THE SKIN is composed of two portions, the EPIDERMIS and the CORIUM.

THE EPIDERMIS is a stratified epithelium. The superficial layers of the
cells composing it are hard and horny, while the deeper layers are soft and
protoplasmic. These latter form the so-called _Retae Mucosum_ of Malpighi.

[Illustration: FIG. 23.--VERTICAL SECTION OF EPIDERMIS (HUMAN). (AFTER
RANVIER) _A_, The horny layer of the epidermis; _B_, the rete mucosum;
_a_, the columnar pigment-containing cells of the rete; _b_, the polyhedral
cells; _c_, the stratum granulosum; _d_, the stratum lucidum; _e_, swollen
horny cells; _f_ the stratum squamosum.]

Commencing from below and proceeding upwards, we find that the lowermost
cells of the rete mucosum, those that are set immediately on the corium,
are columnar in shape. In animals that have a coloured skin these cells
contain pigment granules. Directly superposed to these we find cells which
in shape are polyhedral. Above them, and forming the most superficial layer
of the rete mucosum, is a series of flattened, granular-looking cells known
as the _stratum granulosum_.

Immediately above the stratum granulosum the horny portion of the epidermis
commences. In the human skin this is formed of three distinct layers.
Undermost a layer of clear compressed cells, the _stratum lucidum_.
Next above it a layer of swollen cells, the nuclei of which are
indistinguishable. Finally, a surface layer of thin, horny scales, the
_stratum squamosum_, which become detached and thrown off in the form of
scurf or dandruff. In the skin of the horse, except where it is thickest,
these layers are not clearly defined.