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resolving the difficult markings on some of the finer forms of Diatomaceæ, to a future opportunity.

J. S.

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A CHAPTER ON HAIRS.

Amongst the numerous subjects which animal products supply for the worker with the microscope, there are a few which are more interesting, or which offer a greater variety of structure, than those delicate filamentous processes which constitute the substance we know as hair. To pluck out a hair from the head of a playmate, and look at it with a simple microscope, is one of the first experiments of schoolboy days, although the result of the investigation is not always satisfactory, for reasons which will be presently apparent. Although this epidermal substance is not confined to hair only, but under various circumstances takes the form of horn, hoof, and nail, yet, as an external appendage, hairs are found not only on the superior orders of animals, but are likewise developed on the lower orders, on various insects, and to a limited extent on crustaceous animals; hence it naturally follows that the variety of hair is very great, its limits not being confined to the quills of the porcupine on the one hand, or the delicate hairs of the butterfly on the other; examination having proved that the horn of the rhinoceros is but a mass of hairs firmly united together, while the revelations of the microscope render it difficult to say how small those hairs may be which the insect world possesses.

It will be sufficient for our present purpose to refer more particularly to hair as commonly understood, and we may accept as a type of it, that with which we are most familiar, viz., human hair. This substance originates in a

Fig. 1.—Human Hair.

bulbous root, contained in a depression of the skin, termed a "hair-follicle," and it is by an accumulation of the pulp within the bulb, that the hair is formed and in due course pushed upwards. When viewed through a microscope with reflected light, the external covering only is seen, which is called the cuticle, and consists of a thin membranous substance, freely marked with irregular lines, which result from the imbricated edges of scales or compressed cells with which the shaft is enveloped (fig. 1).

When viewed with transmitted light, a dark line is seen to run up the centre, which for a long time sustained the notion that human hair was tubular. A more careful investigation, however, has show that the dark line is due to air, contained in cells, forming a pith or medullary substance. Surrounding this central column, and forming the bulk of the hair, is a fibrous substance termed the cortex, which, like the medulla, consists of cells, and to this horny portion of the hair is due its peculiar properties of strength and elasticity. There are likewise a number of pigment granules, exceedingly small, but which give to hair its various shades of colour, according as they are more or less numerous.

The microscopist will find a never ending variety in the structure of hair, both with regard to the internal and external arrangements of the scales, cells, and pigment. Thus, while human hair is comparatively smooth, the hairs of many animals are rough, and the scales on their surface looser. This is a principal reason why wools are so useful for weaving and felting, processes for which human hair is not applicable. A reference

Fig. 2.—East India Wool.

to figs. 1, 2, and 3 will illustrate the difference between human hair and wool, which is but a modification of hair, and will explain why Merino wool is so esteemed by the manufacturers.

The wool of some kinds of goats, termed mohair (fig. 4), is of a similar character, but much finer in substance. The hair of the bat tribe affords most interesting objects, from the peculiar arrangement of the cortical scales, which project very much from the