276 



THE SKIN 



forms the nail matrix of Eanvier. In this portion also is a distinct 

 stratum granulosum, a layer which is absent or rudimentary beneath 

 the body of the nail. It is the presence within this layer of numerous 

 keratohyalin granules which renders the root of the nail opaque and 

 thus forms the dull white hunt/a which contrasts with the transparent, 



eleidin-eontaining, stratum 

 lucidum, which latter layer 

 alone covers the germinal 

 layer of the nail body (Unna). 

 The Nail Bed. The nail 

 rests upon a very vascular co- 

 rium or nail bed (nnilnx of 

 authors) which is continuous 

 with the eorium or derma of 

 the skin. The nail bed is some- 

 times regarded as active in nail 

 formation, but this process is 

 now generally believed to be 

 limited to the matrix of the 

 root. The nail bed at the 

 margins of the nail is provided 

 with papillae as in other por- 

 tions of the skin, but beneath 

 the body of the nail its surface 

 is raised into longitudinal 

 ridges which possess only very 

 minute secondary papillae. 



Nail Growth. The growth 

 of the nail occurs in the matrix 



FIG. 269. TRANSECTION THROUGH THE MAR- 

 GIN OF A FINGER NAIL. 



On the left is the skin, on the right the nail. 

 a, a', horny layer; b, b', germinal layer; c, <', 

 corium; d, margin of the nail; s, nail sulcus. 

 Moderately magnified. (After von Brunn.) 



of the nail root. The cells of 



the stratum germinativtim of this portion, having been once formed by 

 active mitosis push obliquely forward and outward toward the nail body. 

 It is thus that the more advanced are constantly carried onward toward 

 the free border. The growth of the nail occurs at the rate of about one 

 thirty-second of an inch per week (Schafer). 



Development. In the fetus the nail appears as a direct formation 

 of the epidermis, which is very early evidenced by a thickening of 

 the stratum lucidum in the nail area. The nail is therefore at first 

 covered by the superficial pcridermal cells of the cuticle. The nail groove 

 is rapidly formed by an invasion of the mesoblast by the epidermal 



