OF THE HAIRS. 



183 



Fig. 71. 



addition of acetic acid. They resemble smooth muscular fibres, although 

 they cannot be completely isolated and actually recognized as true fusi- 

 form fibres with a single nucleus ; on which account, and especially as 

 no contractions of the hair-sacs have 

 in general been observed, I must for 

 the present refrain from positively 

 deciding upon their nature. 



The third layer, lastly (Fig. 71 6), 

 is a transparent structureless mem- 

 brane, which, when the hairs are torn 

 out, invariably remains behind in the 

 hair-sac, and extends from its base, 

 though, as it would seem, without 

 covering the papilla, as far as the 

 inner root-sheath, and perhaps higher. 

 In the uninjured hair-sac it appears 

 only as a pale streak 0-001-0-0015, 

 rarely 0-002 of a line thick, between 

 the outer root-sheath and the trans- 

 versely fibrous layer of the hair-sac ; 

 by preparing an empty hair-sac, how- 

 ever, it can readily be obtained in 

 large shreds, and then appears smooth 

 externally ; internally it is covered 

 with very delicate, transverse, often anastomosing lines, which, like the 

 membrane itself, remain unchanged in acids and alkalies. Neither acids 

 nor alkalies bring out cells or nuclei in this membrane, and it therefore 

 probably belongs to the category of true structureless membranes. 



The papilla of the hair (Fig. 63 i) also, less properly termed the hair- 

 germ, pulpapili, belongs to the sac, and corresponds with a papilla of 

 the cutis. It is generally seen but indistinctly, especially in dark hairs 

 with a colored bulb, either appearing, only as a clear, indistinctly defined 

 spot, or after the tearing out of the hair, remaining so covered by the 

 cells of the bulb that nothing can be made out of it. It is only in the 

 hair-sacs of white hairs, that its outlines can be more frequently distin- 

 guished without wholly isolating it, especially by the help of a little 

 pressure. Reagents, on the other hand, avail nothing, for they attack 

 the papilla to about the same extent as the bulb, with the sole exception 

 of a weak solution of caustic soda, in which it retains its outlines, for a 



Fio.71. A piece of the transverse fibrous layer, and of the structureless membrane 

 (vitreous membrane) of a human hair-sac, treated with acetic acid ; magnified 300 diame- 

 ters: a, transversely fibrous layer with elongated transverse nuclei ; 6, vitreous membrane 

 in apparent section; c, its edges, where the sheath which it forms is torn; d, fine transverse 

 partly anastomosing lines (fibres) on their inner surface. 



