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 beneath it a caseous foetid matter, easily removed by scraping (the perverted secretion of the keratogenous membrane).  When this is carefully scraped away, the sensitive structures appear to be covered with a thin, smooth membrane, gray in colour and almost transparent, while beneath it may be seen the red appearance of normal sensitive structures.

If the horn surrounding the lesion is not touched with the knife, but little is seen of the extent of the disease, for that removed by natural means is often very small in quantity.  To all intents and purposes the disease appears to be confined to the frog.  This appearance is misleading, especially if the disease has been in existence for some time, for it may have easily spread to the whole of the sole, and even to the greater portions of the laminae secreting the wall.