species, and the different regions of the body. In addition, breeding,
care, heat and cold may cause marked variations in the thickness of the
coat. Exposure to cold causes the coat to thicken. High temperatures cause
the short hairs to drop out and the coat to become thin.

Diseases of the skin may be _classified as parasitic_ and _non-parasitic_.
Parasitic skin diseases are caused by animal and vegetable parasites.
Non-parasitic skin diseases are caused by irritation to the skin and
internal causes. Irritation to the skin may be either chemical, thermic or
mechanical. The internal causes may be due to an individual predisposition
together with digestive disturbances and the eating of feeds too rich in
protein. In this chapter parasitic skin diseases produced by insects will
not be discussed.

FALLING OUT OF THE HAIR AND FEATHERS.--Falling out of the hair and feathers
frequently occurs independent of parasitic diseases. This condition does
not occur as an independent disorder, but as a secondary affection. It is
due to faulty nutrition, and irritation to the skin. Intestinal diseases,
insufficient feed and feed of bad quality are common causes. Animals that
are fed a heavy ration, or that lie on dirty, wet bedding frequently lose
large patches of hair. Sheep that are dipped in late fall and early winter,
or exposed to wet, cold weather may lose a part of their fleece. It is not
uncommon for animals toward the latter period of pregnancy, or that sweat
freely, to lose patches of hair.

Falling out of the hair heals of itself within a few weeks.

_The preventive measures_ are of special importance in sheep and horses.
This consists in avoiding conditions that may lead to alopecia and in
correcting the diet. In horses the regions of the mane and tail should be