THE HAIR 



277 



cells which become piled up at the margin of 

 the groove to form an excessive horny layer, 

 the definitive representative of the embryonic 

 eponychium. At the distal extremity of the 

 nail the superficial cells are also accumulated 

 into a considerable mass which forms a promi- 

 nent hyponychium. Further growth of the nail 

 pushes its distal margin forward over the 

 hyponychium so that the border becomes free 

 shortly prior to birth. The peridermal cells are 

 then shed and the nail body finally presents, at 

 about the time of birth, its naked stratum lu- 

 cidum. 



THE HAIR 



Development. The structure of the hair 

 will be most readily appreciated if preceded by 

 a brief introductory sketch of its development. 



THE HAIR GERM. The hairs arise at any 

 time after the third month of fetal life, their 

 earliest anlage appearing as a slightly increased 

 proliferation of the cells of the germinal layer 

 of the epidermis. The further multiplication 

 of the cylindrical cells produces a solid colum- 

 nar ingrowth of the epidermis which pene- 

 trates into, and sometimes through, the primi- 

 tive derma. The spheroidal cells of the inter- 

 mediate layer of the epidermis increase in size, 

 assume a vesicular character, and finally by 

 fatty degeneration form the epidermal hair 

 canal through which the future hair reaches the 

 surface. 



THE HAIR COLUMN. The columnar epi- 

 dermal ingrowths, hair columns or hair pegs, 

 come into early relation with the anlage of the 

 hair papilla which is formed by a proliferation 

 of the mesenchymal cells at the tip of the hair 

 column. Further development of the papilla 



FIG. 270. LONGITUDINAL 

 VERTICAL SECTION OF 

 THE YOUNG NAIL AND 

 NAIL-BED OP AN IN- 

 FANT. 



n. r., nail root or lunula; 

 n., nail; n. b., nail bed; 

 e., eponychium; h., hy- 

 ponychium. (From Dahl- 

 gren and Kepner.) 



