33 



THIRD LECTURE. 



sac shows externally (Fig. 39, a ; 40, i) longitudinally, then 

 transversely arranged connective tissue (40, //), and, finally, 

 internally (39, b ; 40, g), a hyaline boundary layer. At the 

 bottom it juts forward as a vascular papilla (Fig. 39, i). It 

 is the formative and nutritive organ of the whole. 



On the hair itself we distinguish the hair-bulb (Fig. 39, h), 

 resting on the papilla and the shaft (/). Only a slight portion 



of the latter is surrounded by the sac ; 

 the larger, often very long, remaining 

 portion, projects freely from the skin, 

 as, for instance, the hair on women's 

 heads. As the corium is involuted, 

 so also does the epidermis with its 

 two layers, the rete Malpighii and the 

 horny layer, dip down into the sac. 

 These depressions are very appropri- 

 ately named root-sheaths ; and the in- 

 sinuated rete Malpighii is distinguish- 

 ed as the external root-sheath (Fig. 

 39, c, 40, e), the horny layer of the 

 sac as the internal (Fig. 39, d). The 

 former requires no further description, 

 it presents nothing special ; but the 

 latter acquires a modified structure. 

 It consists of two layers of hyaline 

 cells, an external layer of vertically 

 arranged non-nucleated elements, of 

 0.0377 to 0.0451 mm. (Fig. 40, d), 

 between which longitudinal clefts may 

 ; c, inner, and rf, outer layer b e perceived, and an inner layer of 



: inner root-sheath ; c. outer *■ J 



root-sheath ■/, its peripheral layer ra dially arranged cells with nuclei (c). 



of elongated cells : g, hyaline jo \ / 



membrane of the hair sac ; a, its Downwards, in the depth of the sac, the 



middle, and i, external layer. r 



external root-sheath becomes single. 



The substance of the hair bulb (Fig. 39, //) shows the same 



cells as the external root-sheath, with colorless or pigmented 



molecules. Upwards the appearance changes ; it then 



comes, or at least very frequently, to a differentiation into 



Fig 40. — Transverse section 

 through a human hair and its fol- 

 licle ; a, hair; b, epidermis uf the 

 same 

 of the 



