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 SOLE.--The horny sole becomes flattened under the
action of the body-weight. This is most distinct at the solar branches, and
gradually shades off anteriorly and towards the circumference. As might be
supposed, width of hoof and thickness of the solar horn exert an influence
on the extent of this movement. The sinking of the horny sole is most
marked in flat hoofs.


D. THE FUNCTIONS OF THE LATERAL CARTILAGES.[A]