, but that it _is_
parasitic we feel absolutely assured.

It is simply the light that bacteriological advance has made during the
last two decades that enables us to make the statement with such feelings
of assurance. We arrive at our conclusions by reasoning from analogy.
Here we have a disease always exhibiting the same symptoms, more or less
peculiar to one class of animal, always with a similar characteristic
appearance and smell, always obstinately refractory to treatment, showing
always a tendency to spread to the other feet of the same animal, and often
to the feet of other animals _near enough to become_ infected, and always
cured--when cured it is--by a treatment which may be summed up in two words
as 'rigid antisepsis.' Other diseases, with points in common with this,
have been directly proved to be due to a specific cause. Common regard for
logic compels us to admit the same for canker.

[Illustration: FIG. 134.--A FOOT, THE SUBJECT OF CANKER, SHOWING
DESTRUCTION OF THE HORNY FROG, AND A FUNGOID-LOOKING HYPERTROPHY OF THE
TISSUES BENEATH.]

_Symptoms and Pathological Anatomy_.--The symptoms of canker are seldom
noticeable at the commencement of an attack. The disease is slow in its
progress; for some time confines its ravages to the sub-horny tissues
unseen, and is quite unattended with pain. It is not observed, therefore,
until considerable damage has been done, and the disease is far advanced.
What is usually first seen is a peculiar softening and raising of the horn
of the frog. The infective material has set up a chronic inflammation of
the keratogenous membrane, leading to abnormal secretion, and, in place of
the horny cells it should normally secrete, is thrown out an abundance of a
serous fluid.

This upraised and softened horn once thrown off is not again renewed, and
the whole of the sensitive frog and perhaps a portion of the sensitive sole
is left uncovered. In place of the normal horn, however, is often found a
hypertrophy of the elements of the keratogenous membrane leading to huge
fungoid-looking growths with a papillomatous aspect, damp in appearance and
offensive in smell, and readily bleeding when injured (see Fig. 131).

The horn immediately surrounding the lesion is loose and non-adherent to
the sensitive structures. This indicates, of course, that the disease has
spread further beneath the horny covering than is at first sight apparent.
Portions of this loose horn removed reveal beneat