194 THE TOWER MENAGERIE. 
vary with the age of the animal, but correspond in 
general pretty closely with those of the common deer. 
They may be shortly described as fawn above and 
whitish beneath, becoming deeper with age, and lighter 
in the females than in the males. The occasional stripes 
of a lighter or darker colour, which are generally visible 
on various parts of the body, can scarcely be considered 
as occurring with sufficient regularity to allow of their 
being described as characteristic of the species. 
But for these shades of colour, or for any other, we 
should look in vain in the animal of the Tower Mena- 
gerie, which, in consequence of a particular conformation, 
not unfrequent in some species of animals, and occa- 
sionally met with even in the human race, is perfectly 
and purely white. In order to explain this phenomenon, 
which is one of the most curious, but at the same time 
one of the most simple in physiology, it is necessary to 
observe that there exists beneath the epidermis, or outer 
covering of the skin, both in man and animals, a peculiar 
membrane of very fine and delicate texture, which is 
scarcely visible in the Kuropean but sufficiently obvious 
in the Negro, termed by anatomists the rete mucosum. 
In this net-work is secreted, from the extremities of the 
minute vessels which terminate upon its surface, a mu- 
cous substance which varies in colour according to the 
complexion of the individual, of the varieties in which 
it is the immediate cause; and from the substance thus 
secreted the colouring matter of the hairs and of the iris 
is derived. The pure whiteness then of the covering of 
the animal in question, and of all those which exhibit a 
similar variation from their natural tinge, is attributable 
solely to the absence of this secretion from whatever 
