SKIN AND ITS APPENDAGES. 153 



when present, it varies in shape and length. Thus, 

 sometimes it occupies the whole length of the hair ; 

 at others, it ceases at the root ; and again, it fre- 

 quently presents constricted portions, and even entire 

 interruptions (PL XXIV. fig. IV. 6, fig. VII. 3 ; PL 

 XXV. fig. II. 8). The cells by which it is filled con- 

 stitute the medullary substance ; they contain, usu- 

 ally, a very pale nucleus, and fatty looking granules ; 

 according to Kolliker they contain also air bubbles. 



The hair follicle, or sheath, which envelops its root, 

 recalls in its structure that of the skin ; in fact we 

 find, on section of its walls, two layers of connecting 

 tissue similar to those of the skin, and two other 

 strata of cells representing the epidermis. This 

 structure suggests the idea, although the mode of 

 development of the hair proves the contrary, that the 

 hair follicle is formed by an indentation, or an invo- 

 lution, of the skin. The following, however, is its 

 structure : 



Proceeding from without inwards, we recognise : structur 

 1st, a stratum of very loose* connecting tissue, which 

 is continuous with the deep layer of the true skin 

 (PL XXV. fig. II. 1) ; 2d, another stratum, very dis- 

 tinct from the preceding, and similar in structure to 

 the papillary layer of the true skin, with which it is 

 also continuous ; internally this layer is limited by a 

 delicate and structureless basement membrane, as in 

 the skin (fig. II. 2, 3) ; 3d, reposing upon this deli- 

 cate line is the deep epidermic layer, made up of cells 

 whose shape and mode of stratification resemble those 

 of the rete muoosum (fig. II. 4, PL XXVII. fig. V. 4); 

 4th, the internal epidermic layer, which, with its 



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