[Illustration:  FIG. 79.—­HOOF WITH LOCAL OR CORONARY CONTRACTION (AS INDICATED AT THE POINTS a, a).]

The explanation of the restricted nature of this form of contraction is simple enough.  We have only to refer to the lessons taught by the experiments of Lungwitz, described in Chapter III., and the condition almost explains itself.  We remember that, briefly, the coronary margin of the wall resembles a closed elastic ring, which yields and expands to local pressure, no matter how slight.  We remember also that removal of the counter-pressure of the frog with the ground tended to contraction of the wall’s solar edge when weight was applied.  Connect these two facts with the experience that this form of contraction more often than not occurs in hoofs with sloping heels, and we arrive at the following: 

1.  The excessive slope of the heels tends to throw a more than usual part of the body-weight upon the posterior portion of the coronary margin of the wall, with a consequent expansion of that part of the coronary margin implicated.

2.  That the shoeing, in removing the counter-pressure of the frog with the ground, is at the same time tending to bring about contraction of the lower portions of the wall at the heels and quarters.

3.  That this tendency to contraction will at first appear in the thinner portion of the area of wall named—­namely, in that immediately below the bulging coronary margin.

We thus get the appearance depicted in Fig. 79—­a contraction (a, a) of the heels in the horn below the coronary margin, with the coronary margin itself bulging above, and a hoof of apparently normal width below.

We say ‘apparently’ with a purpose, for, as actual measurements will show, the wall near the solar edge is really contracting, for reasons which we have just described connected with shoeing.  Its ‘appearance’ of normal width is accounted for thus:  The contraction at a, a is caused by the dragging inwards of the coronary cushion brought about by the sinking downwards of the plantar cushion, with which body it will be remembered the coronary cushion is continuous.  With the constant dragging in and down of the coronary cushion there is given, to the horn-secreting papillae, studding both the lower third of its outer face and its lowermost surface, a distinct ‘cant’ outwards.  Below the lowermost limit of the coronary cushion, then, by reason of the cant outwards of the coronary papillae in the situations mentioned, the horn of the wall takes a more outward direction than normal, a fact which lessens in effect the contraction as a whole really going on.  It is interesting, too, to note that by this outward cant of the wall below, and the bulging of the coronary margin above it, the contraction (a, a) is heightened in effect, and caused to appear greater than really it is.

From what we have said it follows that contraction of the heels, excepting the extreme coronary margin, is existent generally, and not confined solely to a, a.