OF THE LI A IKS. 183 



§ 59. 



The hair-sacs, folliculi pilorum, are flask-like follicles 

 1 — 3'" long, which embrace the roots of the hair tolerably 

 closely, and, in the lanugo, are lodged in the substance of the 

 upper layers of the corium, while in the stronger or long hairs, 

 they generally project into its deeper portion, and even extend 

 for a greater or less distance into the subcutaneous cellular 

 tissue. These follicles are simply to be regarded as involutions 

 of the skin, with its two constituents, the corium and the 

 epidermis, and there may be distinguished, therefore, in each of 

 them an external fibrous, vascular part, the proper hair-sac, and 

 a non-vascular, cellular investment lining this, — the epidermis 

 of the hair-sac ; or, siuce it immediately surrounds the root of 

 the hair, — the "root-sheath" {vagina pili). 



§ 60. 



The proper hair-sac consists of two fibrous investments, an 

 external and an internal, and of a structureless membrane ; it 

 is on an average 0*03 5 — 0'022'" thick, and contains in its 

 lower part a peculiar structure, the papilla of the hair. 



The external fibrous membrane (fig. 63 h), the thickest of the 

 three layers of the hair-sac, determines its external form, and 

 by its innermost layer is very closely connected with the 

 corium. It consists of common connective tissue with longi- 

 tudinal fibres, without any intermixture of elastic fibres, but 

 with a considerable number of long fusiform nuclei; it contains 

 a tolerably close plexus of capillaries, and exhibits also a few 

 nervous fibrils with occasional divisions. 



The internal fibrous membrane (fig. 71 a) is much more 

 delicate than the external; bounded by smooth surfaces, and 

 everywhere of equal thickness, it extends from the bottom of 

 the hair-sac as far only as the entrance of the sebaceous 

 glands. To all appearance, it contains neither vessels nor 

 nerves, and is composed solely of a simple layer of transverse 

 fibres, with long narrow nuclei, which may be seen particularly 

 well in the empty hair-sacs of both coarse and fine hairs, with 

 or without the addition of acetic acid. They resemble smooth 

 muscular fibres, although they cannot be completely isolated and 

 actually recognised as true fusiform fibres with a single nucleus; 



