The substance of the wall presents a fibrous appearance which is pretty strongly pronounced. The constituent fibres from which this appearance results are directed from the superior border towards the inferior in parallel and regular lines. The _sole_ is a horny plate which occupies the inferior surface of the hoof (Fig. 100). It is situated between the inferior border of the wall and the bars; and, on account of the oblique direction of these latter, it presents a strongly-marked groove of a [V]-form, with the opening directed backwards. In this depression is lodged the frog. The inferior surface is concave, and thus forms a sort of vault, more or less deep, according to the individual. The sole has a scaly, laminated aspect. We have seen (Fig. 93, and p. 249) that on the inferior surface of the claws of carnivora is found a small interval which is filled by a plate of a more friable horny substance, to which has been given the name of the plantar nail. It seems to us that there is an interesting relationship between the said plantar nail and the sole which we have just been studying. [Illustration: FIG. 100.--INFERIOR SURFACE OF A FORE-HOOF OF THE HORSE: LEFT SIDE. 1, Internal border of the wall (toe); 2, wall; 3, quarter; 4, heel; 5, bar; 6, sole; 7, frog; 8, median cavity; 9, prominence of the frog; 10, lateral cavity.] 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_. [Illustration: 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.] [Illustration: FIG. 102.--THIRD PHALANX OF THE HORSE: LEFT POSTERIOR LIMB, INFERIOR VIEW.