164 SPECIAL HISTOLOGY. 



of the nail goes on especially at its root, yet that plates 

 are added to the body of the nail from below, though more 

 slowly and scantily, thus producing the anterior thickening, or 

 at least preventing the necessary thinning of the nail anteriorly. 

 It is to be remarked further, that the development of nail- 

 substance takes place in all parts of the middle line of the nail 

 more rapidly than in the lateral portions, which, anteriorly, are 

 almost as thin as in the root, though they possess longer 

 processes below. But even there, substance must be added to 

 the body of the nail, because it becomes broader anteriorly. 



The plates of the substance of the nail once formed, alter in 

 certain respects, as they are pushed forwards and upwards by 

 those which come after them. In the first place, their nature 

 becomes altered in a manner which is little understood, the 

 change consisting partly in the deposition of more phosphate of 

 lime, partly in a solidification (conversion into horn) of their 

 organic elements, particularly of the cell-membranes, in conse- 

 quence of which, from being soft, as at the root and under surface 

 of the nail, they become gradually harder and harder. In the 

 second place, like the horny cells of the epidermis, they are very 

 considerably flattened, and at the same time increase somewhat 

 in their longitudinal and transverse diameters; finally, they 

 coalesce more completely, so that they cannot be separately 

 recognised, without the action of reagents, in the upper and an- 

 terior parts of the nail, which appear to be composed of nothing 

 but a homogeneous substance which tears in all directions ; 

 whilst in the lower parts the separate nail-plates are, at least indi- 

 cated, and are occasionally tolerably distinct. On the other hand, 

 the nuclei of the nail-plates do not disappear, and in this lies 

 a characteristic distinction between the horny layer of the nail 

 and that of the epidermis. They are to be seen in perpendicular 

 sections, of fresh nails, and after treatment with caustic soda, 

 and even in the most superficial layers, though somewhat 

 smaller and flatter than in the deep layer. It follows then, that 

 certain metamorphoses go on in the proper substance of the 

 nail, which as in the epidermis, are to be ascribed to a 

 peculiar growth and vital process in the nail-cells themselves. 

 These seem to occur, however, almost solely in the lower and 

 posterior parts of the nail, for if, as Schwann states (fig. 91), 

 two points be marked upon the posterior portion of the free 



