This is a disease chiefly of the hairy scalp, although it occurs on other parts of the skin and also in the nails. On the scalp it forms crusts about the size of split peas, which. when removed, are usually seen to be somewhat cap-shaped, the convexity of the cup being next the skin, the latter presenting a corresponding depression. If the cups, which have a characteristic saffron-yellow colour, be divided with a knife they are seen to have a porous appearance, the interstices being filled with air, and they are brittle, so that they can easily be broken down in water.
The crusts are formed of fungus along with epidermic scales and their debris. The fungus is the Achorion Schoenleinii, in the form for the most part of short tubes (Fig. 485) with rows of conidia spores.
The proper mycelium is not jointed, but the receptacula are. The fungus appears to begin its growth in the hair follicles, enveloping the hair and passing into it. It also passes into the hair sheath and epidermis around, separating the cells and destroying them. It has its seat thus in the first instance beneath the horny layer which may be continuous over it. It does not invade the true skin, but the crust may cause inflammation around which may go on to suppuration and ulceration. As a rule, however, it only affects the epidermic structures, the hairs being largely destroyed where the crusts are developed.
When it attacks the nail, it forms a yellow crust, the tissue of the nail being opened out and softened by the fungus penetrating between the horny cells.