1402 



HUMAN ANATOMY. 



Fig. 1168. 



from the matrix surmounting the papilla. In addition to forming the outer root- 

 sheath the peripheral elements contribute the matrix-cells that occupy the fundus of 

 the follicle and surround the papilla. The cells covering the summit and adjacent 

 sides of the papilla are converted into elongated spindles that later gradually become 

 horny and assume the characteristics of the cortical substance of the hair. When 

 present, the medulla is developed by the transformation of the cells occupying the 

 summit of the papilla, which enlarge, become less granular and grow upward as an 

 axial strand that invades the chief substance of the hair and accumulates kerato- 

 hyalin within its cells. At first present as minute drops, this substance increases 

 in quantity until it occupies the cells in the form of large vesicles. The subsequent 

 disappearance of these, followed by shrinkage of the cells and the introduction of 

 air, completes the differentiation of the medulla. The pigment particles, which 

 appear later, are first evident in the hair-bulb and probably arise within the epithelial 

 tissue. The elements of the hair-cuticle and of the inner root-sheath are differentiated 

 from the matrix-cells at the sides of the papilla. The tall columnar elements become 

 elongatejl and converted into the cornified plates of the cuticle both of the hair and 



of the inner root-sheath. The layers 

 of Huxley and of Henle are derived 

 from cells that soon exhibit granules of 

 keratohyalin, so that on reaching the 

 level of the summit of the papilla the 

 process of cornification has been estab- 

 lished. This is especially marked in 

 the elements of Henle' s layer, in which 

 the deposit takes the form of a longi- 

 tudinal fibrillation. 



The growth of the hair takes 

 place exclusively at the lower end of 

 its bulb, where, so long as the hair 

 grows, the conversion of the matrix- 

 cells into the substance of the hair is 

 continuously progressing. By this pro- 

 cess the substance already differentiated 

 is pushed upward by the cells under- 

 going transformation and these in 

 turn are displaced by the succeeding 

 elements. In this way, by the addition 

 of new increments in its bulb, the hair is forced onward and, in the case of those 

 first formed, through the epidermis that still blocks the mouth of the follicle. This 

 eruption begins on the scalp and regions of the eyebrows about the fifth foetal month 

 and on the extremities about a month later. 



Sebaceous gland 



Hair 



Root-sheath 



Bulb 



Papilla 



Developing skin, showing later stage of hair-follicle; hair 

 is now differentiated. X So. 



The hairs covering the foetus 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, are 

 continually falling out and being renewed until puberty, when in many localities, as on the scalp, 

 face, axillae and external genital organs, they are gradually replaced by the much longer and 

 thicker hairs that mark the advent of sexual maturity. Even after attaining their mature growth, 

 the individual life of the hairs is limited, those on the scalp probably retaining their vitality for 

 from two to four years and the eyelashes for only a few months ( Pincus). 



During the years 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 development of additional 

 follicles from the epidermis after the manner of the primary formation. When from age or other 

 cause the hair-follicles loose their productive activity and, therefore, are no longer capable of 

 replacing the atrophic hairs, more or less conspicuous loss of hair results, whether only tem- 

 porary or permanent evidendy depending upon the recuperative powers of the follicles. 



The change of hair that is continually and insensibly occurring in man, in contrast to the 

 conspicuous periodic shedding of the coat seen in other animals, includes the atrophy of the old 

 hair on the one hand, and the development of the new on the other. 



The earliest manifestations of this atrophy, as seen in longitudinal sections of the hair- 

 follicle, are reduction in the size and differentiation of the mass of matrix-cells at the bottom of 

 the follicle and the diminution of the hair-papilla. The progressive reduction of the matrix is 



