242 SKIN, HAIR, AND NAILS, BY ALFRED BIESIADECKI. 



and forms the so-called neck. The excretory duct of the seba- 

 ceous follicle opens at this point. 



Proceeding from the neck of the hair follicle towards its 

 dilated extremity, we distinguish (a) the hair sac or follicle, 

 and (6) the root-sheath. 



The hair sac consists of three layers, the external, the middle, 

 and the internal. 



The external sheath of the hair follicle (d) (external fibrous 

 membrane of Kolliker) consists of compactly arranged fibres 

 of connective tissue running parallel to the axis of the hair, 

 and intimately united above to the fibres of the corium, whilst 

 inferiorly they surround the dilatation of the hair follicle, and 

 accompany for some distance the bloodvessels distributed to 

 the papilla. Externally this sheath, losing its defined contour, 

 passes gradually into the connective tissue of the corium, whilst 

 it appears to form an independent sheath, having a thickness 

 of 0'02 of a millimeter, to that part only of the hair follicle 

 that is imbedded in the panniculus adiposus. 



Within this sheath are two longitudinal bloodvessels (a 

 small artery and vein), that with their transverse anastomoses 

 form a trellis- work surrounding the hair follicle, together with 

 here and there a dichotomously dividing mcdullated nerve fibre. 

 The middle layer of the hair follicle is the internal sheath of 

 the follicle (c), the internal fibrous membrane of Kolliker. It 

 is composed of a few transverse fibres of connective tissue, 

 between which is a homogeneous, slightly granular material, 

 containing numerous rod-shaped, transversely arranged nuclei. 

 After treatment with nitrate of silver, oblong areas bounded 

 by sinuous lines may be distinguished at numerous points 

 (Czerny). The nature of these areas has not yet been satis- 

 factorily determined. They resemble smooth muscular fibres 

 in their fusiform shape and rod-like nucleus, whilst the cir- 

 cumstances that they can only be isolated with difficulty, and 

 that they swell up in water without becoming cloudy, speak 

 in favour of their being of the nature of connective tissue 

 (Henle, Kolliker). 



The strong contraction of this sheath that occurs, for ex- 

 ample, when the hair falls out, demonstrates its muscular 

 nature (fig. 196). 



