunters which are commonly used in districts where the soil contains some gritty or irritating particles which act upon the surface, giving rise to what is known as "mud fever", a disease which is traceable also to the habit of washing the extremities and other parts of the body with warm water after work.

To sum up the causes of skin diseases: - 1. Clianges in the blood induced by poisons, dietetic errors, tuberculous disease, rheumatic tendency, general debility, effects of malaria. 2. Changes in the tissues, due to mal-nutrition, local irritation, whether from chemical or physical agencies, inherited peculiarities of the tissues. 3. Influences which act specially on the nerves, as dirt, exposure to excessive cold or heat, and sometimes organic disease of certain organs which by sympathy excite chronic congestion of different parts of the skin.