We may consider the causes of contraction under two headings—­predisposing and exciting.

Predisposing Causes of Contraction.—­Among these we will first mention heredity, although it is possible it should not be deemed of so great account as it is by some.  That the shape of certain feet, especially those with low heels and abnormally sloping walls, predisposes to contraction no one will deny.  So long, however, as the animal goes unshod, so long does the foot maintain a normal condition of the heels.  In other words, it is not until the tendency to contraction already there is aggravated by careless shoeing and the effects of work that it operates to any noticeable extent.

The degree of contraction will also be very largely governed by the amount of the development of the frog.  With a frog of good size, low down, and taking part in the pressure of the foot on the ground, contraction will be prevented.  On the other hand, an ill-developed frog, one wasted by long-continued and spreading thrush, or one robbed of its normal function by excessive paring in the forge, is a common starting-point of the condition we are considering.  We have already referred to this in Chapter III., when considering the experiments of Lungwitz in this connection.  What we have to bear in mind in these experiments is that the application of a pad to the frog, in such a manner that effective ground-pressure is obtained, results always in a marked expansion of the heels, and that, with counter-pressure with the ground absent, expansion occurs to little or no extent.  This is proof positive of the enormous part the frog plays in maintaining an open and elastic condition of the heels—­a fact so insisted on by Coleman.

It is worthy of mention, however, that loss of the frog’s function does not operate to nearly so serious an extent in horses with high, upright heels as in those with the heels low and excessively sloping.

In illustrating this, Mr. Dollar, in his work on shoeing, mentions the case of a pair of trotting horses of similar age, size, and weight, each having weak fore-heels.  In one case the hoofs were flat, in the other upright.  The horse with the flat hoofs suffered from contraction, while the other did not.

The reason appears to be that in the animal with upright hoofs the proportion of body-weight borne by the heels is considerably less than in those with the hoofs flat and sloping.

Certain conditions of the horn-producing membranes also predispose to contraction.  For example, in horses reared on marshy soils, and afterwards transferred to standing in town stables, we find that a dry and brittle condition of the horn supervenes.  This we may regard as a low form of laminitis, brought about by the heat of the material upon which the animal is standing, and the congestion of the feet engendered by his enforced standing for long periods in one position, as opposed to the more or less continuous exercise when at pasture.  With the hoof in this condition it loses by evaporation the moisture that normally it should contain, and, as we might expect, a certain degree of contraction of its structure is the inevitable result.