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 papillae. These project into the epidermis, which is
moulded on them. For the most part the papillae 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.

Others of the papillae contain nerves, ending here in tactile corpuscles, or
continuing, as we have mentioned before, to ramify as fine fibrils in the
rete mucosum of the epidermis.

THE HAIRS are growths of the epidermis extending downwards into the deeper
part of the corium. Each is developed in a small pit, the _Hair Follicle_,
from the bottom of which it grows, the part lying within the follicle being
known as the _Root_. It is important to note their structure, as it will be
seen later that they bear an extremely close relation to the horn of the
hoof.

Under a high power of the microscope, and in optical section, the central
portion of a hair is tube-like. In some cases the cavity of the tube is
occupied by a dark looking substance f