This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
A Hair consists of a bulb or root imbedded in a follicle, and a shaft or stem ascending therefrom. The follicle is an invagination of the integument lined with a thin prolongation of the cuticle, divisible into two layers, sometimes called the inner and outer root-sheath, and so adherent to the root of the hair that it is liable to be removed with it when a hair is pulled out. At the base of the follicle is an enlarged papilla; and the hair itself may be considered as the exaggerated cuticular investment of this papilla. The root of the hair forms a bulbous enlargement round the papilla, and consists in greater part of polygonal cells, which in dark hairs are loaded with pigment; but towards the upper part, round about, the cellular substance is changed to fibrous, and on the surface there is an imbricated epithelium continuous, below, with the innermost layer of the cuticular lining of the follicle. All these three elements may be represented in the stem. The epithelium on the surface of the root, traced upwards, is seen to form on the stem an extremely thin coat, of which the most easily discernible part is a network formed by the thickened edges of the scales. In many of the larger hairs, the cells which constitute the main bulk of the root are continued in a column up the centre of the stem, and are termed the medulla. But in the smaller hairs of the body, and even often in the hairs of the head, the medulla is absent, or only present in small patches; and in all instances the bulk of the stem is of fibrous substance, which, in contradistinction to the medulla, is termed the cortical part. This cortical substance appears in the natural state nearly homogeneous; but when boiled with potash it is seen to consist of flat fibres, which are derived from the cells of the root by elongation and alteration of consistence.
Fig. 40. Hair, a, Papilla in the centre of the bulb; b, dermic coat of the hair-follicle ; c, d, outer and inner cuticular lining ; e, cortex of the shaft; f, medulla; g, epithelium; g, the same seen in profile, imbricated on the root; A, portion of shaft become white, showing the medulla enlarged by presence of minute air-bells, reflecting the light, and making the hair thicker.
The three elements of the stem of the hair are very differently represented in different animals. In wool the edges of the epithelial cells are prominent, and by their tendency to catch cause the hairs to be easily felted. In some small animals the epithelium is like a series of hollow cones embracing the hairs. In some, as in the mouse, the medulla is thrown into a series of air cavities, and in others, such as the pig, the medulla is entirely absent.