Some knowledge of the structure of the foot and of its functions is necessary to an understanding of the principles of horse-shoeing. The hoof is only a layer of horn covering very sensitive parts and affording a base of support for the limb. A damaged hoof cannot properly protect the parts within, and a deformed hoof places the whole limb at a disadvantage even as a column of support - much more so as a propelling organ, when great effort is required for draught, or quick movements for pace.
The hoof is not a regular geometrical figure, it is an irregular one (fig. 624), and this irregular form must be followed in shoeing. If the two front feet be looked at on the ground it will be seen that they are similar in form and size, that the inner surface is more upright than the outer, and that the hoof is much higher in front than behind.
The Wall (fig. 625) is the part of the horn forming the front and sides of the hoof. It grows downwards from the coronet, and as it slopes forward and is constantly growing, there is a continuous lengthening of the toe. The effect of excessive growth is therefore to bring the bearing surface of the foot out of proper relation to the lea;, and all overgrown feet afford a disadvantageous position for the horse standing or moving. When a horse is shod his hoof continues growing, and if the shoe be retained too long, the hoof gets disproportionate, and may cause either stumbling or injury to the tendons. The angle at which the front of the wall slopes is a useful guide to the proportions of the hoof. It should be about 45 degrees. When the toe is too long the wall slopes too much, when the heels are too high the front of the wall is too upright (fig. 631). The wall is thicker at the toe than at the heels, and as this variation is gradual from front to back, so nails may be driven into it with less danger towards the toe. The wall does not vary in thickness vertically, so a good workman may safely drive a nail to any reasonable height in its substance. The outer layer of the wall is the hardest, and thus most capable of resisting wear. It protects the deeper layers, and by preventing evaporation keeps them tough and pliant. The evil of rasping is that the exposed horn soon becomes hard, and a repetition or excess of the process renders the hoof brittle.
Fig. 624. - Normal Foot: front view, showing slopes of (a) outer wall and (B) inner wall.
The Sole forms the larger portion of the floor of the hoof. It is concave, and firmly attached to the border of the wall. On a smooth, level surface only the outer portion of the sole - that which is immediately connected with the wall - takes a direct bearing. But the sole sustains its share of the weight of the horse just as an arch supports weight although resting only on its abutment.
The frog is the prominent triangular-shaped mass of horn situated at the back part of the under surface of the hoof. It extends forward to a point reaching more than half-way to the toe. Its prominent surface is broken by a depression which should be shallow, but which is too often a narrow, deep fissure. On each side of the frog is a space separating it from the liars. This space permits lateral yielding when weight is placed upon the frog. It must not be supposed that the frog is an extra thick mass of horn resting on a level sensitive foot. Its prominent parts and its depressions follow exactly a similar formation of the sensitive structure under it, and the whole should be left in its full strength. The form of this division of the hoof suggests its use, which is to form a catch when the foot comes to the ground, and so increase the security of foothold. The structure of the frog is a tough elastic horn, and as the back of the foot comes to the ground first during progression, the frog is well constituted to break concussion.
The bars are the ridges of horn which run on each side of the frog-forwards from the heels. They are formed by a turning-in of the wall at its posterior extremity. Between the bars and the wall are enclosed the extremities of the sole, which are often injured by a badly-fitted shoe, especially upon the inner side, and the resulting bruise is called a " corn ". The bars assist in preserving the width of the foot at the heels, and when cut away by the farrier, permit contraction of the hoof.
Fig. 625. - The Wall of the Foot: Hoof showing Insensitive Laminae, etc.
A, Peripolic horn-band. B, Coronary groove, c, Insensitive laminae. D, Horny sole. E, Horny frog.
Bars, sole, frog, and wall form one continuous horny covering to the foot. By long maceration in water they can be separated, but in a healthy living foot they are all firmly united so as to form a sound hoof. Each division should be kept in its most perfect condition, because any long-continued defect of one is certain to affect the other injuriously. If the wall at the heels be left too high, the frog soon shrinks and wastes. If the sole be cut away and weakened, the wall has to support unaided an excess of weight, and it becomes broken and diseased. Wall, sole, and frog must be kept proportionate if the proper relations of the whole hoof are to be maintained.