d the expansion of the coronary edge of the heels
disappears from behind forwards, passing forwards like a fluid wave. In
the lateral and anterior regions of the coronary edge the contraction
disappears; and when the weight is thrown off the foot it passes into a
gentle expansion of the coronary edge of the toe. During the opposite
movement of the fetlock, that of sinking backwards, this change of form is
executed in the converse manner.

In short, the coronary edge resembles a closed elastic ring, which yields
to pressure, even the most gentle, of the body-weight, in such a way that a
bulging out of any one part is manifested by an inward movement of another
part.

In Fig. 37, _b_, the dotted line represents the changes of form in
comparatively well-formed and sound hoofs at the moment of strongest
over-extension[A] of the fetlock-joint.

[Footnote A: The term 'over-extension,' as employed by Lungwitz, is
intended to indicate that position assumed by the fetlock-joint when the
opposite foot is raised from the ground.]

2. BEHAVIOUR OF THE SOLAR EDGE.--Under the action of the body-weight this
is somewhat different from that of the coronary edge. Anteriorly, and at
the sides, as far as the wall forms an acute angle with the ground,
the tendency to expansion exists, but the change of form first becomes
measurable in the region where the lateral cartilages begin. Quite
posteriorly the expansion again diminishes.

Fig. 37, _a_, by the dotted line represents the expansion at the moment of
over-extension of the fetlock-joint. This expansion is itself rather less
than at the coronary edge, and it shows itself distinctly _only when the
weighted hoof is exposed to a counter-pressure on the sole and frog_, no
matter whether the counter-pressure is produced naturally or artificially.
Thus anything tending to the removal of the pressure from below, such as
a decayed condition of the frog or excessive paring in the forge, will
diminish the extent of expansion of the solar edge.

Contraction of the solar edge of the heels occurs at the moment of greatest
over-extension of the fetlock-joint--that is, in a foot with pressure
from below absent. On the face of it, this appears impossible. Lungwitz,
however, has perfectly demonstrated it; and, when dealing with the
functions of the lateral cartilages in a later paragraph, we shall show
reason for why it is but a simple and natural result of the foot dynamics.

3. BEHAVIOUR OF THE S