vessels with which the derma is abundantly supplied; such a cell in fact possesses the vital properties of a primitive embryo cell.

The cells nearer the derma are more immediately and abundantly supplied with nourishment from the dermal blood-vessels, and serve as the focus of growth and multiplication for the whole epidermis; they are in fact the progenitors of the superficial cells which, as they are thrust away by the intercalation of new cells between the last formed and the progenitors, become metamorphosed in form and chemical character, and at last die and are cast off.

And it follows that the epidermis is to be regarded as a compound organism made up of myriads of cells, each of which follows its own laws of growth and multiplication, and is dependent upon nothing save the due supply of nutriment from the dermal vessels. The epidermis, so far, stands in the same relation to the derm as does the turf of a meadow to the subjacent soil.

12. Structures which are rendered clearly distinguishable only by a magnifying power of 300 or 400 diameters must needs be very small, and it is desirable that, before going any further, the learner should try to form a definite notion of their actual and relative dimensions by comparison with more familiar objects. A hair of the human head of ordinary fineness has a diameter of about th (say o'003) of an inch, or o'08 mm. (millimetre). The hairs which constitute the fur of a rabbit, on the other hand, are very much finer, and the thickest part of the shaft usually does not exceed 10th of an inch, i.e. o'oor inch or about o'025 mm. ; while the fine point of such a hair may be as little as th of an inch, about o'oor mm., or even less in diameter.

In microscopic histological investigations the range of the magnitudes with which we have to do ordinarily lies between o'i and o'oor millimetre; that is to say roughly between one two hundred and fiftieth and one twenty-five thousandth of an inch. It is therefore extremely convenient to adopt, as a unit of measurement, o'oor millimetre, called a micro-millimetre, and indicated by the symbol μ, of which all greater magnitudes are multiples. Thus, if the extreme point of a rabbit's hair has a diameter of iμ,

the middle of the shaft will be 25μ, and the shaft of a human head hair 80μ.

Adopting this system, the deep celis of epidermis have on an average a diameter of 12μ or more, the nuclei of 4μ to 5μ, while the superficial cells are plates of about 25μ, the nuclei retaining about the same dimensions. The diameter of a white corpuscle of the blood is about 10μ, that of a red corpuscle being 7μ to 8p. Hence the deep cells of the epidermis are rather larger than white blood corpuscles, and the uppermost ones much larger, at least in superficial area.

13. The epidermis proper everywhere presents substantially the same general characters. Its permeability to water permits, as we have seen, of the transudation of the insensible perspiration, and it thus plays the part of an excretory organ, while, in so far as it continually forms and throws off cornified cells, it might be said to secrete horny matter.

But in many parts of the body the excretory functions of the skin are concentrated and intensified by a very simple modificatio of the epidermis, which is produced inwards into saccular or tubular pouches. These are the so-called cutaneous glands which are of two kinds—sweat glands and sebaceous glands.

The swea glands, as we have seen (Lesson V.), are long tubes, the inner ends of which lie deep in the derma and are coiled up and surrounded by a rich network of capillary vessels. (See Figs. 31, 33, pp. 121, 123, and Fig. 90.)

The sebaceous glands have rather the form of short sacculated pouches; and the substance of their cells undergoes chemical metamorphosis, not into horny but into fatty matter, which, as the cells are thrown off and burst, is poured out through the neck or duct of the pouch.

14. In other regions the cornified cells are not at once thrown off in flakes, but are at first built up in definite structures known as nails and hairs, which grow by constant addition to the surfaces by which they adhere to the epidermis. In the case of the nails, the process of growth has no limit, and the nail is kept of one size simply by the wearing away of its oldest or free end. In the case of the hairs, on the contrary, the growth of each hair

is limited, and when its term is reached the hair falls out and is replaced by a new hair.

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 magn:fied about 4 diameters; Fig. C magnified about 200 diameters.)

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 (1); k, mouths of sebaceous glands; n, horny epidermis of integument.

the nail. It is very vascular, and raised up into numerous parallel ridges, like elongated papillæ (Fig. 91, B, C).

The surfaces of all these are covered with growing epidermic cells, which, as they flatten and become converted into horn, form a solid continuous plate, the nail. At the hinder part of the bed of the nail the integument forms a deep fold, from the bottom of which, in like manner, new epidermic cells are added to the base of the nail, which is thus constrained to move forward.

The nail, thus constantly receiving additions from below and from behind, slides forwards over its bed, and projects beyond the end of the finger, where it is worn away or cut off.

16. A hair, like a nail, is composed of horny cells; but instead of being only partially sunk in a fold of

Part of the shaft of a hair inclosed 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.)

the integument, it is at first wholly enclosed in a kind of bag, the hair-sac, from the bottom of which a papilla (Fig. 92 ), which answers to a single ridge of the nail, arises. The hair is developed by the conversion into horn, and coalescences 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 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