it as hereditary. Seeing further,
however, that it only appears when shoeing is commenced, we may in a
greater degree also regard it as acquired. The lesson, therefore, that this
and other forms of contraction should teach us is the carefulness with
which the shoeing should be superintended in a large stud, or in any case
where the animal is of more than ordinary value.

[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 cus