166 SPECIAL HISTOLOGY. 



tins case, as may be observed in those who are long confined to their 

 beds by sickness, and in the Eastern Asiatics, the nails become 1J- 2 

 inches long (in the Chinese, according to Hamilton, 2 inches), and to 

 curve round the points of the fingers and toes. 



During the growth of the nail, the mucous layer does not change its 

 position at all, but its horny layer is constantly being thrust forward. 

 The formation of the latter goes on continually wherever it is in contact 

 with the stratum Malpighii, in other words upon its whole under sur- 

 face, with the exception of the free anterior edge ; further, in many nails, 

 upon a very small portion of the upper surface of the root, finally, at 

 the posterior edge of the root itself. It is, however, the root portion 

 which grows fastest, whilst the body of the nail is more slowly developed, 

 which is demonstrated especially by the fact that it is not much thinner 

 at the boundary between the root and the body than it is anteriorly upon 

 the body itself, and that the transition of the cells of the stratum Malpigliii 

 into nail-cells is easily shown at the root, but with difficulty in the body. 

 By the constant addition of new cells at the edge of the root, the nail 

 grows forward ; by their addition to its under surface, it is thickened. 

 The longitudinal growth exceeds that in thickness, because the first 

 rounded cells, as they move from behind and below, forwards and up- 

 wards, become more and more flattened and elongated. 



The mode in which the plates of the nail arise from the cells of the 

 mucous layer, is easily demonstrable at the root of the nail. Here, in 

 fact, the uppermost cells of the mucous layer are constructed very dif- 

 ferently from the deeper ones ; they are more or less flattened, and closely 

 resemble the cells of the epidermis, but they possess a nucleus, which, 

 however, is only to be discovered by adding caustic soda, and then with 

 difficulty. If we follow these cells, which form a layer of 0-06-0-12 of 

 a line in thickness, toward the proper substance of the nail, we find 

 that they become more and more flattened, and at last pass without any 

 defined boundary into the latter, uniting together more closely, and 

 taking on a more transparent appearance. 



In the body of the nail, the formation of nail substance is demonstrated 

 with more difficulty, yet here, in opposition to Reichert, we must assume 

 that it does take place, because the nail almost invariably increases in 

 thickness even in the body, from behind forwards. However, there is 

 unquestionably, in this part, a sharper demarcation between the two 

 layers of the nail, than in the root ; but in fine sections it appears by 

 no means so sharp as in those which are commonly examined, and I find, 

 in fact, that the transition of the cells of the mucous layer into the plates 

 upon the body of the nail, is demonstrable with tolerable readiness, par- 

 ticularly on the addition of alkalies, where the ridges of the under sur- 

 face of the proper nail are well developed. Between the ridges also, 

 though no direct transition is recognizable, yet it may be observed that 



