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new epidermic cells are added to the base of the nail, which is thus constrained to move forward.

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FIG. 88.

A, a longitudinal and vertical section of a nail: a, the fold at the base of the nail; b, the nail; c, the bed of the nail. The figure B is a transverse section of the same-a, a small lateral fold of the integument; b, nail: c, bed of the nail, with its ridges. The figure C is a highly-magnified view of a part of the foregoing-c, the ridges; d, the deep layers of epidermis; e, the horny scales coalesced into nail substance. (Figs. A and B magnified about 4 diameters; Fig. C magnified about 200 diameters.)

The nail, thus constantly receiving additions from below and from behind, slides forwards over its bed, and projects

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beyond the end of the finger, where it is worn away, or

cut off.

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FIG. 89.-A HAIR IN ITS HAIR-SAC.

a, shaft of hair above the skin; b, cortical substance of the shaft, the medulla not being visible; c, newest portion of hair growing on the papilla (i); d, cuticle of hair; e, cavity of hair-sac; f, epidermis (and root-sheaths) of the hair-sac corresponding to that of the integument (m); g, division between dermis and epidermis; h, dermis of hair-sac corresponding to dermis of integument (); k, mouths of sebaceous glands; n, horny epidermis of integument.

5. A hair, like a nail, is composed of coalesced horny cells; but instead of being only partially sunk in a fold of the integument, it is at first wholly enclosed in a kind of bag, the hair-sac, from the bottom of which a papilla (Fig. 89, i), which answers to a single ridge of the nail, arises. The hair is developed by the conversion into horn, and coalescence into a shaft, of the superficial epidermic cells coating the papilla, These coalesced and cornified cells being continually replaced by new growths from below, which undergo the same metamorphosis, the shaft of the hair is thrust out until it attains the full length natural to it. Its base then ceases to grow, and the old papilla and sac die away, but not before a new sac and papilla have been formed by budding from the sides of the old one. These

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FIG. 90.

Part of the shaft of a hair enclosed within its root-sheaths and treated with caustic soda, which has caused the shaft to become distorted.-a, medulla; b, cortical substance; c, cuticle of the shaft; from d to f, the root-sheaths, in section. (Magnified about 200 diameters.)

give rise to a new hair. The shaft of a hair of the head consists of a central pith, or medullary matter, of a loose and open texture, which sometimes contains air; of a cortical substance surrounding this, made up of coalesced elongated horny cells; and of an outer cuticle, composed of flat horny plates, arranged transversely round the shaft, so as to overlap one another by their outer edges, like closely-packed tiles. The superficial epidermic cells of the hair-sac also coalesce by their edges, and become converted into root-sheaths, which embrace the root of the hair, and usually come away with it, when it is plucked

out.

Two sebaceous glands commonly open into the hairsac near its opening, and supply the hair with a kind of natural pomatum; and delicate unstriped muscular fibres are so connected with the hair-sac as to cause it to pass from its ordinary oblique position into one perpendicular to the skin, when they contract (Fig. 31, B)."

They are made to contract by the influence of cold and terror, which thus give rise to "horripilation" or "gooseskin," and the "standing of the hair on end."

6. The crystalline lens is composed of fibres, which are the modified cells of the epidermis of that inverted portion of the integument, from which the whole anterior chamber of the eye and the lens are primitively formed.

7. Cartilage. While epithelium and epidermis are found only on the free surfaces of the organs, gristle, or cartilage, is a deep-seated structure (see Lesson VII.). Like them it is essentially cellular in nature, but differs from them widely in appearance on account of the development of a large quantity of the so-called intercellular substance. That is to say, the several cells do not lie closely packed together and touching each other, but are separated from each other by a quantity of material of a different nature from themselves. Just as in indifferent tissue each nucleus is imbedded in a matrix of protoplasm, so in cartilage, each cell, i.e. each nucleus with its allotted quantity of protoplasm, is imbedded in a matrix of intercellular substance.

Inasmuch as during the growth of cartilage the cells remain soft and protoplasmic, while the intercellular substance is converted into a solid semi-transparent hard matter, it comes to pass that the soft nucleated cells appear to lie in cavities in the harder intercellular substance or matrix.

In epithelium it is only the deepest lying cells which undergo division, and so carry on the growth of the tissue. In cartilage, cell-division is much more general; a cell lying in its cavity divides first into two, then into four, and so on, the intercellular substance meanwhile growing in between the young cells and thrusting them apart. It is by means of the repeated divisions of the cells in this way, and subsequent development of intercellular matrix in between the young cells, that cartilage grows. Con

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FIG. 91.

A section of cartilage, showing the matrix (a), with the groups of cells (6) containing nuclei (c) and fat globules (d). (Magnified about 350 diameters.)

A

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B

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FIG. 92.-CONNECTIVE TISSUE.

A, unchanged: a, connective tissue; b, fat cells. B, acted upon by acetic acid, and showing (a) the swollen and transparent gelatine-yielding matter, and (6) the elastic fibres. (Magnified about 300 diameters.)

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