THE NAILS. 253 



ART. XL THE NAILS. 



THE horny appendages of the feet and hands, the nails, do 

 not constitute a distinct structure or type of organisation in 

 themselves, but are merely modifications of one which has 

 already been described, viz. the epidermis. 



Nails, therefore, consist of cells similar to those of which 

 the epidermis is itself constituted, with the difference, that 

 they are harder, drier, more firmly adherent to each other, 

 and that in the majority the nucleus is obliterated. (See 

 Plate XXV.) 



To display the cellular constitution of nails some little 

 nicety is required ; it may be shown, however, in the thin 

 scrapings of any fragment of nail submitted to the microscope, 

 as also by soaking the nail in a weak alkaline solution, and 

 which, acting upon and dissolving the intercellular and uniting 

 substance, sets free the cells. 



The cellular structure of nails may also be shown, even 

 without previous preparation, by a careful examination of the 

 root and under surface of the nail, in which situations young 

 and nucleated cells may usually be detected. 



The younger nail cells, like those of the epidermis in the 

 coloured races, contain pigmentary matter. 



Nails, however, are not simply constituted of superimposed 

 and adherent cells, but these are regularly disposed in layers 

 or strata, each of which probably indicates a period of 

 growth. 



These layers, marked by striae, may be clearly seen on any 

 thin section of nail, whether longitudinal or transverse ; they 

 do not appear to follow any very definite course ; usually in 

 a longitudinal slice, they run from above downwards and for- 

 wards ; sometimes they are horizontal, but I have seen instances 

 in which the striae were directed obliquely backwards, in place 

 of forwards. In the transverse section,- the striae are less 

 strongly marked, and run usually more horizontally. (See 



