Thickenings of the epidermis occur in places where the skin is exposed to unusual pressure or friction. Continued pressure, as by a splint or bandage, causes atrophy, but intermittent pressure, by allowing the parts to recover, and by affording time for increased nutrition, gives rise to hypertrophy. We have thus the horny hands of workmen, and corns, which consist of concentric thickenings of the epidermis. The same law applies to internal parts, but as pressure from within, produced by tumours, aneurysms,*etc, is usually constant, atrophy is much more frequently the result. Hence the original statement of John Hunter is justified, that pressure from without produces thickening, while that from within causes atrophy, although it is not to be taken without reservation.

Literature

Hunter, Palmer's ed., vol. i., pp. 421 and 560; Virchow, Cellular Path.; Paget, Lect. on Surg. Path.; Stanley, Diseases of the bones, 1839; Coats, Compensatory hypert., Proc. Lond. Med. Soc., vol. vii.; Bizzozero, Internat. Med. Congress at Rome, 1894, Report in Brit. Med. Jour., 1894, i., 728; Gies, Arch. f. exp. Path. u. Pharm., vol. viii.; Wegner, Virch. Arch., lv.