In a perfect, well-formed foot, undistorted by shoeing, Bracy Clark observes that " the base of the frog occupies a certain division of the general circle of the hoof, and that this division is about a sixth part of the whole circumference. By knowing this fact we are not only led to entertain more just notions of the form of the foot and the proportions of its parts, but it affords us also an easy means of forming a pretty accurate guess of what in jury or diminution the foot has sustained at any period of the life of the horse without previously seeing the original state of the frog." The wings or lateral processes above de scribed, as extending from the base of the frog, not only enclose the posterior ends or doub lings in of the hoof, but the same horn is con tinued around the whole upper edge or margin of the roof, forming a broad convex band, whose upper edge, projecting higher than the hoof itself, receives and covers over the ter minating edge of the skin, where it meets the hoof, and thus protects this part from external injuries, to which it would otherwise be liable. Posteriorly it is of considerable breadth, and firmly connects the frog with the upper part of the slope of the horny heels, over which it likewise expands. This structure, first de scribed by Bracy Clark, received from him the denomination of the " coronary frog-band." In the centre of the frog, as viewed from the sole, is a considerable cavity, the edges of which are furnished with rising lips or promi nent margins of the horn ; this hollow is termed the cleft of the frog, and extends to a considerable depth. This cavity appears to serve the following useful purposes*: — 1st. It is a safeguard from rupture between the two halves or divisions into which the foot is almost separated. 2dly. By closing when pressure comes direct upon the underside of the foot, it prevents too much condens ation of the horn at this part, and con sequent pressure and a too solid resistance upon the soft parts beneath. 3dly. When the foot bears partially upon the ground, as by one side only, which will happen occasionally where the surface is irregular, it can extend along with that side of the foot without rup turing, by the greater liberty it thus affords to the part, while at the same time the strength of its margin secures it front laceration. 4thly. On loose soils this indent or cavity will doubt less assist in giving the foot a firmer hold by the irregularity it offers to the surface.
It is, however, upon the inner aspect of the hoof that the most remarkable part of this structure is to be observed, for when ex amined internally- it is found that the external cleft is only the hollow base of a cone of stout horn of considerable size, which passes from it directly into the substance of the sensitive frog, and which, though completely imbedded in the soft parts, is nearly or quite as hard and tough as is the horn of the exterior of the frog which is exposed to the air. This re markable provision seems to serve the pur pose of uniting more firmly the two halves of which the foot of the solipeds at this part really consists, there being here an evident tendency, in the tegurnentary defences of the horse's foot, towards that division which in the ruminating quadrupeds is completely carried out. This important cone of horn Bracy Clark named the frogstay or bolt, ob serving that, like an inserted tooth, it more firmly holds the horny to the sensitive frog, for while the sensitive frog falls into the in verted arch of the horny frog, and is thus held most firmly in its place, this part, entering in the opposite direction into the sensitive frog, serves reciprocally to confirm and fix these parts together, and preserve them from external injury and dislocation. An excel
lent view of this piece of anatomy is obtained by making a perpendicular section of the foot extending through the " heels and surrounding elastic matter." The Sole. — This is an irregular plate of horn, which serves to close up the space or great inferior opening described by the lower circumference of the wall, and makes the third member or part of the hoof. It is usually of an arched form, more or less flat tened, its concavity being turned to the ground, so that its centre, which is the thinnest part, is by this means removed from the degree of external pressure which the sides or bottom part of' the arch have to support.
Nature has secured herself, by the arrange ment of this part, in two ways from the re sistance which an arch of common properties would create on becoming condensed under pressure, and forcibly resisting the load brought upon it, which vvould have been sub versive of the leading principles of the me chanism of the hoof. In the first place the sole being cleft to its centre or beyond it by the large triangular opening formed at its posterior part, which, destroying the resist ance of the arch, serves to receive the ends also of the wall of the hoof first, and is then closed and filled up by the inverted arch of the frog ; so that the ends of' the hoof are thus tied in and secured from being forced asunder by the pressure from within, being thus wedged in between the frog and the sole, serving in their places the other offices already noticed, while the sole, being thus broken, has a diminished resistance in the centre.
Again: the lower circumference of the arch of the sole is everywhere found abutting against the sides of the wall, which are ren dered sufficiently flexible outwards to yield to the weight when pressed against by the de scent and flattening of the sole, so that every provision for the elasticity of the foot is thus fully secured.
The horse's hoof is therefore fully provided with the means of preserving its form ; but this power is unfortunately grievously inter fered with by the process of shoeing ; and it is in this country at least a very, rare occur rence to obtain an opportunity of examining a foot in its full-grown natural condition.
From the above description of the foot of the horse it will be seen that, although when viewed in front it appears to be solid and single, the tenns Soliclungula and Solipes con vey but a very imperfect notion of the real nature of this kind of hoof; for though the front be solid the posterior parts possess the greatest degree of elasticity, short of being actually cloven, that can be imagined from the sole being open to its centre and filled up with the frog. In such a foot as the term Solid ungula would imply, or a continuous circle of horn, no animal could long stand, much less move, without great fatigue and pain from compression, which would soon become de structive. If it were necessary to employ any single epithet to express the real nature of this kind of locomotive apparatus, Bracy Clark suggests that the term Semifissipes, or half cloven foot, would be less objectionable, though also not exactly true, on account of the presence of the frog, which, added to the entire hoof in front, seems to afford the most essential character of this kind of foot.