This transparency is a matter of ocular demonstration in white people.

It is conspicuous in every blush; for no one can imagine, that the cuticle becomes red, as often as this happens: nor is it less discoverable in the veins, which are so easy to be discerned; for no one can suppose, that the blue streaks, which he constantly sees in the fairest complexions, are painted, as it were, on the surface of the upper skin.

From these, and a variety of other observations[081], no maxim is more true in physiology, than that _on the mucosum corpus depends the colour of the human body_; or, in other words, that the _mucosum corpus_ being of a different colour in different inhabitants of the globe, and appearing through the cuticle or upper surface of the skin, gives them that various appearance, which strikes us so forcibly in contemplating the human race.
As this can be incontrovertibly ascertained, it is evident, that whatever causes cooperate in producing this different appearance, they produce it by acting upon the _mucosum corpus_, which, from the almost incredible manner in which the cuticle[082] is perforated, is as accessible as the cuticle itself.

These causes are probably those various qualities of things, which, combined with the influence of the sun, contribute to form what we call _climate_.

For when any person considers, that the mucous substance, before-mentioned, is found to vary in its colour, as the _climates_ vary from the equator to the poles, his mind must be instantly struck with the hypothesis, and he must adopt it without any hesitation, as the genuine cause of the phaenomenon.
This fact[083], _of the variation of the mucous substance according to the situation of the place_, has been clearly ascertained in the numerous anatomical experiments that have been made; in which, subjects of all nations have come under consideration.