The inner surface presents a finely pitted appearance which is most marked at the toe and round its border. The part immediately related to the frog shows few pits, and we shall find that the whole surface corresponds to the sensitive parts to which it is attached.
The structure of the sole is, like the wall, fibrous; but the fibres are smaller. They run downwards and forwards in the same direction as those of the wall. The outer layers are the hardest and protect the deeper from injury.
The Frog is the smallest division of the hoof, and is a triangular shaped body filling up the space left between the bars. ([Fig. 6]). Its broad base is rounded and prominent, and is continued laterally by a thin layer which binds together the heels and envelopes the back of the foot. This thin layer is continuous with a horny band extending round the upper part of the wall at its junction with the hair, and sometimes prolonged downwards on the surface of the wall. ([Fig. 7]). It appears to be a continuation of the outer layer of the skin, analogous to the free border of skin at the root of the human nail. ([Fig 8]). It serves the useful purpose of covering and protecting the young horn of the wall at its source of growth.
Fig. 6.—The Frog, detached from the Sole.
Fig. 7.—The Frog and frog-band.