Licence: Public Domain Mark
Credit: A manual of medical diagnosis / by A.W. Barclay. Source: Wellcome Collection.
Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.
604/642 page 580
![wliicli I have seen made hy persons wlio form their diagnosis from pathognomonic signs. A limpet-shell crust is to them the sign of rnpia, and when the descjuamating crust of psoriasis assumes this form, the one is mistaken 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 ini]dies 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 parasitic 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—-favus, or 2^orrigo favosa. It used to be classed among the ptistular erujitions, because it first ajipears 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 odour. 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 purident secretion is sure to be met with; and if removed by a poultice, the moist, exuding sur- face cannot be mistaken. Knowing this fact, we have no need to particularize 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 eai'. One form of j)orrigo 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 ])roves that it is con-](https://iiif.wellcomecollection.org/image/b24989812_0604.jp2/full/800%2C/0/default.jpg)
