We thus see that contraction brought about in this way is not so much caused by the heat of the stable, as it is by the decreased ability of the horn to retain its own moisture.

On the other hand, it cannot be denied that excessive warmth and dryness combined tend also to an undue abstraction of moisture, even from the horn of the healthy foot; and this explains in great measure how it is that lameness, as a rule, and especially that proceeding from contracted heels, is far more frequent and of greater intensity in the hot, dry months of summer, than in the cooler and more humid atmosphere of winter.  It is interesting to note, too, that an alternation of humidity and dryness is far more liable to injure the quality of the horn and tend to its contraction than the long-continued effects of dryness alone.  A common illustration of this is to be found in the effects of the ordinary poultice.  Everyone knows that when, after a few days’ application, they are discontinued, we get as a result an abnormally dry and brittle state of the horn.  This is doubtless due to the poultice removing the thin, varnish-like, and protective pellicle known as the periople, and thereby allowing the process of evaporation to act on the water normally contained in the hoof.

Exciting Causes of Contraction.—­Among these, first place must undoubtedly be given to shoeing.  This does not necessarily imply shoeing more than ordinarily faulty, nor a faulty preparation of the foot, but shoeing as it is generally practised.  No ordinary shoe, except a few devised for the purpose, such as the Charlier or the tip, allows the frog to come in contact with the ground.  This we take to be the main factor in the causation of contracted heels, especially with a predisposition already present in the foot itself.  In the words of Lungwitz:  ’Regarded from this point of view, there is no greater evil than shoeing.  It abolishes the necessary counter-pressure, and thus interferes with expansion.  Bars, sole, and frog cannot perform the functions that naturally belong to them as they would do without the shoe.’

In addition to the evil of the shoe itself, errors of practice in the forge contribute to the causation of contraction.  Taking first the preparation of the foot, we find that often the heels are lowered far too much, and the toe allowed to remain too long.  This can have but one effect—­that of throwing a greater proportion of the animal’s weight upon the heels than properly they should bear, with, what we now know to be the consequence of that, a corresponding pushing inwards and downwards of the horn; in other words, contraction.

Excessive paring of the bars, to which we have already partly alluded, is also an active agent in bringing about an inward growth of the horn of the heels and quarters.  The bar, or inflexion of the wall at the heel, by means of its close contact with the frog, communicates the outward movements of that organ to the wall of the hoof.  With the bar removed, the outward movements of the frog under pressure are naturally rendered of no account, and a proper and intermittent expansion of the wall denied it.  The same evil follows, though to a less extent, excessive paring of the sole.