Opening into each hair-follicle are usually one or more sebaceous, or oil, glands. These consist of groups of minute pouches lined with cells producing an oily material which serves to oil the hair and keep the skin moist and pliant.

238. The Nails. The nails are also formed of epidermis cells which have undergone compression, much like those forming the shaft of a hair. In other words, a nail is simply a thick layer of horny scales built from the outer part of the scarf skin. The nail lies upon very fine and closely set papillæ, forming its matrix, or bed. It is covered at its base with a fold of the true skin, called its root, from beneath which it seems to grow.

The growth of the nail, like that of the hair and the outer skin, is effected by the production of new cells at the root and under surface. The growth of each hair is limited; in time it falls out and is replaced by a new one. But the nail is kept of proper size simply by the removal of its free edge.

239. The Sweat Glands. Deep in the substance of the true skin, or in the fatty tissue beneath it, are the sweat glands. Each gland consists of a single tube with a blind end, coiled in a sort of ball about 1/60 of an inch in diameter. From this coil the tube passes upwards through the dermis in a wavy course until it reaches the cuticle, which it penetrates with a number of spiral turns, at last opening on the surface. The tubes consist of delicate walls of membrane lined with cells. The coil of the gland is enveloped by minute blood-vessels. The cells of the glands are separated from the blood only by a fine partition, and draw from it whatever supplies they need for their special work.

Fig. 101.—Concave or Adherent Surface of the Nail.