NAILS 



389 



are low near the root of the nail but increase in height toward its free 

 distal border; there they abruptly give place to the papillae of the skin. 

 The epithelium consists of a stratum germinativum and a stratum corneum. 

 The latter, according to Bowen (Anat. Anz., 1889, vol. 4, pp. 421-450), 

 represents a greatly thickened stratum lucidum, but this opinion requires 

 confirmation. In the embryo the horny substance is entirely covered by 

 a looser layer, the eponychium, and this name is applied in the adult to 

 the skin-like tissue which overlaps the root and sides of the nail (Fig. 391). 

 The eponychium is the stratum corneum of the adjoining skin. 



It is now generally considered that the cells of the stratum germinati- 

 vum covering the greater part of the "nail bed" do not produce any of the 

 overlying horny material. This function is reserved for the germinative 

 cells at the root of the nail, beneath the crescentic white area, the lunula, 

 and its extension backward under the nail fold. The 

 latter is a fold of skin which is deep at the root of the 

 nail, but becomes shallower as it extends forward on 

 either side, bounded by the nail wall (Fig. 391). It is 

 now stated that cornification in the nails takes place with- 

 out the formation of kerato-hyalin granules, and a fibrillar 

 arrangement of the keratin has been thought to account 

 for the whiteness and opacity of the lunula. The corni- 

 fied cells of the nail may be separated by placing a frag- 

 ment in a strong solution of caustic potash and heating to boiling. The 

 cells differ from those in the outer layers of the skin by retaininig their 

 nuclei (Fig. 392). 



FIG. 392 CELLS 

 OF A HUMAN 

 NAIL. X 240. 



Epidermis. 



Epithelial column. 



HAIR. 



Development. The hairs arise as local thickenings of the epidermis. 



They soon become round columns of 

 ectodermal cells extending obliquely 

 downward into the corium (Fig. 393). 

 As the columns elongate the terminal 

 portion becomes enlarged, forming the 

 bulb of the hair, and a mesodermal 

 papilla occupies the center of the bulb. 

 On that side of the epithelial column 

 which from its obliquity may be called 

 the lower surface, there are found two 

 swellings (Figs. 394-396). The upper 

 is to become a sebaceous gland, dis- 

 charging its secretion into the epithelial 

 column; the lower or deeper swelling is called the "epithelial bed," and 



Mesenchyma. 



Mesenchyma. 



Mo E NTHs. K x F 2 A 3^ UMANEMBRYOOF FIVE 



