Etiology

The general question of the treatment of diseases of the skin involves the consideration of the different and several indications, and of the means at our command for the fulfilment of these indications. Before, however, we can properly appreciate the special features appertaining to any one given morbid condition, it is better to devote a little time to the discussion of the general factors which lead to the development of cutaneous disease; in other words, to consider the question of etiology.

Primarily it may be stated that cutaneous lesions are due to influences or forces from without, or from those which exist or arise within the body. To the former class may be assigned such as depend on temperature and climate, such as are due to traumatisms of various kinds, such as result from various parasitic invasions, etc. This is in reality but a limited class; on the other hand, the etiological factors which arise within the body itself are very numerous. We will, however, first consider the external causes. Excessive heat or direct exposure to the sun may excite undue activity of the sudoriferous glands, and result in the production of sudamina, or the extremely annoying papular affection known as lichen tropicus; or to an erythematous or even vesicular inflammation, commonly known as sunburn; or to the more trivial affection called ephelis, or freckles. Excessive cold may result in absolute congelation of exposed portions of the integument, followed by death and sloughing off of the parts; or a less degree of cold may excite the condition known as pernio, or chilblains, or in some persons produce chapping or fissuring of the skin; or, by depressing the general vitality, promote an outbreak that otherwise would not have occurred. Of the animal parasites that may infest the body, the different varieties of pediculi, or lice; the acarus, or itch insect; the leptus, etc., produce affections more or less annoying, but usually readily removable. On the other hand, the infinitely minute and to the naked eye invisible parasites of vegetable origin, as the achorion and the trichophyton, give rise to affections which are exceedingly tenacious, and sometimes well-nigh incurable.

The internal causes of cutaneous diseases, however, are far more frequently in operation, and are far more im­portant than those of external origin. In this class we may place those affections of the skin which are due to preexisting lesions of some part of the nervous system, as, for instance, zoster, which results from an anterior lesion of the ganglion attached to the posterior root of a spinal nerve; or some of the lesions of leprosy, which succeed certain degenerations of the spinal cord. The nervous system, however, may be in a perfectly sound condition, and yet act as the medium for the transmission of some internal irritation to the surface; thus, the gastric irritation resulting from the ingestion of shell-fish may manifest itself chiefly on the skin in the form of urticaria, or a chronic irritation of the pelvic viscera may be the active cause of rosacea. Again, cutaneous lesions may be due to internal changes which have in turn arisen from external causes as, for example, the cutaneous manifestations of syphilis, which are due to an internal dyscrasia produced by the entrance into the body of a certain form of morbific matter from without. Leprosy may be placed in the same category.Still another internal cause of cutaneous lesions will be found in that condition of ill nutrition or imperfect assimilation known as scrofula or struma.


Finally, we may have external lesions resulting from the accumulation in the blood of certain materies morbi. Most of the so-called medicinal rashes are due to this. Or, again, we may see the materies morbi generated within the body itself through imperfections in the digestive, assimilative, or excretory functions. As a matter of fact, I believe that fully one-third of the cases of cutaneous disease which come under the physician's eye are due to this last-named cause. If this be true, a somewhat brief considera­tion of this topic will not be out of place at this point.

In order that nutrition may be healthily carried on in any part there must be -

1. a proper state of the blood;
2. a proper condition and behavior of the tissues to be nourished; and
3. a right exercise of the controlling influence exerted by the nerves.

And these three must work harmoniously together. Deviations from health may Originate consequently from a flaw in any one of the three conditions above named. The theoretical origin, therefore, of diseased changes in the skin may be specially in the blood, as we see in xymotic affections, and here the skin affection is only symptomatic or a part of a more general disease; in the tissues themselves, as seen in the case of warts, cancer, keloid, psoriasis; or in the nerves, as in pruritus, and, it is thought and now generally taught, herpes, pemphigus, and urticaria. If the exact origin of disease be not as stated, the parts of the system chiefly concerned in the production of diseased conditions may be emphatically in one case the blood, in a second the tissues, and in a third the nerves. But of course, inasmuch as the ordinary action of these three agencies is bound up and related in the closest manner in health, the misbehavior in disease of one affects, secondarily, the proper action of others of the three agencies concerned in healthy nutrition. So that all are more or less involved in disease when fully developed, but primarily one or the other is mainly concerned in it.

Now there is much readiness to ascribe disease to changes in the blood, but not to sufficiently recognize the influence of perversions in the inherent cell-life of the skin structures, nor the controlling supervision of the nerves in the generation of cutaneous disease. Cancer is an example of disordered tissue-life.


It is more than probable that the origin of some diseases of the skin may really be in the central nervous system, and the cutaneous trouble is the effect of a general disturbance of the nervous system; or in the nerves themselves that run to the affected part; at any rate the nerves are mainly concerned, or they may constitute the agency by which the morbid changes in the skin are produced.