280 



NORMAL HISTOLOGY. 



FIG. 315. 



cylindrical epithelial mass assume the character of the external root- 

 sheath. Subsequent differentiation in the central mass of 

 formative cells produces the individual layers 

 of the inner root-sheath and of the hair. 

 The young hair, or lanugo, at first lies com- 

 pletely embedded within the epidermis, 

 its exit being opposed by the cells occupy- 

 ing the neck of the follicle ; these cells 

 soften and undergo fatty degeneration, when 

 the young hair forces its way against the 

 superficial epithelial layers. The epidermal 

 scales at first are raised, but afterwards they 

 are broken through by the pointed ex- 

 tremity of the growing hair-shaft. The 

 eruption of the hairs on the head and the 

 eyebrow occurs about the close of the fifth 

 month of fcetal life, and is completed about 

 the sixth month on the extremities. The 

 fcetal hairs, forming the downy covering, 

 or the lanugo, never possess a medulla, and 

 are short-lived, ceasing to grow towards 

 the end of gestation ; shortly after, or even 

 before, birth these embryonal hairs are 

 largely shed and replaced by more per- 

 manent successors ; on the face and a few 

 other places, however, the lanugo remains. 

 The general renewal of the hairs after birth 

 corresponds to the periodical change of coat 

 so common among the lower animals ; such 

 renewal is very unusual in man, the replace- 

 ment of the effete hairs continually taking 

 place. As soon as the growth of a hair 

 is arrested the pressure induced by the 

 surrounding soft elastic structures is no 

 longer resisted, and in consequence the 

 hair is separated and lifted from its papilla ; 

 such hairs possess knob-like extremities, 

 which are lodged in corresponding expan- 

 sions of the outer root-sheath. Beneath 

 the terminal knob the cells of this outer 

 root-sheath grow out as a new mass 

 towards the base of the follicle ; from these 

 young cells in due time the new hair is formed, the details of the 

 process corresponding with those of the development of the primary 



Section of hair-follicle from 

 human scalp, exhibiting changes 

 accompanying growth of new 

 hair: a, old hair, terminating in 

 expanded degenerating end (a')\ 

 b, inner root-sheath ending in 

 atrophic area at b' ; c, outer root- 

 sheath ; e, glassy membrane ; f t 

 lateral projection marking attach- 

 ment of arrector pili muscle (g ) ; 

 h, mass of new cells derived from 

 root-sheath of old follicle from 

 which formation of new hair will 

 proceed. 



