Hair arises from the bottom of small pits, or follicles, situated in the true skin, in some cases passing completely through it into the tissues beneath (4, fig. 260). Each hair is composed of a bulb, a portion of which is concealed in the follicle, and commonly described as the root, and a free portion or shaft which projects from the surface. The bulb of the hair consists of an aggregation of young epithelium. At its lower extremity it is hollowed out, and fits on to a small papilla at the bottom of the hair follicle.
Hairs possess a covering of flattened scales or cells arranged, like the outer layer of the cuticle, in the form of tiles on a roof. The outline of each cell is perfectly well defined on the surface of the hair, as can be seen by examination under a moderate magnifying power. The epidermis extends into the follicle very near to the bulb. Within the epidermis or outer layer is the cortical substance of the hair, in which the pigment granules, to which it owes its colour, are distributed. The cortical substance forms the chief part of the bulk of the hair. It is composed of long elliptical fibres, which may be reduced to their elements, i.e. epithelial lamellae with their nuclei. Under the microscope this portion of the hair exhibits the appearance of longitudinal stripes or fibres. The third portion of the hair is the so-called medullary substance; this occupies the narrow cavity in the centre, which extends from the bulb upward towards the point. The hair follicle, in which the root is embedded, is really an involution or doubling down of the skin itself. It presents for notice, proceeding from the inside of the follicle, a membranous structure, consisting of cells similar to those forming the deep layer of the cuticle, rete mucosum, on which is imposed the internal sheath of the hair, in reality the involuted epidermis. Besides hair of the ordinary kind as described, there are certain varieties which present special characters. For example, what is ordinarily described as " horse hair", and employed for the purpose of stuffing cushions and weaving into coverings, is an extremely coarse variety, occurring in the forelock between the ears, passing along to the top of the neck as far as the withers, constituting the mane, existing also upon the margin of the eyelids, eyelashes, and growing here and there on the outside of the lips and below the eyes, described as tentaculae. These coarse hairs are developed in the most prominent form upon the tail, from which they grow to an extraordinary length, reaching almost to the ground if left uncut. Similar hairs also grow at the back of the fetlock joints, investing the horny growth which is known as the ergot.
The variety of hair which is described as wool, is distinguished by its fineness and softness. It does not, however, under the microscope, present any elements which differ from those already described.