184 



SPECIAL HISTOLOGY. 



Fig. 71. 



.a 



on whicli account, and especially as no contractions of the hair- 

 sacs have in general been 

 observed, I must for the pre- 

 sent refrain from positively 

 deciding upon their nature. 



The third layer, lastly (fig. 

 71 b), is a transparent struc- 

 tureless membrane, 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 in- 

 ner root-sheath, and perhaps 

 higher. In the uninjured 

 hair-sac it appears only as a 

 pale streak 0-001 — 0-0015'", 

 rarely 0-002" thick between 

 the outer root-sheath and the 

 transversely fibrous layer of 

 the hair- sac ; by preparing. an 

 empty hair- sac, however, 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, pulpa pill, belongs to the sac, and corresponds 

 with a papilla of the cutis. It is generally seen but indistinctly, 

 especially in dark hairs with a coloured 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 



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

 (vitreous membrane) of a human hair-sac, treated with acetic acid, x 300 : a, traus- 

 versely fibrous layer with elongated transverse nuclei; b, vitreous membrane in ap- 

 parent section; c, its edges, where the sbeath which it forms is torn; d, fine traus- 

 \< rse partly anastomosing lines (fibres) on their inner surface. 



