tain instances during
suppression of the renal secretions a small quantity of urea. The skin is
also the chief organ for the regulation of animal heat, by or through
conduction, radiation, and evaporation of water, permitting of loss of
heat, while it also, through other mechanisms, is able to regulate the heat
lost. The hair furnishes protection against extreme and sudden variations
of temperature by reason of the fact that hairs are poor conductors of
heat, and inclose between them a still layer of air, itself a nonconductor.
The hairs are also furnished with an apparatus by which the loss of heat
may be regulated; thus, in cold weather, through the contraction of
unstriped muscular fibers of the skin, the hairs become erect and the
external coat becomes thicker. Cold, too, acts as a stimulus to the growth
of hair, and we find, in consequence, a thicker coat in winter than in
summer. The hairs also furnish protection against wet, as they are always
more or less oily from the secretion of sebaceous glands, and thus shed
water. Through their elasticity they furnish mechanical protection, and
through the thickness of the coat, to a certain degree, resist the attacks
of insects. Finally, the hairs assist the sense of touch.

The sweat glands are constantly discharging a watery secretion in the form
of insensible perspiration, and by their influence act as regulators of the
temperature of the body; hence, in warm weather, the secretion of the skin
is increased, which tends to prevent overheating. Sweating, in addition to
regulating heat, is also an active agent in removing effete material from
the blood; therefore this secretion can not be checked without danger. If
the skin is covered with an impermeable coating of grease or tar, death
results from blood poisoning, owing to the retention of materials destined
to be excreted by the skin.

All secretion poured out by the skin is not only modified by the condition
of the atmosphere but also by the character and quantity of the food, by
the amount of exercise, and especially by the quantity of fluid taken.

The sebaceous secretion is intended to lubricate the skin and hairs. It
consists of soft, fatty material suspended in water, and is characterized
by an odor peculiar to the animal by which it is secreted.

I will not attempt to classify the various diseases of the skin, for in a
work of this kind it would serve only to confuse the reader.

We shall first consider a cl