The whole horny hoof, if unravelled by maceration or long continued exposure, is found to be essentially composed of longitu dinal corneous threads or hairs matted, and, as it were, strongly glued together,— a structure preeminently adapted to combine all the re quirements of strength, elasticity, and tough ness.
As it approaches the quarters and heels, the horny helmet encasing the foot diminishes in its thickness as well as in height, affording, by this means, a degree of pliancy, which here becomes as necessary as firmness and unyield ing solidity were in the front of the organ ; yet, even here, by the doubling in of the hoof towards the sole, a strong horny margin is left, which is admirably adapted to receive the principal bearing of this part of the foot, and to protect and defend the sole enclosed within its curvature.
Frog.— The triangular chasm left by the inflections of the wall towards the centre of the foot, is filled up by a very remarkable organ, named, in the language of farriery, the " frog," either from some fancied gross resem blance which it bears to that animal, or, more probably, by corruption, from the French 6' fourche " or " fourchette," Anglice " fork," applied to the same structure. By Latin writers, it is generally known under a similar appellation, " furca," and by the Greeks was named " xviuSova," from a similarity between its shape and that of a swallow.
This body, which externally has the ap pearance of a triangular mass of elastic horn, may not be inaptly compared to an elastic key-stone received into an elastic arch com municating in some cases, and admitting in all, the springing movements of which such an arch would be capable. Its bar, which, to wards the heels, is thin and broadly spread out, possesses a considerable degree of flexi bility, which is gradually lost in approaching the centre of the foot, where there is less occasion for movement.
The base of the frog lies between and con nects together the two posterior incurvations of the hoof ; it then passes over and en ve'opes those parts, restraining their action. The sides of the frog are united by applied surfaces to the upper edge of the arch formed by the sole, or more truly the bar formed by the continuation of the wall. Its point extends to or beyond the centre of the sole.
The frog recedes from pressure in the na tural foot, by having its !eve/ within the level of the other parts of the under surface of the foot, taking a third rate or degree of bearing upon the ground : the wall first ; the bar next projecting beyond it : its base also retires further from pressure than the other parts of it, and is protected by the projecting angles of the horny or lower heel.
On either side, the frog is bounded by deep longitudinal excavations or channels, named the commissures of the frog ; the bottom or deepest part of these channels, forming the line of union of the frog with the bar, a space is thus afforded on either side of the frog, which, as an elastic body, would have been useless without it; for in vain would elasticity have been given to any part, unless sufficient room was also given for its expansion. To wards the heels, these commissures are of considerable width, and they are there arched over by horny prolongation, from the base of the frog, named the arch of the commissure. The other extremity of the commissure grow ing, by degrees, shallower, is lost in the level of the sole, before it approaches the arch of the frog.
Seen from without, the frog makes a bold projecting appearance, as though it were a solid body of horn ; and the smiths, deceived by this appearance, entertain but indifferent notions of its real structure, and use their paring knives upon it much more freely than its thickness vs arrants ; for it is in reality only an inverted arch of horn that is turned downwards and reversed in respect to the general arch formed by the sole and bar, so that its real thickness in horn is by no means so considerable as on a first view it would appear to be.
Examined from within—that is to say, w hen the foot has been dravvn forth from the.f oof— the frog presents an inverted triangular arch, so intimately connected with the bar and sole, that no one would suspect it of being a distinct or divisible part, one uniform uninterrupted surface being everywhere observable on this inside : it may, however, be exhibited as a dis tinct inserted part by making a horizontal sec tion of the foot through the union of the bar with the side of the frog, when the difference of their structure and appearance, and the line of their applied surfaces become sufficiently visible and distinct. A hoof exposed to the weather vvill also be seen in its decay to se parate at this part first, and thus readily show its distinctness from the rest of the hoof.