THE NAILS. 395 



These cells correspond here to the basilar layer of the stratum Mal- 

 pighii. The stratum granulosum is not uniformly present, although 

 occurring as isolated areas in the region of the nail root and lunula, 

 the white area of demilunar shape at the proximal portion of the 

 nail. Unna has demonstrated that the pale color of the lunula and 

 root of the nail is due to the presence of keratohyalin. Formerly, 

 this peculiarity was attributed to a difference in the distribution of 

 the vessels in the various portions of the nail bed. The body of 

 the nail, with the exception of the lunula, is transparent a con- 

 dition which may be explained by the fact that the elements of the 

 nail correspond to those of the stratum lucidum. As a consequence, 

 the vessels of the matrix shine through, except at the lunula, where 

 the keratohyalin granules render the nail opaque. 



The nail itself consists of elements homologous to those of the 

 stratum lucidum. They are flat, transparent cells, closely approxi- 

 mated, and all contain nuclei. The cells overlie each other like 

 tiles, and are so arranged that each succeeding lower layer projects 



Corium. 



. J Blood-vessel . 



Fig. 318. Transverse section through human r.ail and its sulcus ; X 34- 



a little further distalward than the preceding. At the period when 

 the nails are formed, about the fourth month of fetal life, sulci are 

 already present. The first trace of the nail is seen as a marked 

 thickening of the stratum lucidum in the region which later be- 

 comes the body of the nail ; in this stage the structure is still cov- 

 ered by the remaining layers of the stratum corneum, constituting 

 the eponychium. The embryonal nail then spreads in all directions 

 until it finally reaches the sulcus. Henceforward the growth is 

 uniform. The eleidin normally present in the stratum lucidum of 

 the skin also occurs in the nail, and is derived, as we have already 

 seen, from the keratohyalin. It may readily be conceived that later, 

 when growth is confined to the root of the nail, keratohyalin is also 

 present. As soon as the nail begins to grow forward, in the ninth 

 month, the greater, part of the eponychium is thrown off; but 

 during the entire extrauterine life, a portion of the eponychium is 

 retained at the nail wall, and as hyponychium on the anterior and 

 under surface of the nail. 



