266 THE SOLIDS. 



hair is included in a sheath consisting of two distinct layers, 

 an outer and an inner. (See Plate XXVIII. fig. 1.) 



The outer layer is an inversion of the epidermis : it first 

 merely encircles the portion of the stem of the hair beneath 

 the level of the epidermis : it next surrounds the inner layer 

 of the sheath, to which it soon becomes intimately adherent ; 

 finally, it forms a cul de sac around the bulb of the hair. 



The fact of the inverted sheath of the epidermis forming a 

 pouch around the bulb of the hair, may be inferred from the 

 circumstance, that when the epidermis peels off as the result 

 of decomposition, the hairs usually come away entire with it ; 

 its continuation moreover around the bulb may be shown in 

 transverse sections of the skin of the axillse and whiskers, in 

 which the hairs penetrate into the subcutaneous fatty sub- 

 stance. (See Plate XXVIII. fig. 1.) 



This outer layer is colourless, is possessed of considerable 

 thickness, and is evidently made up of granular cells similar 

 to young epidermic cells. 



In most hairs, whether coloured or uncoloured, which have 

 been forcibly removed from the skin, this outer layer is usually 

 torn across, the rupture occurring almost invariably at a little 

 distance from the bulb of the hair : the root of the hair then 

 below this breach of continuity consists only of the inner layer 

 of the sheath and of the stem of the hair ; and at this situa- 

 tion, the root to the naked eye appears contracted : this is, 

 however, but an appearance, the result of the absence of the 

 outer layer, and of the expansion of the stem into the bulb. 

 (See Plate XXVIII. fig. 2.) 



The inner layer of the sheath is an eversion or revolution 

 of the epidermis, and is an offset or continuation of the outer 

 layer, commencing at the lower part of the bulb : it is colour- 

 less, possessed of considerable thickness, its diameter being 

 about one-third of that of the stem of the hair in its thickest 

 part : it tears readily in the longitudinal direction with a 

 somewhat uneven fracture ; and hence may be inferred to be 

 of a fibrous constitution, as may be shown to be the case : its 

 inner surface is marked with reticulated lines, the impressions 

 of the cortical scales of the shaft of the hair. 



