The hair-bulb or root of the hair embraces the papilla and is in turn surrounded by the hair-follicle. It is composed of three structures, the cuticle, the cortex and the medulla. The thin outer covering or cuticle is a transparent membrane, composed of non-nucleated, imbricated lamellae. As the root nears the surface, the cuticle of the hair appears to be formed of imbricated scales with elevated edges which, in the shaft of the hair, gives it the characteristic serrated appearance. The cortical mass or main body of the hair is composed of delicate, flat, nucleated, firmly attached, fusiform scales which give to the hair its great strength and elasticity. These qualities are further assured in the larger hairs by a central marrow or medulla, which is composed of loosely packed embryonal corpuscles, fatty and pigment matter, and extends from the root of the hair to its point. It is now generally believed that what was formerly thought to be pigment and fat, are in reality air- vesicles.
The shaft of the hair extends from the place of cutaneous exit to its free extremity. When uncut and healthy, the latter always ends in an acuminate point, well illustrated in the hairs of normal eyelids. Previous mention has been made of the color, caliber, length and straightness or curl of hairs in different individuals. The structure of the hair-shaft, in that it presents cuticular, cortical and medullary elements, is similar to the make-up of the hair-root, minus the root-sheaths. The flattened plates of the cuticle are non-medullated, non-pigmented and overlaid, like adherent fish-scales. The cells of the cortex, composing its bulk, are nucleated, pigmented and fusiform. The medulla is wanting in the lanugo hairs and is best developed in the strong hairs of the beard and eyelashes.
The hair-papilla is composed of a delicate fibrous or myxomatous connective tissue, freely supplied with pigmented and non-pigmented connective tissue corpuscles and a variable number of blood-vessels. Papillae are club-shaped or spherical formations, arising from the corium and expanding into the hair-bulbs. They are nearly twice as long as they are broad and their breadth, according to Robinson, is in direct proportion to the length of the hairs.
Normally a hair has a limited existence and is shed by a process of separation which takes place about the bulb, accompanied by a contraction of the hair- follicle at this point. The new hair is regenerated from the inner root-sheath about the papilla and, as it grows, pushes the dead hair before it until it is shed or accidentally removed by traction from without. Hair is not only derived from the epidermis but its production is a process that forms a cornified cylinder, which is projected from the cutaneous surface and is very analogous to real epidermal cornification.
Nails are dense, horny, translucent, elastic, concavoconvex plates derived from the epidermis and imbedded in the corium on the dorsal aspect of the distal phalanges of the fingers and toes. The convex surface of the nail is exposed while the concave is implanted on the nail bed beneath. They appear from without to be curved downward from side to side and less markedly so from the root to the anterior or free edge. This latter, if uncut, extends beyond the end of the fingers or toes. The nail consists of horny epithelia like the corneous layer of the epidermis, only more dense in structure. According to Bowen, the nail is a modification of the stratum lucidum. About the third month of fetal life, two or three rows of epithelia from the mucous layer appear at the site of the future nail-root. In the fourth month one or two additional rows of epithelia are projected between the first. As these cells mature, they are gradually forced forward between the rete mucosum and the horny layer of that part of the epidermis which becomes the nail-bed. By the beginning of the seventh month, the nail pushes its way through the corneous layer and at the end of intrauterine life is usually well developed.
Various terms are applied to designate the anatomic portions of the nail and the surface upon which it rests and grows. Thus the nail proper or uncovered part is known as the nail-body while the imbedded portion is called the nail- root. The matrix is that portion from which the nail grows. It extends from the floor of the nail-fold at the root as far forward as the anterior convex border of the lunula (that portion of the matrix not concealed by the nail-fold), represented externally by the relatively light-coloured part of the nail. The derma of the matrix is composed of dense fibrous tissue, which is blended with the periosteum of the last phalanx beneath and rises above into rows of papillae parallel with the long axes of the fingers and toes. These papillae decrease in size as they press forward and are the least developed at the lunula. The stratum mucosum rests on the papillae and in the furrows between the rows, and these epithelia are gradually transformed into horny epithelia. According to Unna the most posterior part of the matrix produces the horny plates of the nail-surface; the middle of the matrix, the middle of the nail; and the anterior part of the lunula, the deepest part of the nail.