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.