SECT. 64.] 



HAIRS. 



117 



Fig. 52. 



Thus a diversity of shade arises in the hitherto homogeneous hair-rudiment, 

 and a central, conical mass, broad below and running out into a point above, 

 1. lines distinguished from an external portion, which is narrow below and 

 thick above. When the hair-rudiment is 0-22'" long, this demarcation becomes 

 still more evident, as the somewhat longer, and especially broader, inner cone 

 acquires a clearer aspect. 

 Lastly, in hair-rudiments of 

 o - 22"' long, the iunercone be- 

 comes distinguished into two 

 parts : a central, somewhat 

 darker, and an external, 

 wholly transparent, and hya- 

 line portion, the hair and the 

 inner root-sheath, ; whilst the 

 peripheral cells, which have 

 remained opaque, form un- 

 doubtedly the outer root- 

 sheath (fig. 52). At the same 

 time, the hair-papilla, of which 

 there were before slight traces 

 visible, becomes more dis- 

 tinct, and the proper hair- 

 follicle more perceptible, as 

 the cells which are situated 

 external to its structureless 

 membrane begin to pass into 

 fibres, which may already be 

 recognised by their decussa- 

 tion. The hair-follicles and 



hairs are developed, in other fouicle . n0 riul iments of sebaceous glands are as yet present. 

 places, exactly in the Same «• hair-bulb ; /. hair-shaft ; g. point of the hair ; n. rudiments 



, , , of the sebaceous glands. 



way as in the eyebrows, ex- 

 cept that their formation takes place at a later period. In the fifteenth week, 

 there are no hair-rudiments to be seen, except on the forehead and eyebrows 

 in the sixteenth and seventeenth weeks, they appear over the whole head, 

 back, chest, and abdomen ; in the twentieth week, on the extremities. The 

 hairs themselves are never visible earlier than three to five weeks after the 

 formation of the rudiments. Thus, for example, in the nineteenth week, no 

 hairs are to be seen anywhere in the rudiments, except on the forehead and 

 eyebrows ; in the twenty-fourth week, they are still wanting upon the hand 

 and foot, and, in part, upon the fore-arm and leg. 



When once formed, the hairs and hair-follicles increase in size ; the former 

 sometimes penetrate the epidermis immediately (eyebrows, eyelashes), 

 sometimes, in company with the inner root-sheath which likewise becomes 

 lengthened, they push their points between the horny and mucous layers, 

 or even into the substance of the horny layer itself, and continue to grow for 

 some time longer under the epidermis (chest, abdomen, back, extremities [?] ) 

 which they at last penetrate. Involutions of the skin, which grow towards 

 the penetrating hairs, are never seen, and the notion that such involutions 

 exist, has no real foundation. 



The downy hairs (lani/f/o), the eruption of which is completed by the 



A. Hair-rudiment, with just developed but not erupted hair, 

 of 0'28"' in length. The inner root-sheath somewhat sur- 

 mounts the point of the hair, and laterally, at the neck of the 

 follicle, two papilli-form outgrowths of the outer root-sheath 

 present the first rudiments of the sebaceous glands. B. Hair- 

 follicle from the eyebrows, with a hair which has just erupted. 

 The inner root-sheath projects into the aperture of the hair- 



