Simple atrophy is not uncommon in the skin. The most frequent example of it is afforded by Senile atrophy, resulting in the wrinkled skin of old people. In this ease the connective tissue of the cutis loses in bulk, especially the papillary layer. The epidermis is also thinner and dry, and there is often desquamation in the form of dry scales or larger membranous pieces. The hairs are atrophied, as their papillae have taken part in the general atrophy of the papillae of the skin. The hair-follicles are shortened and the sebaceous gland.s on this account may be brought close to the surface so as to be very visible in the thin skin.

A general atrophy also occurs in emaciated persons, and it may closely resemble the condition in senile atrophy.

Of the Local atrophies the most familiar is that which occurs after Pregnancy. White lines are found in the abdomens of persons who have been pregnant, and similar lines occur in persons whose abdomens have been distended by tumours, by ascites, or even by accumulation of fat. The white lines have a cicatricial appearance, and they seem to owe their origin to the connective-tissue fibres of the cutis being dissociated by the stretching. Somewhat similar white lines or striae sometimes occur idiopathically, especially over the buttocks, trochanters, pelvis and thighs.

The Hairs are liable to atrophy, and two forms may be distinguished according as either the hair itself or the pigment diminishes. Alopecia or Baldness is atrophy of the hair itself. All through life a continuous falling out of the hair is occurring, and is due to an atrophy of the bulb. But the papilla remains, and a healthy new hair is produced on the site of the old one. In some people as life advances the new hairs are not reproduced of normal size, and they become gradually finer and finer, till there are only the finest silky hairs on the bald part, or even none at all.

Besides being produced in this way, baldness may be secondary to syphilis, to inflammations, and to certain parasitic diseases; in these cases it depends on interference with the nutrition of the sheath and papilla.

Special attention has been paid to Alopecia areata, in which baldness occurs in circular patches. These patches are not entirely bald, but are covered with fine woolly hairs, and the papillae are not destroyed. The nature of this disease is obscure, some regarding it as parasitic, and others as a trophoneurosis. Its habit and mode of extension render the parasitic view very probable. It is very likely that, as Thin contends, the parasite here is a microbe (a micrococcus), which is much more difficult to detect than the other parasites of the skin, which are fungi.

Canities or Greyness of the hair is also for the most part a natural atrophy of advancing life. But it also sometimes comes on prematurely. It depends for the most part on a deficiency of pigment in the individual hairs at their original formation, so that the pigment granules in the cells in the cortical layers of the hair are diminished. But there may be a temporary blanching of the hair from air getting infiltrated among the cells of the cortex. Cases of sudden permanent blanching have received no satisfactory explanation.

Albinism is a congenital absence of pigment which affects the iris and choroid of the eye as well as the hair and skin. It is not infrequent in negroes, the peculiar result being a white negro. The albinism is sometimes partial, so that, in the case of the negro, a piebald appearance is produced.

The Nails are liable to atrophy, which may be congenital or acquired. In the course of acute illnesses, such as fevers or maniacal attacks, the formation,of the nails is often partially suspended, and the illness is marked for a time by a transverse depression, which, with the growth of the nail, proceeds from the root outwards, and disappears in due course. The nails also atrophy sometimes in consequence of various parasitic or inflammatory skin diseases.