ass of diseases which are of an inflammatory
type; next, those caused by faulty secretion and abnormal growth; then,
diseases of parasitic origin; lastly, local injuries of the skin.

PRURITIS (ITCHING).

We shall consider pruritis first as a distinct subject. It is not a
disease, only a sensation, and therefore a symptom. It is one of the
symptoms accompanying the majority of the diseases which we will consider
in this chapter. It is, then, a functional affection produced by slight
irritation from without or by an internal cause acting upon the sensory
nerves of the skin. Nothing characteristic is seen except the secondary
lesions, produced mechanically by scratching or rubbing.

There are various forms of itching, the result of specific skin diseases,
where the pruritis is a secondary symptom. In such cases it should not be
regarded as an independent affection.

_Causes._--Many causes may induce the condition which we recognize here as
pruritis. The most common one is dirt on the skin, resulting from
insufficient care. If the ceiling of the stable is open, so that dust and
straw may fall, the skin is irritated and pruritis results. It also occurs
in some forms of indigestion.

The parts of the body most exposed to this condition are the croup, the
back, the top of the neck, and the root of the tail.

Another cause is found in affections of the liver and of the kidneys, when
an increase of effete material has to be thrown off by the skin. Morbid
materials circulating in the blood may produce a tickling or smarting
sensation of the skin in their passage from the blood to the free surface
of the skin. Certain irritating substances when eaten may be excreted by
the skin, and coming thus in direct contact with the sensory nerves produce
itching, or may go further and cause distinct inflammation of the skin. In
another class of cases the pruritis may be ascribable to an atrophy,
contraction, or hardening of the skin, when the nerves become irritated by
the pressure. These conditions may be so slightly marked in a thick skin
like that of the ox that they can not be recognized. It is frequently
noticed that cattle rub themselves as soon as they pass from the stable
into the open air--changing from a warm to a cold atmosphere. Again, we may
find one that does all its rubbing in the stall. We may look for lice, but
fail to find them. These conditions are generally attributable to high
feeding and to too close confinement