THE NAILS 



299 



C). The nail fold also extends laterally on either side of the nail anlage and forms 

 the lateral nail fold of the adult {A, B). 



The material of the nail is developed in the lower layer of the proximal nail 

 fold (C). In certain of the epidermal cells, which according to Bo wen represent a 

 modified stratum lucidum, there are developed keratin or horn fibrils during the 

 fifth month of fetal hfe. These appear without the previous formation of kerato- 

 hyahn granules as is the case in the cornification of the stratum corneum. The 

 cells flatten and form the plate-like structure of which the soHd substance of the 

 nail is composed. Thus the nail substance is formed in the proximal nail fold as 



--■Sole plate 



Eponychium 



Nml plate 



Nail fold 



~^ Natl bed 



Fig. 303. — Figures showing tlie development of the nail. A, From a 40 mm. human fetus (X 20); 

 B, from a 100 mm. fetus (X 13); C, longitudinal section through the nail anlage of a 100 mm. fetus 

 (X 24). (KoUmann.) 



far distad as the outer edge of the lunula (the whitish crescent at the base of the 

 adult nail). The underlying epidermis distal to the lunula takes no part in the 

 development of the nail substance. The corium throws its surface of contact with 

 the nail into parallel longitudinal folds which produce the longitudinal ridges of 

 the nail. The nail is pushed toward the tip of the digit by the development of 

 new nail substance in the region of the nail fold. The stratum corneum and the 

 epitrichium of the epidermis for a time completely cover the nail matrix and are 

 termed the eponychium (Fig. 303 C). Later, this is thrown off, but a portion of 

 the stratum corneum persists during Hfe as the curved fold of epidermis which 

 adheres to the base of the adult nail. During hfe the nail constantly grows at its 



