STRUCTURE OF THE SKIN. 83 
It has been supposed by some, that this layer owes it 
colour to the action of the solar rays; but as it 1s often 
found coloured in those parts secluded from the light, into 
which it enters along with the cuticle, such as the palate, 
tongue and ears, we must attempt to trace its origin to 
some other cause. Biumensacu refers the colour to car- 
bon, precipitated on the mucus, and combining with it. 
The proofs, however, are still wanting, of the presence of 
this ingredient in the particular state of colour which char- 
coal exhibits. Were the mucous membrane always found 
either colourless or black, we might be induced to admit 
the colouring matter to be charcoal ; but when this layer, 
in different animals, exhibits ail the tints of the prismatic 
spectrum, we are inclined to reject the explanation as hy- 
pothetical, and wait the result of more decisive experi- 
ments. Dr Bepposs, and afterwards Fourcroy, ascertain- 
ed, that oxymuriatic acid deprived the skin ofa negro of its 
black colour; but in a few days it returned with its former 
intensity. That the colour depends on an animal oil secret- 
ed by the true skin, is, perhaps, the most plausible conjec- 
ture which can be advanced on the subject. 
This layer of the skin occurs not only im the warm-blood- 
ed animals, but, perhaps, in all the mferior classes *. Cu- 
VIER conjectures, that the shell of molluscous animals 
and the crust of lobsters, and other crustacea, occu- 
py the place of this layer, as they are in immediate contact 
with the cuticle. But as these parts are employed as or- 
gans of protection and of support to the muscles, purposes 
to which the mucous web is never applied in its most per- 
fect state, analogy appears in opposition to the conjecture. 
3. Corium.—This is known by the name of the cutes 

* It is necessary to add, that the existence of this layer is even denied by 
some.—Edin. Phil. Journ. i. p. 213. 
Fr 2 
