What is superfluous is expelled from the stronger parts of the body, and cannot remain anywhere, it arrives at whatever is the weakest point...The skin is created not for any specific activity, but for its use alone, so it should be expected that it is weaker than those parts that do something. It can be likened to some natural covering or clothing on the body, but affording neither digestion, nor assimilation of food, nor conversion into blood, nor pulsation, nor respiration, nor self-generated movement, nor in short any activity in the body. At the same time, since it is situated outside everything else, it receives the waste products of the whole body.

Galen, On the Causes of Diseases, from Grant M. Galen on Food and Diet. Routledge, London and New York, 2000. P. 55