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 of the whole of the foot.  Its longitudinal and transverse diameters become enormously increased, and the whole foot apparently flattened from above to below (see Fig. 136).  This indicates that not only has the horny sole been entirely destroyed, but that the destructive process has also extended to the greater part of the lower half of the wall, with a consequent hypertrophy of exposed soft structures, and a sinking of the bony column, similar to that which occurs in laminitis, but not so pronounced.

[Illustration:  FIG. 136.—­FOOT WITH ADVANCED CANKER.]

A further aspect of the badly-cankered foot is to be found in an apparently enormous increase in the length of the wall.  This we have seen protruding for quite 5 inches beyond the plane of the sole.  It simply indicates that, in order to keep the animal at work, the smith has at every shoeing spared the wall, so that the diseased structures might be kept from contact with the ground.

As we have said before, pain and other symptoms of distress are quite absent.  Animals affected with canker for a long time maintain their condition, feed well, and are quite capable of performing work under ordinary conditions.