THE NAILS. 255 



Nails are, to a considerable extent, hygroscopic, becoming 

 by the imbibition of fluid soft and yielding. 



DEVELOPMENT OF NAILS. 



Nails are developed somewhat differently from the epider- 

 mis of which we have stated that they are a modification ; 

 thus they do not increase by the equal development of new 

 cells on the entire of their under surfaces, but they grow from 

 a point, from the base or root of the nail. 



The reality of this mode of development becomes evident 

 from the following considerations : 



1st. That the younger nail cells are found principally at 

 the root of the nail. 



2nd. That if the relative situations of two spots or stains 

 be observed, it will be seen that during the growth of the 

 nail, they preserve precisely the same relation with each 

 other, only that they gradually approach the end or free 

 margin of the nail, which at length they reach, and from 

 which finally they are worn away. This observation proves 

 the absence of interstitial growth, and shows that the nail 

 increases in length by additions made to the root. 



Although it is certain that the longitudinal growth of nail 

 occurs by the development of cells at the root, yet it is also 

 evident that its thickness is increased by the formation of 

 new cells on its under surface, where they may usually be 

 detected with the microscope in a partially developed state. 

 This double development explains why the nail is thinnest at 

 the root, where only a single method of growth prevails. 



The junction of the root with the body of the nail is in- 

 dicated by a semicircular line, and the former is not merely 

 thinner than the latter, but it is also softer and whiter; 

 whiter in consequence of the subjacent dermis containing 

 in that situation fewer blood-vessels, and its papilla being 

 smaller. 



From the preceding account of the development of nails, 

 it follows that when these sustain any loss of substance on 

 their upper surfaces, this loss is never repaired, but remains 

 without alteration until it reaches the free margin of the nail 



