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. Illustration: Fig. 101.—Concave or Adherent Surface of the Nail. A, border of the root; B, whitish portion of semilunar shape (the lunula); C, body of nail. The continuous line around border represents the free edge. Illustration: Fig. 102.—Nail in Position. A, section of cutaneous fold (B) turned back to show the root of the nail; B, cutaneous fold covering the root of the nail; C, semi lunar whitish portion (lunula); D, free border. With few exceptions every portion of the skin is provided with sweat glands, but they are not equally distributed over the body. They are fewest in the back and neck, where it is estimated they average 400 to the square inch. They are thickest in the palms of the hands, where they amount to nearly 3000 to each square inch. These minute openings occur in the ridges of the skin, and may be easily seen with a hand lens. The length of a tube when straightened is about 1/4 of an inch. The total number in the body is estimated at about 2,500,000, thus making the entire length of the tubes devoted to the secretion of sweat about 10 miles.