898 ANATOMY [ORGANS OP SENSE. finger, and toe is situated a firm horny curved plate, the nail. Each nail rests on a bed, the surface of which is formed of the cutis, which also overlaps the side and root of the nail ; thus the nail fits into a groove formed of the cutis something after the manner in which a watch-glass fits into its rim. A nail is merely a special modifica tion of the cuticle, the cells of the superficial stratum of which are more horny, harder, and more firmly adherent to each other than in the cuticle proper. Deeper than the horny stratum is the rete Malpighii of the nail, the cells of which are soft, as in the cuticle itself. The cutis forming the bed of the nail is studded with papilla?, which are arranged in almost parallel rows, and are highly vascular. Xails grow both in length and thickness : the increase in thickness is due to the formation of nerve cells on the bed of the nail ; the increase in length takes place through the formation of nail cells at its root, and as the nail is thus slowly pushed forward it requires to be cut at intervals. At the root, sides, and below the free border of the nail the cuticle is continuous with the substance of the nail itself. Hair. Projecting from the surface of the skin are multi tudes of elongated cylindrical horny structures, the hairs. In the skin of the scalp, the armpits, and the pubis, they are long and numerous ; but in the eye-brows, eye-lashes, vibrissce of the nostrils, and surface of the body generally, they are short. They are stronger and thicker in the skin of man than of woman, more especially on the cheeks, lips, and chin. Hairs do not grow from the skin of the palms and soles, the back of the ungual phalanges, and the surface of the upper eye-lids. Each hair is partially em bedded in a depression of the skin, called a hair follicle. The deeper end of the follicle is somewhat dilated, and has in it a papilla, the hair papilla. The wall of the hair follicle is formed of the constituent structures of the skin ; the outer part of the wall belongs to the cutis, and has been described as arranged in three layers, the external, middle, and inner layer of the hair follicle. The external and middle layers are formed of connective tissue, with blood-vessels ; whilst the inner, sometimes called the vitreous layer, is transparent and homogeneous, and continuous with the so-called base ment membrane of the cutis. The inner part of the wall of the hair follicle, or the root-sheath, belongs to the cuticle, and consists of two layers, the outer and inner root-sheaths. The outer root-sheath is continuous with the rete Malpighii, and consists of cells similar to those of that stratum. The inner root-sheath is continuous with the horny stratum of the cuticle, arid consists of elongated scale-like translucent cells in which no nuclei can be seen. A hair possesses a root, a shaft, and a tip ; the root is embedded in the hair follicle, whilst the shaft and tip form the free projecting part of the hair. In the human hair the substance of the hair is composed of a fibrous-looking horny material, which by the action of strong sulphuric acid is resolved into elongated, closely compacted, fusiform cells, which in coloured hairs contain pigment granules. In the thicker hairs the cells in the axis of the hair are polygonal, contain air, and form a central pith or medulla. The hair is invested by imbricated scale-like cells, which form the hair cuticle. In different animals the size and relative pro portion of the cells of the cuticle, medulla, end fibrous part of the hair present many modifications. The wool of the sheep has its cuticle scales, with well-defined serrated margins, so that the hair of this animal is well adapted for felting into cloth ; in the bat, also, the cuticle cells are largo and strongly serrated. The bristles of the pig, again, have the fibrous part of the hair largely developed. In the deer tribe the hair consists of polygonal medulla-like cells, which contain air. The root of the hair dilates at its deeper end into a bulb which embraces the hair papilla. It is softer in texture than the shaft, so that the cellular structure of the hair is more easily demonstrated. Next tho papilla the cells are like those of the rete Malpighii, but when traced onwards to the shaft they are seen to become differentiated, both in structure and composition, into the proper hair cells. The root is enveloped in a special sheath, termed the sheath of Huxley, composed of nucleated cells, which sheath, in the more superficial part of the follicle, blends with the internal root-sheath. The hair papilla bears to the hair the same relation as a papilla of the cutis has to its investing cuticle, so that a hair is to be regarded as a specially modified cuticular structure. . The human hair papilla is vascular, but no nerves have been traced into it. In the tactile hairs of the mammalia, how ever, nerves have been traced into their papilla?. The bristles, feathers, claws, hoofs, the horny envelope of the horn cores in the hollow horned ruminants, and various tegumentary spines and scales, present in many animals, are, like hairs and nails, special modifications of the tegumentary system. Each hair follicle has opening into it the excretory duct of a small gland, named a sebaceous gland. This gland consists of the excretory duct, and of from two to twenty grape-like saccular expansions which open into the duct. The wall of the sacculi and of the duct is continuous with the vitreous layer of the outer wall of the hair follicle. Capillary blood-vessels are distributed on the outer wall of the sacculi. The sacculi are almost entirely filled with polygonal cells containing drops of fat, which cells are continuous with the epithelial lining of the gland duct and the cells of the outer root-sheath. These glands secrete a fatty material, which lubricates the surface of the hair. Sometimes a small parasite, called Acaru-s folliadorum, is found in a sebaceous gland. Some years ago Kollikcr described one or two bundles of smooth muscular fibres extending from the wall of the hair follicle to the deep surface of the cutis; these muscles, named arrectores pili, by their contraction erect the hairs, that is, cause them to become more prominent, and produce the condition of skin, called cutis anserina or goose skin, well known to occur when cold is applied to the surface of the body. Hairs are developed about the 4th month of embryo life, within depressions in the cutis, which form the future hair follicles, filled with cells similar to, and continuous with, those of the rete Malpighii. A papilla forms at the bottom of this depression, around which the cells become arranged in a bulbous expansion. The cells, in line with the bulb, elongate and harden, and group themselves so as to form the shaft of the young hair, which at this stage is com pletely buried within the follicle. A rapid production of new cells takes place at the bulb, the hair consequently increases in length, and is pushed outwards through the superficial horny stratum of the cuticle, which had closed in the mouth of the depression or follicle in which the hair is produced. At the same time, the more external cells within the follicle are pushed outwards towards its wall, and form the cells of the root-sheath. When a hair is pulled out of its follicle the cells of the root-sheath are drawn out along with it. A new hair will be developed at the bottom of a follicle from which the hair has been shed as long as cells continue to be formed around the papilla. When the growth of cells ceases within the hair follicles then permanent baldness is the result. The sebaceous glands are developed as bud-like offshoots from the hair follicles, filled, like the follicles themselves, with cells continuous with those of the rete Malpighii. Instead of the cells in these buds differentiating into a hair, they become filled with fatty particles, and the wall of the bud assumes the .characteristic sacculated form of
tho gland.
