Hair

A hair is a development, in the form of a cylinder, of a cap of corneous epidermis surmounting a papilla of the dermis sunk to the bottom of a tubular pit, or involution of the skin, called a hair-follicle. In the upper part of the hair-follicle the walls consist of the ordinary skin with all its parts, dermis, Malpighian layer, and corneous layer, the latter as usual of considerable thickness. At some little distance from the mouth of the follicle the corneous layer suddenly ceases, and in the follicle below this, the epidermis is represented by the Malpighian layer, now called the outer root-sheath, and two layers of peculiar cells, forming the inner root-sheath, of which the outer is called Henle's and the inner Huxley's layer; these may, perhaps, be con­sidered as corresponding to the stratum granulosum and the stratum lucidum respectively. The dermis of the wall of the follicle is at the same time developed into an outer layer with bundles of connective tissue disposed chiefly longitu­dinally, and an inner layer of peculiar nature, the arrange­ment of which is transverse, and which at least simulates, if it really be not, a muscular transverse coat. Between this dermis of the follicle and the outer root-sheath or Mal­pighian layer is a very conspicuous definite hyaline base­ment membrane, so thick that it presents a very easily re­cognized double contour.

At the bottom of the follicle the dermis of the wall of the follicle is continuous with the substance of the (dermic) papilla, while the outer root-sheath or Malpighian layer, which here becomes extremely thin and reduced to one or two layers, is reflected over the papilla, and there expands again into a mass of cells, which like the cells of the Malpighian layer in the rest of the skin multiply, and by their multiplication give rise to the corneous body of the hair. It is said that in those hairs which possess a medulla the vertically disposed lowermost cells of the Malpighian layer are at the actual summit of the papilla continued upward in the axis of the hair, as the medulla.

The layer of Henle, following the Malpighian layer or outer root-sheath on which it rests, is similarly reflected and forms over the hair a single layer of flat transparent imbricated scales known as the cuticle of the hair; Huxley's layer, similarly reflected, forms a similar layer of similar scales, but this is considered as belonging to the root-sheath, and is called the cuticle of the root-sheath.