162 SPECIAL HISTOLOGY. 



than the epidermis, whence they derive their hardness : this 

 may be correct, although, as Mulder states ( f Phys. Chemie,' 

 p. 536), both yield about the same proportion of ash (1 per 

 cent.). 



As regards the lamellar structure of the proper nail, it is to 

 be regarded in the same light as that of the horny layer of the 

 epidermis; but it is not so distinct, because the plates of the 

 nail are more intimately connected than the elements of the 

 epidermis. Reagents, however, render the lamellar structure 

 very evident, and it is also clear in pathologically thickened and 

 curved nails. 



§ 51. 



Growth of the Nails. — The nails grow continually, as long as 

 they are cut ; on the other hand, if uncut, their growth is 

 limited. In this case, as may be observed in those who are long 

 confined to their bed by sickness, and in the Eastern Asiatics, 

 the nails become 1 \ — 2 inches long (in the Chinese, according 

 to Hamilton, 2 inches), and curve round the points of the fingers 

 and toes. 



During the growth of the nail, the mucous layer does not 

 change its position at all, but its horny layer is constantly being 

 thrust forward. The formation of the latter goes on continually 

 wherever it is in contact with the stratum Malpighii, in other 

 words upon its whole under surface, with the exception of the 

 free anterior edge ; further, in many nails, upon a very small por- 

 tion of the upper surface of the root, finally, at the posterior 

 edge of the root itself. It is, however, the root portion which 

 grows fastest, whilst the body of the nail is more slowly deve- 

 loped, which is demonstrated especially by the fact that it is not 

 much thinner at the boundary between the root and the body 

 than it is anteriorly upon the body itself, and that the transition 

 of the cells of the stratum Malpighii into nail-cells is easily 

 shown at the root, but with difficulty in the body. By the 

 constant addition of new cells at the edge of the root, the nail 

 grows forwards ; by their addition to its under surface, it is 

 thickened. The longitudinal growth exceeds that in thickness, 

 because the first rounded cells, as they move from behind and 

 below, forwards and upwards, become more and more flattened 

 and elongated. ' 



