182 SPECIAL HISTOLOGY. 



59. The hair-sacs, folliculi pilorum, are flask-like follicles 1-3 

 lines 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 subcu- 

 taneous cellular tissue. These follicles are simply to be regarded as invo- 

 lutions of the skin, with its two constituents, the corium and the epi- 

 dermis^ 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, 

 since it immediately surrounds the root of the hair, the "root-sheath" 

 (vagina pili). 



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

 ternal and an internal, and of a structureless membrane ; it is on an 

 average 0-015-0-022 of a line thick, and contains in its lower part a 

 peculiar structure, the papilla of the hair. 



The external fibrous membrane (Fig. 63 A), 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 longitudinal 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 a long narrow nuclei, which may be seen particularly well 

 in the empty hair-sacs of both coarse and fine hairs, with or without the 



the very darkest hairs ; lastly, under favorable circumstances, this reagent raises up a definite 

 basement membrane from the outer surface of the lowest part of the bulb, in immediate 

 contact with the rounded " nuclei" of this part, and this basement membrane may be traced 

 upwards into direct continuity with the homogeneous portion of the cuticle above-described. 

 (In the " Edinburgh Monthly Journal of Medical Science/' for March, 1853, Mr. Dalzell 

 states that the papilla of the hair has a basement membrane. Is it this structure to which 

 he refers ?) In all cases in which, in man or in animals, we have isolated the hair-bulb 

 from its sac, it seemed to have a definite limiting outer line down to the narrow neck by 

 which it passes into the hair-sac, though it was not often easy to obtain evidence that this 

 limiting line was the expression of a distinct basement membrane. However, the same 

 difficulty would occur with any dermic papilla ; and it seems to us that there is sufficient 

 evidence to show that the cuticle of the hair is not the product of any direct metamorphosis 

 of cells, but represents a modified basement membrane with a subjacent layer of peculiarly 

 altered blastema, corresponding precisely with the " Nasmyth's membrane" and the enamel 

 of the teeth. Vide infra, on Teeth. TES.] 



