external skin, or hair skin, in
having no sweat or oil glands, but, like it, is richly supplied with
blood vessels and sensitive nerves. And, just as the derm of the hair
skin produces upon its outer surface layer upon layer of horny cells
(epiderm), which protect the sensitive and vascular derm, so, likewise,
in the foot the pododerm produces over its entire surface soft cells,
which, pushed away by more recent cells forming beneath, lose moisture
by evaporation and are rapidly transformed into the corneous material
which we call the hoof. It is proper to regard the hoof as a greatly
thickened epiderm having many of the qualities possessed by such
epidermal structures as hair, feathers, nails, claws, etc.

The functions of the pododerm are to produce the hoof and to unite it
firmly to the foot.

There are five parts of the pododerm, easily distinguishable when the
hoof has been removed, namely: (1) The perioplic band, a narrow ridge
from one-sixteenth to one-eighth of an inch wide, running along the edge
of the hair from one heel around the toe to the other. This band
produces the perioplic horn, the thin varnishlike layer of glistening
horn, which forms the surface of the wall or "crust," and whose purpose
seems to be to retard evaporation of moisture from the wall. (2) The
coronary band, a prominent fleshy cornice encircling the foot just below
and parallel to the perioplic band. At the heels it is reflected forward
along the sides of the fleshy frog, to become lost near the apex of this
latter structure. The coronet produces the middle layer of the wall, and
the reflexed portions produce the "bars," which are, therefore, to be
regarded merely as a turning forward of the wall. (3) The fleshy leaves,
500 to 600 in number, parallel to one another, running downward and
forward from the lower edge of the coronary band to the margin of the
fleshy sole. They produce the soft, light-colored horny leaves which
form the deepest layer of the wall, and serve as a strong bond of union
between the middle layer of the wall and the fleshy leaves with which
they dovetail. (4) The fleshy sole, which covers the entire under
surface of the foot, excepting the fleshy frog and bars. The horny sole
is produced by the fleshy sole. (5) The fleshy frog, which covers the
under surface of the plantar cushion and produces the horny frog.

The horny box or hoof consists of wall and bars, sole and frog. The wall
is all that part of the hoof wh