Human anatomy, including structure and development and practical considerations . ules ofkeratohyalin, so that on reaching thelevel of the summit of the papilla theprocess of cornification has been estab-lished. This is especially marked inthe elements of Henles layer, in whichthe deposit takes the form of a longi-tudinal fibrillation. The growth of the hair takesjilace exclusively at the lower end ofits bulb, Mhere, so long as the hairgrows, the conversion of the matrix-cells into the substance of the hair iscontinuously progressing. By this pro-cess the substance already differentiatedis pus
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Human anatomy, including structure and development and practical considerations . ules ofkeratohyalin, so that on reaching thelevel of the summit of the papilla theprocess of cornification has been estab-lished. This is especially marked inthe elements of Henles layer, in whichthe deposit takes the form of a longi-tudinal fibrillation. The growth of the hair takesjilace exclusively at the lower end ofits bulb, Mhere, so long as the hairgrows, the conversion of the matrix-cells into the substance of the hair iscontinuously progressing. By this pro-cess the substance already differentiatedis pushed upward by the cells under-going transformation and these inturn are displaced by the succeedingelements. In this way, by the additionof new increments in its bulb, the hair is forced onward and, in the case of thosefirst formed, through the epidermis that still blocks the mouth of the follicle. Thiseruption begins on the scalp and regions of the eyebrows about the fifth foetal monthand on the extremities about a month later. Sebaceous gland Hair Root-sheath Bulb Papilla—. ->v^. Developing skin, showing later stage of hair-follicle; hairis now differentiated. X 80. The hairs covering the fcetus are soon shed, during the last weeks of gestation and immedi-ately following birth, and are replaced by the stronger hairs of childhood. These latter, too, arecontinually falling out and being renewed until puberty, when in many localities, as on the scalp, face, axilke and external genital organs, they are gradually replaced by the much longer andthicker hairs that mark the advent of sexual maturity. Even after attaining tiieir mature growth, the individual life of the hairs is limited, those on the scalp probal)ly retaining tlieir vitality forfrom two to four years and the eyelaslies for only a few montlis ( Pincus). During the >ears of greatest vitality not only are the discarded hairs replaced by new ones, but the actual number of hairs may increase in consequence of the deve