KIDNEYS, SKIN, AND GENERAL EXCRETION. 401 



the flesh and the transition from the skin is so gradual as to 

 be indeed difficult to follow. The fibrous nature of the 

 corium may be a matter of easy observation on any piece of 

 leather, a torn edge here displaying frequently very satis- 

 factorily the tanned shreds of fibers which go to make up 

 the material. The process of tanning consists in nothing 

 more than taking plexuses of connective fibers, forming the 

 corium or true skin, and treating these with a substance 

 called tannin, by means of which they are hardened and 

 transformed into a substance which increases their strength 

 as well as their resistance to decay. It need not be added 

 that the epidermis does not figure at all in this process. 



SPECIAL MODIFICATIONS OF THE EPIDERMIS. 



1. Nails. The epidermis is in certain portions of the 

 body specially modified to form hairs or nails. A nail is a 

 portion of very much-thickened epidermis, the component 

 cells of which are much more closely packed, and the chem- 

 ical change of which into keratin or horn is more complete. 

 The posterior part of a nail is concealed in a groove of the 

 skin, and it is at this point that the nail increases in length 

 by the formation of new epithelial cells. The nail grows in 

 thickness by the proliferation of cells from the bed of the 

 nail, the thickest portion of the nail being the exposed end. 

 The nail usually presents near its root a lens-shaped white 

 area known as the lunula. This whitish appearance is due 

 to an opacity at this point, resulting from the thickness of 

 the nail bed immediately under it, the cells of which are in 

 very active division to increase the thickness of the nail. 

 The cells of the root and bed of the nail, in active process 

 of division are soft, while those longest formed and next the 

 surface or end of the nail are in increasing degrees horny, 

 differing, however, from the horny cells of the ordinary 

 epidermis in the fact that they seem to possess spiny pro- 

 cesses, by means of which they are firmly interlocked and 

 united. 

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