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THE SWEAT-GLANDS.

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keratinized epithelial cells, which have entirely lost the appearance of ever having been nucleated.

The corium consists of two layers of fibro-elastic tissue; the external, a stratum papillæ, is quite dense and at its upper surface projects into the epidermis as papillæ, the upper surface of which contains the tactile nerve-organs. The internal layer, or stratum reticu= lare, is much looser in texture and merges gradually into the underlying tissues.

THE SEBACEOUS GLANDS.

The sebaceous glands are small racemose glands, situated in the corium, their excretory ducts discharging into the hair follicles from the free surface of the skin in localities where hair is absent.

The acini are lined by one or two layers of cuboid epithelial cells, the lumen being filled with a mass of degenerated cells, granular detritus, and a fatty, waxy secretion of the cells.

THE SWEAT-GLANDS.

The sweat-glands, or sudoriferous glands, are coiled, tubular glands which occur in great numbers (2,000,000) throughout the skin. The deeper portions of the glands lie in the stratum reticulare as a roundish, coiled mass, the excretory duct being straighter. The gland tubules consist of a single layer of pyramidal columnar cells resting upon a distinct basement membrane. In many of the glands, especially those of larger size, a more or less complete layer of smooth. muscle exists between the cells and basement membrane.

The excretory duct is slightly smaller in size and lined with low cuboid cells up to the epidermis, where

the course of the tubule becomes more spiral, its walls being formed by the epithelial cells until it terminates in a funnel-shaped expansion at the surface.

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Fig. 55.-Skin and longitudinal section of hair: a, Epidermis; b, corium; c, sebaceous gland; d, fibrous root sheath; e, glassy membrane; f, outer root sheath; g, inner root sheath; h, expanded bulbous end of hair; i, papilla of hair; j, arrector pili; k, adipose tissue.

THE HAIR.

The hair is derived from the skin, and consists of two divisions-that within the skin called the root, and that external to the skin, the shaft.

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The root lies embedded in a deep depression called the hair follicle.

The shaft consists of three layers-the cuticle, cortical, and medulla, all of which are composed of more or less modified epithelial cells.

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Fig. 56.-Cross-section of hair and follicle at level of root: a, Cut end of longitudinal fibers of sheath; b, circular fibers; c, glassy membrane; d, outer root sheath; e, f, Henle's and Huxley's layers of inner root sheath; g, hair cuticle; h, cortical portion of hair substance; i, medullary portion of hair substance (sometimes absent).

The cuticle is the outermost covering, and consists of a single layer of horny, nonnucleated cells which overlap one another like shingles.

The cortical or middle layer is the thickest, and is

composed of many strata of greatly elongated, indistinctly nucleated, horny epithelial cells, which are very intimately united with one another.

These cells become broader and shorter in the root, where, not having become entirely altered, they resemble the cells of the stratum mucosum.

Scattered through the cortical portion is a greater or less amount of pigment upon which depends the color of the hair.

The medulla or pith is often absent, but when present, lies in the center of the hair fiber. It consists of two or three columns of cuboid cells which are usually filled with minute air-spaces, which give the medulla a silvery appearance when viewed by reflected light, but by transmitted light it appears black.

The hair follicle is a flask-shaped depression of the epidermis which extends down into the corium or subcutaneous tissue, according to the size of the hair, is surrounded by a fibrous sheath from the corium, and tightly encircles the hair root.

At the bottom of the follicle a small projection of the fibrous sheath (the hair papilla) extends a short distance into the hair bulb, carrying with it blood-vessels and pigment cells.

The outermost coat of the follicle is the fibrous sheath, which is derived from the corium, and consists of three layers-the outer longitudinal, the middle circular, the inner glassy or hyaline layer, a delicate structureless membrane upon which the epithelium of the middle coat rests.

The middle coat of the follicle is called the outer root sheath. It consists of a number (8 to 10) of rows of cells directly continuous with those from the rete mucosum of the epidermis. The outer cells are columnar; those more internal become polyhedral.

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The inner coat is called the inner root sheath, and consists of two layers. The outer of these is called Henle's layer, and is composed of a layer of elongated, nonnucleated, granular polyhedral cells. The inner layer is called Huxley's layer, and is composed of one or two layers of polyhedral cells containing granules of eleidin and small nuclei. The inner cells of Huxley's layer rests directly upon the cuticle of the hair root.

The arrectores pilorum are small bundles of smooth muscle-fibers which extend from the fibrous coat of the hair follicles to the upper surface of the corium.

The contraction of these muscles raises the hair from an oblique to a vertical position.

THE NAILS.

The nails consist of three portions-the root, the body, and the free margin.

The nail groove surrounds the root and sides of the body, the free margin projecting beyond the finger tips.

The floor of the nail groove upon which the body rests is called the nail bed, that portion supporting the root being called the matrix.

The nail groove has the same structure as the skin. The nail bed consists of a prolongation of the corium, the upper surface of which is thrown into longitudinal ridges (instead of papillæ, as in the skin) that are covered by a number of layers of epithelial cells, resembling and continuous with the stratum Malpighii of the skin.

The nail is virtually a greatly hypertrophied stratum lucidum which is formed only above the matrix and pushed forward as the cells of the matrix multiply.

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