150 SKIN AND ITS APPENDAGES. 



of the epidermis, and continuous with them without 

 any line of demarcation (fig. II. 2, 7). In the 

 superficial stratum, the distinctive characteristics in 

 which it differs from the corresponding layer of the 

 cuticle are : the persistence of the nuclei of its cells, 

 the greater transparency of their contents, and a 

 firmer cohesion of the cells to each other. 



The epidermis of the fold at the root of the nail 

 adheres closely to its surface (fig. II. 10), but as its 

 component cells are not exactly identical with those 

 of the surface of the nail, there results a very distinct 

 line of demarcation between the two otherwise con- 

 tinuous layers (fig. II. 4). 



Development. As early as the third month, the groove which 

 receives the root and sides of the nail is apparent. 

 Both the true skin and epiderm become slightly 

 hypertrophied, and by the fifth month, the laminated 

 ridges of the derma have become visible, and" the nail 

 is distinguishable from the surrounding cuticle with 

 which it. is continuous. 



Growth. The growth of the nail takes place at the expense 

 of the rete mucosum, the more superficial cells of 

 which become successively transformed into scales of 

 horn. Its increase in length is effected by the active 

 vegetation of the cells at the bottom of the groove 

 which lodges its root, they becoming continuous with 

 its substance. Whilst it is thus pushed forwards, the 

 deep surface of the nail appropriates a portion of the 

 cells generated from the surface of its matrix, and it 

 thus grows in thickness ; but the growth in length is 

 by far the more rapid of the two processes. 



In the study of its structure, similar preparations 



