jacent structures rendered deficient. The
tissues of the bone and bursa are insufficiently nourished, and the
secretion of synovia lessened. In this way it is conceivable that navicular
disease may follow the condition of simple contracted heels.

In common with the other structures, the lateral cartilages also suffer
from the continual pressure. Their blood-supply is lessened, their
functions interfered with, and side-bones result.

_Causes_.--Upon the causation of contraction a very great deal has been
written, both by early veterinarians and by those of the present day. Many
and widely differing opinions have been advanced, but a careful resume of
only a few will lead one to certain fixed conclusions.

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,