h 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.

It is, in fact, not until the pressure exerted by the normal horn
is removed by its breaking away that the vascular structures of the
keratogenous membrane begin to swell, and the perverted secretions to
enlarge in size. Once the pressure is removed, however, this quickly
comes about, and the characteristic fungoid growths rapidly make their
appearance.

This tendency to spread is highly indicative of canker. The serous matter
exuding from the diseased keratogenous membrane appears, in fact, to be
highly infective. Once its flow is commenced, it slowly, but surely,
invades the sensitive structures near it, appearing, as Elaine has put it,
to 'inoculate' them. What is really the case, of course, is not that the
discharge itself is infective, but that it is contaminated with infective
material.

The fungoid-looking growths to which we have before referred are, in
reality, nothing more than the villi of the sensitive frog and sole greatly
hypertrophied and irregular in shape. At times the hypertrophy is as a huge
and compact enlargement occupying the position of the frog. Sometimes,
however, it occurs as numerous elongated and twisted fibrous bundles,
separated from each other by deep clefts, and the clefts filled with the
offensive cankerous discharge (see Fig. 134).

[Illustration: FIG. 135.--LOWER ASPECT OF CANKERED FOOT, SHOWING
DESTRUCTION OF WALL.]

At a very advanced stage canker leads to destruction of much of the horny
sole and frog; or even parts of the wall may become separated from the
tissues beneath, and break away from the foot (see Fig. 135). At other
times the disease brings about a deformity