It is the Malpighian layer of the epidermis that is most active in cell
division. As they are formed the new cells push upwards those already
there, and the latter in their progress to the surface undergo a chemical
change in which their protoplasm is converted into horny material. This
change, as we have already indicated, takes place above the stratum
granulosum.

In addition to its constant formation of cells to replace those cast
off from the surface, the active proliferation of the elements of the
Malpighian layer is responsible for the development of the various
appendages of the skin, the hairs with their sebaceous glands, the sweat
glands, horny growths and the hoof, and, in the human subject, the nail.
These occur as thickenings and down-growths of the epithelium into the
corium.

The epidermis is devoid of bloodvessels, but is provided with fine nerve
fibrils which ramify between the cells of the rete mucosum.

THE CORIUM is composed of dense connective tissue, the superficial layer
of which bears minute papillæ. These project into the epidermis, which is
moulded on them. For the most part the papillæ contain looped capillary
vessels, rendering the superficial layer of the corium extremely vascular.
Why this must be a moment's reflection will show. The epidermis, as we have
already said, is devoid of bloodvessels. It therefore depends entirely for
its nourishment upon the indirect supply it receives from the vessels of
the corium. The need for extreme vascularity of the corium is further
explained when we call to mind the constant proliferation and casting off
of the cells of the epidermis, the growth of the hairs, the production of
the horn of the hoof, and the work performed by the numerous sweat and
other glands.