and nails are originally de rived from the epidermis, and are essentially cellular structures. A hair is formed by a folding or dipping inwards of the skin. A depression or furrow is thus formed, the inner walls of the depression consisting of the folded epidermis. The depression takes the shape of a sac, and is called the hair sac or hair follicle. At the bottom of the follicle is an enlarged papilla of the true skin (c, Fig. 165) pushed downwards by the folding-in process. Like the other papilhe of the skin, it is covered with the active cells of the deep layer of the epidermis, which form a bulbous enlargement over the papilla, that is, the root of the hair. As the cells in direct contact with the papilla grow and multiply, those above them are pushed upwards to make room for them, and owing to the shape of the hair sac the cells become packed together so as to form a cylinder or stem, which finally, as the growth from below goes on, is pushed out beyond the skin as the shaft of the hair. A hair thus consists of a peculiar ment of the cells covering the true skin. So closely are the cells packed to form the cylinder that a fibrous appearance is presented, except in the centre of the hair—the medulla—where the cells still retain their shape, and make the hair appear different in the centre from the circumference. The hair is thus not a tube but a solid rod posed of cella packed tightly at the ference and loosely in the centre. Sometimes little spaces exist in the centre owing to sorption of cells, and the spaces are filled with air, giving in some parts the appearance of a tube wben the tisk is examined under thg microscope. The different colour of hair is due to pigment present in the cementing substance between the cells as well as in the cella them selves.
Fig. 166 shows the appearance under the microscope of a hair, the cells overlapping one another like tiles on a roof.
Glands of off from each hair sac are one or two glands (f, Fig. 164), the sebaceous glands (Latin, sebum, tallow). They are also shown in Fig. 165, d, and con sist of groups of minute sacs lined with cells, which produce an oily material to lubricate the hair and skin. Connected with each hair sac, especially if of a good size, is a bundle of in voluntary muscular fibres (e, Fig. 165). The
bundle passes in such a direction that, when it contracts, the hair sac, which is placed obliquely in the skin, is caused to become more upright, and thus the hair is made "to stand on end." It is this that causes the appearance of "goose's akin." A Nail is also a compact mass of epidermal cells. At the bottom of a fold on the skin is the root of the nail, at which growth takes place by multiplication of cells. The nail is thus continually pushed forward by the growth behind. The bed on which the nail rests, and from which it also receives additions, is formed by numerous papillae of the true skin.
Just as in man hair and nails are altered epidermal structures, so the feathers of birds and the claws of animals are formed from the surface layers of the skin.
ridges and furrows caused by the true skin being thrown into the papillae described on p. 412, are not obliterated by the layers of cuticle above them. On the contrary the cuticle is moulded, so to speak, accurately over the elevations and depressions of the true skin. The surface of the skin is not therefore smooth, but irregularly marked. On most of the body the furrows are shallow and irregular, and intersect one another; but on the palm of the hand and sole of the foot they form more or less parallel lines and curves, producing well-marked patterns. It has been shown that these patterns, specially well seen on the tips of the fingers and front of the thumb, are different in different individuals, no two persons having identically the same arrangement of curves. It has also been proved that the arrangement never changes throughout life, unless by injury of the skin, and that the pattern on the infant's thumb is the same as the pattern will be when the infant has become an old man. An impression of the marks, therefore, on a person's thumb, taken on wax or in ink, will serve as a means of identify ing the same person years after. Similarly the marks of greasy fingers, left on a polished sur face, such as glass or silver, can be used as a means of identifying the person. This method of identifying criminals has been adopted in Great Britain, since 1902 an impression of the finger-prints of every criminal being taken and carefully classified and preserved.