Henry Gray (18251861). Anatomy of the Human Body. 1918.
the finger it is connected with the under surface of the nail a little behind its free edge. The cuticle and the horny substance of the nail (both epidermic structures) are thus directly continuous with each other. The superficial, horny part of the nail consists of a greatly thickened stratum lucidum, the stratum corneum forming merely the thin cuticular fold (eponychium) which overlaps the lunula; the deeper part consists of the stratum mucosum. The cells in contact with the papillæ of the matrix are columnar in form and arranged perpendicularly to the surface; those which succeed them are of a rounded or polygonal form, the more superficial ones becoming broad, thin, and flattened, and so closely packed as to make the limits of the cells very indistinct. The nails grow in length by the proliferation of the cells of the stratum mucosum at the root of the nail, and in thickness from that part of the stratum mucosum which underlies the lunula.
FIG. 943 Longitudinal section through nail and its nail groove (sulcus). (See enlarged image)
Hairs (pili) are found on nearly every part of the surface of the body, but are absent from the palms of the hands, the soles of the feet, the dorsal surfaces of the terminal phalanges, the glans penis, the inner surface of the prepuce, and the inner surfaces of the labia. They vary much in length, thickness, and color in different parts of the body and in different races of mankind. In some parts, as in the skin of the eyelids, they are so short as not to project beyond the follicles containing them; in others, as upon the scalp, they are of considerable length; again, in other parts, as the eyelashes, the hairs of the pubic region, and the whiskers and beard, they are remarkable for their thickness. Straight hairs are stronger than curly hairs, and present on transverse section a cylindrical or oval outline; curly hairs, on the other hand, are flattened. A hair consists of a root, the part implanted in the skin; and a shaft or scapus, the portion projecting from the surface.
The root of the hair (radix pili) ends in an enlargement, the hair bulb, which is whiter in color and softer in texture than the shaft, and is lodged in a follicular involution of the epidermis called the hair follicle(Fig. 944). When the hair is of considerable length the follicle extends into the subcutaneous cellular tissue. The hair follicle commences on the surface of the skin with a funnel-shaped opening, and passes inward in an oblique or curved directionthe latter in curly hairsto become dilated at its deep extremity, where it corresponds with the hair bulb. Opening into the follicle, near its free extremity, are the ducts of one or more sebaceous glands. At the bottom of each hair follicle is a small conical, vascular eminence or papilla, similar in every respect to those found upon the surface of the skin; it is continuous with the dermic layer of the follicle, and is supplied with nerve fibrils. The hair follicle consists of two coatsan outer or dermic, and an inner or epidermic.
The outer or dermic coat is formed mainly of fibrous tissue; it is continuous with the corium, is highly vascular, and supplied by numerous minute nervous filaments. It consists of three layers (Fig. 945). The most internal is a hyaline basement membrane, which is well-marked in the larger hair follicles, but is not