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Fig. 12.—Under Surface of Foot showing Sensitive Frog and Sole.

The Sensitive Frog in structure resembles the sensitive sole, but its papillæ are very much smaller, and the surface therefore is smoother. The irregular prominent surface of the frog, with its cleft and the space at each side of it, is exactly reproduced on the sensitive frog, as might be expected, for the one is moulded on the other. There is one difference between the sensitive frog and the other portions of the sensitive foot which I may here mention. It is not attached to the bones of the foot except by its point, but is situated behind the bone, and has as a basis a mass of soft tissue which forms an important cushion or pad, to be referred to later.

Growth of Hoof.

Like every other part of an animal body, the hoof is constantly changing. Wear and tear cause waste of the horn, which is replenished by growth. When wear exceeds growth the foot becomes denuded of horn, and lameness[Pg 20] results. When growth exceeds wear the hoof becomes disproportionately long, and some parts suffer by the overgrowth of others—for instance, whenever the heels are unduly high the frog becomes small and weak. In a state of nature the horse's foot keeps itself of proportionate form. On hard ground the hoof is worn away as quickly as it grows. On soft ground it may, for a time, become overgrown, but this is rectified by the soft horn becoming fractured and broken off. In enclosed cultivated grounds the movements of the horse, even on grass land, are too limited to ensure a proportionate form of hoof. When horses are turned out without shoes the feet should not be left to take care of themselves, unless th