The Art of Horse-Shoeing: A Manual for Farriers

by William Hunting

The details I have given of the structure and uses of each separate part of the foot will, I hope, be sufficient to enable us to understand the form and action of the organ as a whole.

No one part of the foot is of greater importance than another, each is dependent for its highest development and soundest condition upon the integrity of neighbouring parts.

A weak wall allows of the flattening and spreading of the sole, whilst a weak sole permits contraction of the wall. Overgrown heels cause wasting of the frog, but low weak heels are usually accompanied by excessive development of frog.

The special function of the foot is to sustain the weight of the animal whilst standing or moving. The horse standing squarely on all four feet rests his weight chiefly on the lower circumference of the wall. On level ground the sole, on account of its arched form, takes no direct bearing, but if sole and wall be sound a proportion of all pressure applied to the wall is transmitted to the sole. So also must all weight imposed on the arch of the sole be transmitted, through its abutments or union with the wall, to the wall. If the sole be so thin that it yields to pressure then its proper action is destroyed, and instead of acting like an arch and supporting weight imposed on it, it yields and injury results. The arched form of the sole indicates that it was not intended to take a direct bearing on hard ground. On a soft surface the edge of the wall sinks and the whole under surface of the foot takes a direct bearing. Pressure of the sole on the soft surface does no harm because it is diffused evenly over the whole of the sole. We take advantage of this when the wall is diseased or injured, and we desire to throw on the sole a larger share of weight. We turn such animals out into a soft field or stable them on sand or saw-dust. Any system of shoeing founded upon the true form and action of the foot must recognise the arch, and not endeavour to force the sole to take a bearing for which it is not adapted. There is only one part of the sole which should act as a bearing surface, viz., that outer border which is firmly joined to the wall. This part—the abutment of the arch—is destined by nature to take a bearing and through it the whole of the sole supports its share of weight.

The frog takes a bearing on the ground but it has a weight sustaining function quite secondary to the harder and firmer parts of the hoof. It is formed of a softer horn, and it has above it only soft tissues which permit yielding. The frog then, when weight is placed upon it by the standing horse, recedes from pressure, and leaves the heels (wall and bars) to sustain the primary weight. Wall, sole, and frog, each take their share in supporting weight, but this function is distributed over them in different degrees, and it is fulfilled by each in a varying manner. During progression the foot is repeatedly raised from and replaced on the ground. It has not only to support weight but to sustain the effects of contact with the ground at each step, and the effects of being the point of resistance when the body is carried forward and the foot is again raised from the ground.

What part of the foot comes first to the ground? Many different answers have been given to this question. It has been said by some that the toe first touches the ground, by others that the foot is laid flat down, and by a few that the heel is the first part to come in contact with the ground. Fortunately it is not now necessary to argue this question on a purely theoretical basis. Instantaneous photography has shown that on level ground, at all paces, the horse touches the ground first with the heel. This fact gives significance to the structural differences we find between the front and back portions of the foot. At the back part of the foot we have the wall thinner than elsewhere, we have the moveable and elastic frog, the lateral cartilages, and the frog-pad. We have in fact a whole series of soft and elastic structures so arranged as to provide a mechanism best adapted to meet shock and to avoid concussion. Whilst drawing heavy loads, or ascending or descending hills, the horse may vary his action to suit the circumstances, and then we have the exception which proves the rule—then we have sometimes the heel, sometimes the toe brought first to the ground.

At the time when the foot first touches the ground, the leg is extended forward and the pastern is in the same oblique position to the shank as when a horse is standing. This obliquity of the pastern is another safeguard against concussion, and it renders impossible the first contact with the ground at any point other than at the heel. As the leg becomes straightened, the weight of the body is imposed upon the foot, but the greatest strain arrives just before the toe leaves the ground, for then there is not only weight to sustain, but the friction to be borne which results from the toe being the fulcrum upon which falls the whole effect of the muscular effort necessary to raise and carry forward the body of the animal. The front part of the foot is structurally well adapted for its use. It presents the thickest and strongest part of the horny covering, and, as an inside basis, it has the unyielding coffin bone. Thus we have at the toe strength and rigidity—at the heels strength and elasticity.

Another important point in the action of the foot is implied by the question—does it expand when weight is thrown on it? The principles of horse-shoeing require that this question should be answered. There are those who say that the foot does not alternately expand and retract as weight is placed upon or removed from it. There are others who assert that the expansion of the foot is an important natural function that must be provided for in any system of shoeing. It is agreed by most observers that at the upper border of the hoof, more particularly at the heels, expansion does occur. It is when we come to the lower border of the foot that the statements are most conflicting. Ordinary measurements taken at this part with calipers or by tracings on paper of the foot when raised from the ground and when resting upon it, show no variations in the width of the foot. These methods of measurement are not sufficiently delicate to be trustworthy. Experimentalists in Germany and in this country have recently used an apparatus by which the slightest variations are detected by electrical contact, and the results are very interesting. These experiments show that in a well-formed, healthy foot the hoof throughout its posterior two-thirds does expand to pressure, and perhaps that the arch of the sole is slightly flattened. This expansion is, however, comparatively slight—about equal to the thickness of a sheet of writing paper—and may practically be disregarded in considering the best methods of shoeing sound feet.

One result of these experiments is to show what an important part the frog plays in the foot, and also how the action of one part depends upon the conditions of others. When the frog rests firmly on the ground and weight is placed upon the foot expansion occurs, especially at the upper or coronary border of the hoof. When the frog does not touch the ground and weight is imposed upon the foot, contraction occurs. The explanation of this difference seems to be as follows. When weight is placed upon a foot, the coronet bone is depressed upon the soft mass of the frog-pad. With a sound frog taking a bearing upon the ground, the frog-pad cannot descend, and the compression to which it is therefore submitted causes it to bulge laterally and so expand the back of the foot. When the frog does not reach the ground and weight is placed upon the frog-pad, there is nothing to prevent it yielding downwards, and in so doing the fibrous bands connecting together the two lateral cartilages of the foot are depressed and the cartilages drawn together—hence the contraction of the foot. No better illustration could be given of the unity of all parts of the foot, and how one or many parts may suffer if the structure or function of one be defective.

There is one more movement of the hoof which is possible and which must be referred to, as it has been made the basis of a grave error in shoeing. I have said the back part of the foot is elastic and yielding. If you examine a shoe, so applied to a foot that an inch or more of its extremity has no contact with the hoof, you will find that when weight is rested on that foot the horn yields downwards and comes in contact with the shoe. This simply demonstrates that when there is nothing to support it the horn at the heels may be forced downwards. It is not a normal action, and in an unshod foot cannot occur on a level surface. The effect of this downward movement of the heels is to put a strain on the horn of the quarters. A shoe so fitted as to permit this evil is in common use, and no fault is more serious than thus forcing an unnatural action upon the hoof at every step. With unintentional irony this piece of bad work has been called "easing the heels."