The skin is not only the inside and outside border of the body, but also has a role to control the amount of body fluids and the condition of heat by getting loosened in summer and tightened in winter. If two thirds of the gross area of the whole skin is covered by something like tar, the body is filled with heat, and the person dies.
Apart from nails grown too long, nails are together with the skin. So, the skin is also related with adjusting the heat and other roles, if you consider that the fingertips are sensitive as antennas of the body, you should better not to fill the nail such as a nail polish by nature.
*This article is translated from "Yamai hitokuchi Memo" written by Shoji Kobayashi, 2005.