The Hoof.
Everyone is familiar with the general appearance of the hoof. It is not a regular geometrical figure. Each of the four feet of the horse shows some peculiarity in form, by which a farrier can at once identify a fore from a hind or a left from a right.
The fore feet should be similar in size and shape. Disease may be suspected when any marked difference exists. But a healthy hoof which has been broken, or much rasped, does not retain its proper form and may thus confuse a novice.
The hind feet should be proportionate in size to the fore, and then it is not of much practical consequence whether the whole are large or small.
The front feet are rounder and less pointed at the toe than the hind; they are also more sloping in front. The two fore feet and the two hind should be in pairs. The right and left feet are distinguished from each other by the inner side being more upright or, if examined on the under surface, by the outer border being more prominent.
Although to a casual observer the hoof appears as one continuous horny structure, it may easily be separated into three distinct parts by prolonged soaking in water. The division takes place so as to leave the sole, frog, and wall separate portions. These may now be considered.
Fig. 1.—A Fore Foot.
Fig. 2.—A Hind Foot.
The Wall is that portion of the hoof seen whilst the foot rests upon the ground. It covers the front and sides of the foot. It extends from the coronet downwards and slightly outwards so that its lower circumference is greater than its upper. The front portion shows its greatest height and obliquity, diminishing in these respects as it passes backwards. At the heels the wall is turned in upon itself, and passes forward towards the centre of the foot until it becomes lost in the structure of the sole. These turned-in portions of the wall are called the bars, and serve two purposes; they increase the bearing surface of the wall, and by embracing a part of the sole on each side, they afford an increased solidity to the union of the wall with the rest of the hoof.
If we detach the wall its inner surface is seen to consist of a number of thin horny projections running parallel to each other from above downwards and forwards. These are called the horny laminæ. They number from five to six hundred and correspond to similar processes on the sensitive foot. ([Fig. 3]).
Fig. 3.—Half of a Hoof, showing the inside.
Round the upper circumference on the inside of the wall is a depression or groove presenting innumerable small pits or openings. This corresponds to a part of the sensitive foot called the coronary band, which will be noticed again.
A section of wall enables us to see variations in its thickness. ([Fig. 4]). It is thickest at the toe, becoming gradually thinner towards the heels; thus affording strength and solidity to resist wear at one part, as well as pliancy at another to ward off concussion.
Fig. 4.—Transverse Section of Wall showing variation in thickness.
The structure of the wall is fibrous—the fibres running parallel to each other, and with the same obliquity as that presented by the front of the wall. Although the wall varies in thickness from before backwards, it does not from above downwards. It maintains the same thickness from the coronet to its lower circumference.
The layers of the wall are hardest externally, becoming softer as they approach the inner surface—a condition due to the outer layers being exposed to friction and evaporation. This is a simple and valuable provision of nature which should not be interfered with. The hard outer layer is best adapted to withstand wear, and its density protects the deeper layers from evaporation. This maintains the whole wall at the degree of softness and toughness which best preserves elasticity and strength of horn.
The Sole is that division of the hoof which forms the floor of the foot. It is situated within the lower border of the wall, and is slightly arched so that on a hard level surface its central part takes no bearing. ([Fig. 5]). Posteriorly the sole is divided by a triangular space into which the frog fits, and thus its continuation to the heels consists of two angular portions embraced between the bars and the wall. The unmutilated sole is throughout of nearly equal thickness, but a slight excess round the circumference gives firmer attachment to the wall.
Fig. 5.—The Sole with Frog removed.
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.
Fig. 8.—The frog band detached from wall by a small wedge.
The point of the frog, much the harder part, extends forward to the centre of the sole. Though situated between the bars the frog is only attached to their upper border—the sides remaining free and separate. Thus on each side is formed a deep fissure which permits the frog to expand laterally when compressed, without such force being continued to the sides of the foot. The frog is elastic, and when pressed upon must expand slightly. If these spaces between frog and bars did not exist, the foot would be injured when the frog was compressed by the weight of the horse—either the sensitive parts within would be bruised or the heels would be forced apart.
The centre of the frog presents a depression or "cleft" caused by the doubling in of the horn. Few shod feet exhibit it of natural appearance, and the term cleft, by implying a narrow deep fissure, keeps up the false notion. The cleft should be shallow and rounded. It serves two purposes—it increases the mobility of the frog, and by breaking the regularity of surface affords a secure foot-hold on level ground.
The prominence of the frog might lead a superficial observer to consider it a thick solid mass; and I believe this mistake is the cause of its too frequent mutilation. It is merely a layer of horn following the outline of the structures within, which are similarly prominent and irregular in surface. ([Figs. 9] and [10]). The diagrams show a section through the point and through the cleft of the frog.
Fig. 9.—Section of Foot at cleft.
Fig. 10.—Section at point.
The frog is fibrous, though not to such a marked degree as the other portions of the hoof. Its chief qualities are elasticity and toughness.