THE NAILS. 251 



appearing in transverse sections as pointed processes, 75^3 to gj^ 

 of an inch in length, and even -5^ to ^J^ of an inch at the edge of 

 the nail. The upper surface also frequently shows distinct parallel 

 longitudinal streaks, appearing as the almost effaced impressions of 

 the laminae below. 



The nail increases in thickness from the root to near the free 

 edge, being at least three times as thick anteriorly as posteriorly 

 (iff to "sS f an incLf). 



Unprepared sections of nails give very little indication of any 

 structure whatever. But on boiling them in dilute caustic soda, they 

 at once display a beautiful arrangement of cells, like a scaly epi- 

 thelium, but nucleated; these being flatter in the superficial than 

 in the deeper layer, and not more than one half as thick. One or 

 several layers of these cells constitute a lamella; and the lamellas 

 closely united and not sharply defined, form the whole of the horny 

 substance. The greater hardness of nails as compared with epi- 

 dermis, is said by Lauth to be due to a greater proportional amount 

 of phosphate of lime in the former. 



The nails continue to grow only so long as they are cut. Re- 

 maining uncut, they attain to the length of one and a half to two 

 inches, and curve over the ends of the fingers and toes. Among 

 the Chinese, of whom the literary class never cut the nails, the 

 length is, according to Hamilton, two inches. The growth takes 

 place at the expense of the cells in the Malpighian layer, both at the 

 edge of the root and under the body of the nail. Thus, the latter 

 becomes longer and thicker at the same time. The longitudinal 

 growth is, however, by far the most rapid ; since the first round 

 cells become more and more flattened and elongated as they move 

 forwards and upwards from their first position. 



The time necessary for a nail to grow its whole length, varies in 

 different parts from twelve to twenty or more weeks ; and hence 

 this length of time is required for the formation of a new nail. 1 

 The nail is thicker on its most convex portion than at its edges. 



If the changes in the nail cells are investigated as compared with 

 those of the epidermis, a striking similarity is discovered. 1. The 

 original cell-membranes (those of the Malpighian layer) become 



1 According to M. Beau, the nails of the fingers grow four times as rapidly as 

 those of the toes ; the thumb growing two-fifths of a line per week, and its whole 

 length in twenty weeks while the nail of the great toe requires ninety-six weeks, 

 or nearly two years, to grow its length. The portion of a nail growing during a 

 disease is thinner than the rest, as is shown by a transverse groove or depression 



