THE EPIDERMIS. 



37 





Fig. 38.— Cells of the 

 horny layer of the nail ; 



a, a, view from above ; 



b, b, seen from the side. 



The nail consists of two strongly demarcated layers, a deep 

 and a superficial. The former is the ordinary 

 Malpighian mucous reticulum, such as is 

 shown by any other part of the skin. The 

 superficial layer, corresponding to the horny 

 layer of the epidermis, has gone through the 

 process of hornification in a much higher 

 degree than occurs elsewhere. At the first 

 consideration, we perceive only a brittle, 

 homogeneous substance. The power of refrac- 

 tion of all its constituents has 

 become equal. Reagents, and 

 above all, solutions of the alka- 

 lies, are here of inestimable 

 value. With them we dissolve 

 the cement substance and re- 

 store a recognizable appear- 

 ance to the cell. The latter, an 

 originally flat thing, measures 

 0.0375 to 0.0425 mm., but, dif- 

 fering from the ordinary epider- 

 mis cell, contains a lens-shaped 

 granular nucleus, as may be 

 readily recognized from Fig. 38, 

 a and b. 



Concerning the duration of the 

 life of the nail cell, see p. 12. 



The highest variety of epider- 

 moidal tissue is represented in 

 man, by the hair. 



An extreme complication of 

 structure welcomes us here, all 

 at once. 



The hair (Fig. 39), lies in an 

 obliquely directed sac, an invo- 

 lution of the corium, and fre- 

 quently likewise of the subcutaneous cellular tissue 



Fig. 39. — Human hair ; <?, the sac : /', its 

 hyaline inner layer ; r, the external, d, the 

 inner root-sheath ; e, transition of the outer 

 sheath to the hair-bulb ; /, epidermis of the 

 hair (at_/, in the form of transverse fibres) ; 

 g, lower portion of the same ; //. cells of the 

 hair-bulb ; /, the hair papilla; ; k, cells of the 

 medulla ; /, cortical layer ; m, medulla con- 

 taining air ; n. transverse section of the lat- 

 ter ; o, the cortex. 



The 



