Whether this disease be of shorter or longer duration, there is always redness of the skin, the cuticle rising in separate blisters, generally rounded and prominent, and filled with serum ; the duration of each blister is not long, but great difference is ob served in the rapidity with which the subsequent healing process goes on. There can be no question as to whether it be a vesicu lar eruption, because of the immense difference in size ; indeed, the only disease in which similar blisters are ever seen is ery sipelas ; and for this it cannot be mistaken.
In the sequel the appearance of the skin depends much on the rapidity of the healing process ; and when fresh bullte have ceased to appear, or the mor bid action is for a time suspended, there may be considerable doubt as to the true nature of the disorder. The skin may be merely morbidly red in patches, or it may be covered with rounded spots over which the cuticle has been re moved, and new epidermis is forming ; or again, these spots may be covered with a sero-purulent discharge, which in some is converted into a thin crust : in such instances the history of the case will best explain the meaning of what is seen.
Rupia, although it be said to commence as pemphigus does, in a bulla of some size, presents but few analogies to it. It is far more nearly related to ecthyma ; it is not a blister full of clear liquid, but a very large pustule, which does not come from be neath the skin like a boil, but yet goes deeper into its texture than the mere pustular eruptions, leaving an ulcerated surface, of con siderable depth covered by a thick crust. This, its ulterior stage, is the characteristic one of rupia. It has been compared to a limpet-shell, and the resemblance is in some instances not far fetched. In its earlier stage the spots are few and large, and there is not much it edness around ; they contain unmixed pus, not bloody or sanious matter : but it is only when there remain soli tary spots of ulceration, of a circular form, covered with a thick crust, around which the skin is moderately elevated, that the dis ease deserves the name of rupia ; and in whatever way. it have commenced, whether like ecthyma or even impetigo, it is now rupia ; and the name is of importance, because it at once points to a condition of system.
We shall have to refer to this form of disease when speaking of syphilitic affections, and it is of vast importance to be able to say whether rupia be or be not specific. But there is one mistake which I have seen made by persons
who form their diagnosis from pathognomonic signs. A. limpet-shell crust is to them the sign of rupia, and when the desquamating crust of psoriasis as sumes this form the one ispistaken for the other. It is surely needless to say that the shape is only accidental, the history and the condition of skin are perfectly distinct, and not less so the condition of system.
§ 7. Vegetable Parasites.—The distinction which this name im plies is only of recent date; perhaps microscopical researches may yet extend the class, and at all events we may hope for more definite knowledge of the relation of the various forms of para sitic life to the eruptions with which they are associated; such as in acne and sycosis.
The most important of this class is one that has its seat in the hair follicles—favvs or porrigo favosa. It used to be classed among the pustular eruptions, because it first appears as a small yellow spot, the sheath of the hair filled with the fungous growth ; but it has no tendency to suppurate. It grows with great rapidity, and when neglected forms large, hard, dry crusts, which have a peculiar mouse like odor. It is most liable to be confounded with impetigo, but it requires only moderate care to determine whether the crust be hardened pus or an independent growth. The distinction is based on the presence or absence of secretion: be the crust of impetigo never so dry, some trace of purulent secretion is sure to be met with; and if removed by a poultice, the moist, exuding surface cannot be mistaken. Knowing this fact, we have no need to varticularize the rounded form, the cracked, broken-looking surface, and all the other characters which older writers were obliged to enumerate. It is met with commonly in the head, but sometimes also down the back of the neck or in front of the ear.
One form of porrigo decalvans belongs to this class. The hair falls out in a patch of a circular form, leaving the skin of the head perfectly smooth : the absence of cutaneous eruption of any kind proves that it is connected simply with disease of the hair and not of the skin : this, too, is found to be a microscopic fungus.