dmit as a cause, and in so doing
partly explain the comparative, almost total, immunity of the hind-feet
from the disease. The fore-limbs, as we have already pointed out, are
little more than props of support, and the force of the propelled
body-weight is transmitted largely down their almost vertical lines, to
end largely in concussion in the foot. With the hind-limbs matters are
different. 'These,' as Percival explains it, 'have their bones obliquely
placed, so as to constitute, one with the other, so many obtuse angles, to
the end, that by forming powerful levers, and affording every advantage for
action to the muscles attached to them, they may be fitted for the purpose
of propulsion of the body onward.'

The effect of these several obtuse-angled joints in the limb is to absorb
the greater part of the force exerted by the body-weight before it reaches
the foot. When with this we take the facts that the fore-limbs have to
carry the head and neck, and that they have to bear this added weight, plus
a propelling force from behind, we see why it is that they should be so
subject to the disease, and the hind-limbs so exempt.

As pointing out the part that concussion plays in its causation, we may
mention that navicular disease is a disease of the middle-aged and the
worked animal. It is interesting to note, too, that it occurs in animals
with well developed frogs--in feet in which frog-pressure with the ground
is most marked. This at first sight appears to flatly contradict what we
have said with regard to frog-pressure in other portions of this work. With
this, however, must be reckoned other predisposing causes. In this case it
is not to frog-pressure alone we must look, but to the condition of the
frog itself, and that of the neighbouring parts. It is when we have a frog
which, though well developed and apparently satisfying all demands as
to size and build, is at the same time composed of a hard, dry, and
non-yielding horn that we must look for trouble.

The foot predisposed to navicular disease is the strong, round, short-toed
or clubby foot, open at the heels, with a sound frog jutting prominently
out between them. Here is a frog exposed to all the pressure that might
be desired for it, bounded at its sides by heels thick and strong, and
indisposed to yield, and itself liable, from its very exposure, to become,
in the warm stable, hard and dry, and incompressible' (Percival).

Here, instead of acting, as normally