Indeed, these two horny structures appear to be homologous. Is not the lamina of the claw comparable to the wall of the hoof? And does not the interval which occurs at the inferior part of this latter, and is filled by the sole, recall that which, in extremely reduced form, is filled by the plantar portion of the claws?
The frog ([Fig. 100]) is a mass of horn, in form of a wedge, with its apex in front, which occupies the space limited laterally by the recurved portions of the wall (the bars) and the posterior border of the sole.
It covers the plantar cushion previously described ([p. 252]) and reproduces its form.
Its inferior surface is hollowed out in the middle by an excavation, which is known as the median lacuna. This cavity separates the branches of the frog, which terminate posteriorly by two swellings which are known as the prominences of the frog, forming two rounded elevations situated above the claws. These same branches unite in front of the median lacuna to form the body of the frog. This latter, in its anterior part, gradually narrows, and terminates in a point which occupies the bottom of the hollow limited laterally by the bars of the wall and the posterior border of the sole.
Between the lateral surfaces of the frog and the bars are found two angular cavities—the lateral lacunæ, or the commissures of the frog.
Fig. 101.—Third Phalanx of the Horse: Left Anterior Limb, Inferior View.
1, External border; 2, internal border; 3, semilunar crest; 4, 4, re-entrant processes.