STRUCTURE OF THE SKIN:—EPIDERMIS. 49 
outwards, until we arrive at layers composed entirely of dry 
flat scales, which show hut little indication of ever having 
been cells. There is no doubt, however, that all these forms 
are but different stages of the existence of one and the 
same set of epidermic cells; these taking their origin in the 
formative fluid exuded on the surface of the basement-mem¬ 
brane, and being progressively carried towards the surface by 
the. successive development of new layers beneath them, 
whilst the layers above them are thrown off, or are worn 
away ; and at the same time undergoing a change of form, in 
the first instance from mutual pressure, and afterwards from 
the loss of their contained fluid. At the same time they are 
rendered more firm in texture, by the formation of a homy 
secretion in their interior ; so that' the outer layers of epi¬ 
dermis form a consistent membrane, which is raised from the 
surface of the Cutis when fluid infiltrates between them (as 
when the hand has been long soaked in water), or is poured 
out by the vessels of the latter (as when a blister is applied); 
whilst the soft internal layers remain in contact with the 
basement-membrane.—The number of layers varies greatly in 
different parts, being usually found to be greatest where 
there is most pressure or friction, as if the irritation deter¬ 
mined an increased supply of blood to the spot, and thus 
favoured an augmented development of epidermic cells. 
Thus, on the soles of the feet, particularly at the heel and 
the ball of the great toe, the Epidermis is extremely thick; 
and the palms of the hands of the labouring man are 
distinguished by the horny hardness of their thick cuticle. 
—It was formerly supposed that a special layer of a soft 
spongy tissue, termed the rete mucosum, intervenes between 
the Cutis and the Epidermis; and that this was the special 
seat of the colour of the skin in the dark races. It is 
now well ascertained, however, that this supposed rete con¬ 
sists of nothing else than the newly-forming soft layers 
of the true • epidermis; and that the colouring matter is 
diffused through the epidermic cells, so as to tinge the 
entire thickness of the cuticle, although its presence is 
particularly obvious in the deeper layers.—The Nails may 
be considered as nothing more than an altered form of 
Epidermis; when examined near their origin, they are found 
to consist of cells which gradually dry into scales that remain 
E 
