Hair (The Structure Of The).
2. The hair differs considerably in length, thickness, shape, and colour; according to situation, race, family, sex, and age.
3. As hair is a bad conductor of heat, it is obviously one of the most appropriate coverings for the bodies of animals, or the head of man, because heat escapes very slowly through it. The surface of the body is protected from the influence of excessive heat, moisture, and electricity, by means of the hair.
4. " The hair," says Mr. Paget, the eminent anatomist, "in its constant growth, serves, over and above its local purposes, for the advantage of the whole body, in that, as it grows, it removes, from the blood the bisulphide of protein and other constituents of its substance, which are thus excreted from the body." It is therefore evident that the hair performs an important part in the animal economy. It has been remarked that shaving or cutting the hair assists in the removal of carbon and hydrogen from the system ; consequently long hair is injurious.
5. If we but look at the back of our hands we shall observe the hair issuing from small depressions in the skin. These depressions are the orifices of the hair - follicles, which extend to various depths, in the corium, and are always lined with cells of the same kind as those found in the epidermis.
6. Here is a diagram that will explain how the hair is retained in the skin, and if you examine it attentively you will be able to understand the relative positions of the various parts. The diagram represents a section of the human scalp, showing the manner in which the hair penetrates it ; a, is the hair-follicle; b, the hair within the follicle: c, the epidermis; d d, the sebaceous glands opening into the hair-follicle ; e, the fatty tissue, with the cellular tissue underneath it, in which the base of the hair-follicle is embedded.
7. As we have seen above, the sweat-glands are connected with the hair - follicles; and, in the accompanying figure, you will observe that there are no less than six of these glands open-ing into the hair-follicle which belongs to the beard. 8. The shaft of the hair is that part which you discern above the surface of the skin (fig. 1, b), and if you follow it into that membrane, you will see that it is lodged in a fold of what is termed basement membrane, or its follicle, which is larger or bulbous at the lower part, like the hair which is inside it. The accompanying diagram will enable you to understand the relative positions of the adjacent parts to the hair. 9. The hair-follicle is merely a turning-in of the skin, so as to invest the hair, and its inside is a continuation of the epidermis; for the cells are of the same nature (fig. 2, c), the deeper ones being somewhat round, while the superficial ones are flattened and scaly (e). The follicle is kept in its place according to the size and strength of the hair, by means of the adjoining tissues (sec. 6, fig. 1), and the small vessels, called capillaries, which afford materials for increase.
10. The hair grows from the bottom of the follicle, being formed by the secretions of these cells, and being gradually pushed upwards by them, so as to increase its length.
11. As the cells ascend in the bulb of the hair, they become larger until they reach the central part - hence the increased size of the bulb; out when the shaft commences, the cells become longer, denser, and, in fact, fibrous.
* Fig. 3. a, mass of cells in the centre of the hair, filled with pigment; b, basement membrane of the hair follicle; c, laver of epidermic cells; d, imbricated cells, loaded with pigment at the lower par , and becoming gradually compressed as they approach the surface of the skin; e. layer of imbricated cells.
12. By simply crushing the hair, we are able to discern its fibrous nature ; but this may be more readily demonstrated after the hair has been softened by maceration in an acid.
13. Hair consists of a cortical or fibrous horny texture which invests it, and a me-dullary or pith-like substance, which is observed on the inside.
14. The cortex, or bark of the hair, is formed by a single layer of cells being imbricated (fig. 2, e), and forming a thin layer outside the fibrous tissue of the shaft. These cells overlap each other, so that their edges give a serrated feel and appearance to the hair. If you rub your finger from root to point over a hair, and then rub it backwards, you will find that its surface is serrated; and, if the hair is very large, you will find that the roughness is greater, - because there is usually a double series of imbrications in the large hairs.
15. If we make a longitudinal section of the shaft of a hair, we find that the centre is made up of a series of cells, filled up with pigment, and contained in the fibrous part of its substance. In order to observe this appearance, we must use a sharp razor to make the section, and a magnifying power of about 150 ; but as every person is not able to do so, we have given a diagram of the appearance of the longitudinal section of the hair.
16. If we take a hair, and cut it across with a sharp razor, so as to make a very thin section of it - a mere shaving, in fact - we shall observe three parts: first, a thin varnish-like layer of flattened cells; then a set of fibres, which are placed further apart as they approach the centre, which is dotted here and there with pigment-cells in some hairs, but is always loose, and looks like pith.