DEVELOPMENT OF HAIR, 



255 



little hair ; and the residuary 

 cells contained within the rudi- 

 mentary follicle form the root- 

 sheath, the inner layer of which 

 (/), lying next to the hair, is 

 soon distinguished by its trans- 

 lucency from the more opaque 

 outer layer (c) that fills up the 

 rest of the cavity. The young 

 hair, continuing to grow, at last 

 perforates the epidermis (c), 

 either directly, or after first slant- 

 ing up for some way between the 

 Malpighian and the horny strata. 

 In the former case it may, per- 

 haps, be aided in its progress by 

 the harder inner layer of the 

 root-sheath, which accompanies 

 the hair, and makes way for it 

 through the cuticle. Prof. Kol- 

 liker further thinks it not impro- 

 bable that the eruption of the 

 hairs is facilitated by the gene- 

 ral desquamation and shedding 

 of the superficial part of the epi- 

 dermis, which occurs from time 

 to time during foetal life ; more 

 especially as the period of most 

 thorough desquamation begins 

 at about the same time as the first 

 eruption of hairs. A shedding of 

 the first-formed hairs, or lanugo, 

 is known to take place before 

 birth ; but according to Prof. 

 Kolliker, only to an inconsidera- 

 ble extent. On the other hand, 

 he has observed that the infan- 

 tile hairs are entirely shed and 

 renewed within a few months 

 after birth ; those of the general 

 surface first, and afterwards the 

 hairs of the eyebrows and head, 

 which he finds in process of 

 change in infants about a year 

 old. The new hairs are gene- 

 rated in the follicles of the old, 

 as previously seen, by Heusinger 

 and Kohlrausch, in quadrupeds ; 

 but Kolliker describes the steps 

 of the process somewhat differ- 

 ently from his predecessors. He 

 conceives that an increased 

 growth of cells takes place in the 

 soft hair-bulb and in the adjoin- 

 ing part of the root-sheath (Fig. 

 46, A.) ; the growing mass pro- 



Fig. 45. 



Development of the Hair in the eyebrows : A, first 'distinct 

 separation of the inner and outer portions of the hair-matrix; 

 B, first formation of the hair, whose point has not yet appeared 

 above the skin ; c, the hair soon after its first emersion : 

 a, horny layer of the epidermis ; b, its mucous layer ; c, outer 

 root-sheath ; d, inner root-sheath ; e, hair-bulb ; /, hair-shaft ; 

 g, point of the hair ; h, hair papilla ; i, structureless mem- 

 brane on the exterior of the matrix ; n, commencement of the 

 sebaceous glands. 



Fig. 46. 



Development of second Eyelashes in an Infant of a year old : 

 A, incipient formation of matrix of second hair ; B, incipient 

 development of the young hair, the outer and inner portions 

 not yet distinct ; c, the young hair, more advanced, and push- 

 ing up the old hair ; its proper substance distinct from the root- 

 sheath ; D, the young hair emerged from the opening, and its 

 predecessor about to fall out : a, external root-sheath ; b, in- 

 ternal root-sheath of young hair ; c, cavity for the reception 

 of the formative papilla ; d, bulb of the old hair ; e, its shaft ; 

 /, bulb of the young hair ; g, its shaft ; h, its point ; i, i, se- 

 baceous glands ; k, k, sweat-canals ; I, passage of the external 

 root-sheath into the mucous layer of the epidermis ; m, first 

 appearance of young hair. 



