180 SPECIAL HISTOLOGY. 



upper part, arranged perpendicularly. At the bottom of the 

 hair-sac, the outer root-sheath, its cells becoming gradually 

 rounded, passes continuously, and without any sharp line of 

 demarcation, into the round cells of the hair-bulb which cover 

 the papilla. The outer root-sheath is generally about 3 — 5 

 times as thick as the inner ; but not unfrequently it becomes 

 thinned towards its upper part, and below invariably passes 

 into a very thin lamella, In the coarse hairs it measures 

 in the middle of the root 0*018 — - 03'", and presents 5 — 12 

 layers of cells. 



The inner root-sheath (fig. 68 — e. g.) is a transparent mem- 

 brane which extends from almost the very bottom of the hair- 

 sac, over more than two thirds of it, and then suddenly ceases. 

 It is closely connected externally with the outer root-sheath, 

 internally with the cuticle of the hair (its outer layer), so that 

 normally there is no space between it and the hair ; further it 

 is distinguished by its great density and elasticity, and it 

 consists in all but its lowermost part, of two or even three layers 

 of polygonal, elongated, transparent, and somewhat yellowish 

 cells, all of which have their longitudinal axes parallel to that 

 of the hair (fig. 68). The outermost layer (fig. 72, A), which 

 alone was formerly known, the inner root-sheath of Henle, is 

 formed of elongated cells without nuclei, 0*016 — , 02 /// in 

 length, and 0004 — 0-006'" in breadth, which are intimately 

 connected, and in the ordinary mode of investigation, after the 

 addition of acetic acid, caustic soda or potass, which swell up 

 the hair, or after the hair has been teased out, present elon- 

 gated fissures between them, whence they appear like a 

 fenestrated membrane. In quite recent hairs, however, if all 

 reagents and mechanical injury have been avoided, we see 

 hardly any trace of apertures in the upper half of the layer in 

 question, and in the lower half (from the finely fibrous part of 

 the cortex upwards), at most mere indications of them, in the 

 form of striae, clear or dark, according as they are in or out of 

 focus, and similar to those of the cortex of the shaft. We can 

 hardly avoid supposing, therefore, that the openings as they are 

 commonly seen (0005 — 0-008'" in length, and 0-001 — 003"' in 

 breadth), are produced artificially by the teasing out of the 

 membrane. Secondly, cells also occur in the root-sheath, 

 between which gaps are never visible. These (fig. 72, B) 



