330 



NORMAL HISTOLOGY. 



The hairs covering the foetus are soon shed, during the last weeks of gestation 

 and immediately 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. The change of hair, that 

 is continually and insensibly occur- 

 ring in man, includes the atrophy 

 of the old hair and the development 

 of the new one. 



The earliest manifestations of 

 this atrophy are reduction in the 



Sebaceous gland 



Hair 



Root-sheath - 



Bulb 



Papilla 



FIG. 375. Later stage of developing follicle ; the hair is now 

 differentiated. X 80. 



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 accom- 

 panied by the production of a club-shaped enlargement of the hair, between which and 

 the shrunken matrix a strand of atrophic epithelial cells for a time remains. With 

 the continued progress of these changes, the root of the club-hair, as the degenerating 

 hair is termed, shortens so that the bulbus enlargement recedes from the bottom ol 

 the hair-sac until it lies just below the narrow neck of the follicle, where it remains 

 for a longer or shorter period until the hair is dislodged and finally discarded. While 

 the old hair is still lodged in the upper part of the follicle, the first steps towards its 

 replacement are initiated by the stratum germinativum of the old hair-sac, the deepest 

 follicle-cells contributing by proliferation the material from which the new hair is 

 developed in a manner essentially the same as that by which its predecessor was formed. 



THE NAILS. 



The nails, the horny plates overlying the ends of the dorsal surfaces of 

 the fingers and toes, correspond to the claws and hoofs of other animals and, 

 like them, are composed exclusively of epithelial tissue. They are special- 

 izations of the epidermis and, therefore, may be removed with the cuticle 

 without mutilation. The entire nail-plate is divided into the body, which 

 includes the exposed portion, and the root, which is embedded beneath the 

 skin in a pocket-like recess, the nail-groove. The modified skin supporting 

 the nail-plate, both the body and the root, constitutes the nail-bed, the 

 cutaneous fold overlying the nail being the nail-wall. During life the nail 

 shows color-zones, its projecting portion being immediately followed by a very 

 narrow yellow band that corresponds to the line along which the stratum 

 corneum of the underlying skin meets the under surface of the nail-plate. 

 The succeeding and larger part of the nail is occupied by the broad pink 

 zone which owes its rosy tint to the blending of the color of the blood in the 

 underlying capillaries with that of the horny substance. On the thumb 

 constantly, but on the fingers often only after retraction of the cuticle, is seen 

 a transversely oval white area, the lunula, which marks the position of the 

 underlying matrix. 



