cause in their turn
eruptions on the skin. The relations--nervous, secretory, and
absorptive--between the skin and internal organs are most extensive and
varied, and therefore a visible disorder in the skin may point at once
and specifically to a particular fault in diet, to an injudicious use of
cold water when the system is heated, to a fault in drainage,
ventilation, or lighting of the stables, to indigestion, to liver
disease, to urinary disorder, etc.

[Illustration: PLATE XXXVIII.

THE SKIN AND ITS DISEASES.]

STRUCTURE OF THE SKIN.

The skin consists primarily of two parts: (1) The superficial
nonvascular (without blood vessels) layer, the cuticle, or epidermis;
and (2) the deep vascular (with blood vessels) layer, the corium,
dermis, or true skin. (See Pl. XXXVIII, fig. 1.)

The cuticle is made up of cells placed side by side and more or less
modified in shape by their mutual compression and by surface evaporation
and drying. The superficial stratum consists of the cells dried in the
form of scales, which fall off continually and form dandruff. The deep
stratum (the mucous layer) is formed of somewhat rounded cells with
large central nuclei, and in colored skin containing numerous pigment
granules. These cells have prolongations, or branches, by which they
communicate with one another and with the superficial layer of cells in
the true skin beneath. Through these prolongations they receive nutrient
liquids for their growth and increase, and pass on liquids absorbed by
the skin into the vessels of the true skin beneath. The living matter in
the cells exercises an equally selective power on what they shall take
up for their own nourishment and on what they shall admit into the
circulation from without. Thus, certain agents, like iodin and
belladonna, are readily admitted, whereas others, like arsenic, are
excluded by the sound, unbroken epidermis. Between the deep and
superficial layers of the epidermis there is a thin, translucent layer
(septum lucidum) consisting of a double stratum of cells, and forming a
medium of transition from the deep spheroidal to the superficial scaly
cuticle.

The true skin, or dermis, has a framework of interlacing bundles of
white and yellow fibers, large and coarse in the deeper layers, and fine
in the superficial, where they approach the cuticle. Between the fibrous
bundles are left interspaces which, like the bundles, become finer as
they approach the surface, and inclose