In this disease there are patches of a yellowish or dark brown colour with a scaly surface, and occurring chiefly on the trunk in adults. If the surface be scraped* and the scaly products examined under the microscope (best with the addition of liquor potassse, both in this case and the others), there will be found epidermic cells with the elements of a fungus, the Microsporon furfur. There are usually abundant mycelium threads, and the spores are in rounded masses, which have been compared to bunches of grapes (see Fig. 486).
Fig. 485. - The fungus in favus. Short threads are shown, x 350.
Of the lesions due to Animal parasites the most important is scabies.
This disease is characterized by the formation of papules, vesicles, pustules, and other signs of inflammation due to the effects of the Acarus scabiei or Sarcoptes hominis (see under Animal Parasites). The female of this insect penetrates into the substance of the epidermis, forming tunnels there in which the ova are deposited.
The inflammatory conditions are the result partly of the irritation of the acarus and its embryoes, which, as they develop from the ova, begin to travel along the tunnels, and partly the effect of the scratching induced by the itching. The primary lesion is a papule running into which a minute furrow is visible to the naked eye. The papule is produced by the opening up of the epidermis and swelling of the papillary layer, which is infiltrated with serous fluid and round cells. Vesicles and pustules are produced when the inflammation is more severe, and are met with chiefly in children and persons with delicate skins.
The Acarus folliculorum is a parasite sometimes met with in the sebaceous follicles.
The Parasitic insects are referred to under Animal Parasites.
The Larvae of insects are occasionally met with in the skin. The ova are deposited in the skin, and the larvae, having developed, may traverse the skin and subcutaneous tissue for a considerable distance till they present at some part and are squeezed out or extracted. They may produce considerable inflammations in the form of boils or swellings.
According to M. Sabouraud, who has succeeded in producing cultures of the fungi, there are two principal forms of the trichophyton, one with small spores (trichophyton microsporon) and one with large spores (trichophyton megalosporon). The former is chiefly that of childran and its special seat of growth is the substance of the hair, while the latter is chiefly that of the beard and smooth parts of the skin. The cultures were most successful on jellies consisting of gelatin.1.5. peptone 08, maltose 37, and water 100. (See Brit. Med. Jour., 189.), ii., 470; and Sabouraud, Les trichophytcs humaines, 1894; also Rosenbach, Ueb. die tief. eiternd. Schimraelerk. d. Haut, 1894).
Fig. 486. - Fungus in pityriasis versicolor. Threads are seen with groups of spores. x 500. (DCHRING).