With its bearing surface too wide, the shoe immediately exerts direct pressure upon the sole with every movement of the animal. The sole normally is not made to receive this, and harm is bound to result.
Among other ill-fitting shoes we may mention the one with branches too short, and the one with the extremities of the branches too pointed. In the first case, as wear of the shoe proceeds, the thinned end is far more likely to turn in under the seat of corn than is a shoe with branches of ordinarily correct length. It is evident in the second case that the pointed branch, when thinned, is a more dangerous agent than the branch which is nearer the square at its end.
The matter contained in the first half of the foregoing paragraph explains in a large measure the rarity of corns in the hind-feet. Here there is nothing to prevent a shoe with branches of full length being used. The correct bearing is thus maintained, even with a shoe excessively thinned with wear, and the liability to injury from it decreased. An exception is to be found in the case of a feather-edged shoe, such as is used to prevent cutting or brushing. The thinning by wear from above to below of the branch already purposely thinned from side to side leads to the formation of a thin and narrow piece of iron admirably calculated to bend over and injure the sole.
Even with a shoe of correct length, with a flat-bearing surface at the heels, and other conditions favourable to correct application, evil may still result from the shoe itself being made too narrow. As a result of this, the branch of each side is set too far under the foot, with consequent injury to the sole. This is, of course, sheer carelessness on the part of the smith. When practised, however, it is not easy of detection, as in all cases the foot is rasped down to cover what has been done. In other words, the foot is made to fit the shoe and not the shoe the foot.
Recognising this close fitting of the shoe as a cause, we are able to explain in some measure how it is that corns should occur with greater frequency in the inner than in the outer heel. There is no doubt that the inner branch of the shoe is nearly always fitted closer than is the outer. In the fore-foot it is also often shorter. Take these two evils and add to them the fact that the inner heel is called upon to bear more of the body-weight than is the outer, and the frequency of corns in the inner heel will no longer be wondered at.
Indirectly, the shoe may still be a cause of corn by reason of the irritation set up by gravel and small pieces of flint becoming firmly fixed between the sole and the web of the shoe. In nearly every case of this description the part to be injured is the white line.
Corns may also result from the animal picking up a stone. The stone becomes firmly wedged in between the inner border of the branch of the shoe and the bar or the frog. With every step the animal takes it becomes wedged more tightly into position. Projecting below the level of the lower surface of the shoe, it imparts the concussion it thus obtains directly to the sole. A bruise - and a bad bruise - is the result.
Finally, it cannot be denied that the work the horse is put to is largely responsible for the causation of corn. In country animals corns are comparatively rare, while in animals in town, almost constantly upon hard paving, they are common. This seems to point strongly to the fact that concussion through constant work upon unyielding roads is a great factor in their production.
Symptoms. - Unless the discoloration of the horn is accidentally discovered by the smith, the simple, dry corn may go undetected. The disturbance excited by it is so small, and the pain occasioned so slight, that the patient may offer no indication of its existence.
Ordinarily, however, the first symptom is that of pain. The animal goes feelingly with one or both feet, in some cases even showing decided lameness. The lameness, however, is in no way diagnostic, and the lesion itself must be discovered before an exact opinion can be pronounced.
As an aside, it is well to observe in this connection that a negative opinion as to the existence of corn should never be given unless the superficial layers of horn have first been removed with the knife.
When standing at rest the animal exhibits signs more or less common to all foot lamenesses. He 'points' the foot - in other words, the limb is slightly advanced, the fetlock partly flexed, and the heels from off the ground. When both feet are affected they are pointed alternately, and the animal often manifests his uneasiness by repeated pawing movements, and by scraping his bedding behind him.
Should the injury run on to suppuration, the lameness becomes most acute. The pawing movements become more pronounced, and there is evident disinclination on the part of the animal to place the foot squarely on the ground. One is then led to manipulate the foot. The hoof is hot to the touch. Percussion causes the animal to flinch, and to flinch particularly when that portion of the wall adjoining the corn is struck. Finally, exploration with the knife reveals the serious extent to which the injury has developed. In a neglected case of this description it is even possible to detect the presence of pus by the amount of swelling and fluctuating condition of the coronet. The suppurative process has advanced in the direction of least resistance, and is on the point of breaking through the tissues immediately above the horn.
Lameness due to corn is oftentimes intermittent. With a simple corn, dry or moist, this intermission is largely dependent on the degree of dryness of the hoof or the road, and also on the character of the road surface. With a neglected, suppurating corn, on the other hand, variation in the degree of lameness, in addition to depending on circumstances such as these, is dependent to a larger extent upon the changes occurring with the suppuration. In this case the time of greatest lameness is immediately before the pus gains outlet. Immediately after its exit at the coronet the animal will go almost sound. Soundness continues so long as the opening at the coronet remains clear. The tendency, however, is for the opening thus made to quickly close again. Pus again accumulates, lameness arises as before, and disappears again with the second discharge of the contents of the sinus now formed.