120 EXCRETION. 



The connections of the nails with the true skin resemble 

 those of the epidermis ; but the relations of these structures 

 to the epidermis itself are somewhat peculiar. Up to the 

 fourth month of foetal life, the epidermis covering the dorsal 

 surfaces of the last phalanges of the fingers and toes does 

 not present any marked peculiarities ; but at about the 

 fourth month, the peculiar hard cells of the horny layer of 

 the nails make their appearance between the Malpighian 

 and the horny layer of the epidermis, and at the same time 

 the Malpighian layer beneath this plate, which is destined 

 to become the Malpighian layer of the nails, is somewhat 

 thickened, and the cells assume more of an elongated form. 

 The horny layer of the nails constantly thickens from this 

 time ; but until the end of the fifth month, it is covered by 

 the horny layer of the epidermis. After the fifth month, 

 the epidermis breaks away and disappears from the sur- 

 face ; and at the seventh month, the nails begin to increase 

 in length. Thus, at one time, the nails are actually included 

 between the two layers of the epidermis ; but after they 

 have become developed, they are simply covered at their 

 roots by a narrow border of the horny layer, the epidermis 

 commencing again under the nail where the free border 

 leaves the bed. The nails are therefore to be regarded as 

 modifications of the horny layer of the epidermis, possessing 

 certain anatomical and chemical peculiarities. The Malpig- 

 Tbian layer of the nails is continuous with the same layer 

 of the epidermis, but the horny layers are, as we have seen, 

 distinct. 



One of the most striking peculiarities of the nails is in 

 their mode of growth. The Malpighian layer is stationary, 

 but the horny layer is constantly growing, if the nails be 

 cut, from the root and bed. It is evident that the nails 

 grow from the bed, as their thickness progressively increases 

 in the body from the root to near the free border ; but their 

 longitudinal growth is by far the more rapid. Indeed, the 

 nails are constantly pushing forward, increasing in thickness 



