nd upon the withers. Poll evil is a fistula upon the poll, and in no
sense differs from fistulous withers except in location. The description
of fistula will apply, then, in the main, to poll evil equally well.
Quittor presents the characteristic tubular passages of a fistula and
may, therefore, be considered and treated as fistula of the foot.
Fistulous passages may also be developed upon the sides of the face,
through which saliva is discharged instead of flowing into the mouth,
and are called salivary fistulas. A dental fistula may arise from the
necrosis of the root of a tooth. Again, a fistula is sometimes noted at
the umbilicus associated with hernia, and recto-vaginal fistulas have
been developed in mares, following difficult parturition. Fistulas may
arise from wounds of glandular organs or their ducts, and thus we have
the so-called mammary or lacrimal fistulas.

Fistulous tracts are lined with a false, or adventitious, membrane and
show no disposition to heal. They constantly afford means of exit to the
pus or ichorous material discharged by the unhealthy parts below. They
are particularly liable to develop at the withers or poll because of the
exposed positions which these parts occupy, and, having once become
located there, they usually assert a tendency to further extension,
because the vertical and laminated formation of the muscles and tendons
of these parts allows the forces of gravitation to assist the pus in
gaining the deeper-lying structures and also favors its retention among
them.

_Causes._--Fistulas follow as a result of abscesses, bruises, wounds, or
long-continued irritation by the harness. Among the more common causes
of fistula of the poll (poll evil) are chafing by the halter or heavy
bridle; blows from the butt end of the whip; the horse striking his head
against the hayrack, beams of the ceiling, low doors, etc. Fistulous
withers are seen mostly in those horses that have thick necks as well as
those that are very high in the withers; or, among saddle horses, those
that are very low in the withers, the saddle here riding forward and
bruising the parts. In either of these locations ulcers of the skin, or
simple abscesses, if not properly and punctually treated, may become
fistulas. They are often caused by bad-fitting collars or saddles, by
direct injuries from blows, and from the horse rolling upon rough or
sharp stones. The pus burrows and finds lodgment deep down between the
muscles, and