Skin
Aiton's Encyclopedia (1910)
in physiology, the outer covering of the body. The skin varies in thickness from an eighth of an inch in the palm of the hand and on the sole of the foot to possibly the one-hundredth part of an inch in other places. The skin of a fair-sized adult is about two yards in area. The mucous membrane, or lining of the interior passages, is a continuation of the skin. The skin consists of two layers, an outer and an inner. The outer skin is known as the epidermis or cuticle. The outer cells of the cuticle are scaly. They are most numerous on the palm of the hand and the sole of the foot where protection is most needed. The inner part of the cuticle contains the pigment that gives color to the complexion and forms one of the distinguishing marks of races. The epidermis of the hand is raised in parallel ridges arranged in patterns. As these patterns remain the same for life they serve for the identification of criminals. The inner layer of the skin is known as the dermis cutis, and true skin. It contains the nerves of touch, the sweat glands, the sebaceous glands from which hair grows, and a system of blood vessels. The office of the outer skin, like the outer bark of a tree, is chiefly protective, and, as in the case of bark, the outer scales-the older cells-are thrown off and are replaced by others. The inner skin or cutis is protective also, but it is the seat, as stated, of essential systems of organs. The prick of a pin is not felt till the dermis is reached. No blood flows from a cut in the epidermis only. A blister is an accumulation of water between the outer and the inner skin. See Hair; Thorn; Perspiration; Albino; Leather.
What Has Changed in the Last Century
Aiton's 1910 description of the anatomy — a scaly epidermis shed and renewed like bark, a dermis holding nerves, sweat glands and blood vessels, and ridge patterns that "remain the same for life" — is still what students are taught, and its estimate of about two square yards of surface matches the modern figure of roughly 1.5 to 2 square metres. What has been added is function. Skin is now treated as an organ in its own right: it makes vitamin D under ultraviolet light (worked out in the 1920s and 30s), carries its own immune cells, and hosts a dense microbial community that was only catalogued in detail after DNA sequencing surveys began around 2007. Two other shifts stand out: fingerprints, which Aiton's mentions only for "the identification of criminals", are now routine biometric keys on passports and phones; and pigment, presented in 1910 as "one of the distinguishing marks of races", is understood as melanin varying continuously with ancestral ultraviolet exposure, carrying none of the racial-classification weight once placed on it.
Written for this edition, 2026.