It is in this way: We have only to refer to the chapter on anatomy to see
that the whole of the foot is covered with a tissue of extreme vascularity.
Thus we find papillæ--the over the coronary cushion; enlarged and modified
papillæ sensitive laminæ--covering the anterior face of the os pedis; and
numberless papillæ again covering the sole. There can be no doubt that the
quantity of fluid brought by the bloodvessels of these papillæ to the foot
acts largely as a means of hydraulic protection to the soft structures.[A]
In like manner as that delicate organ, the brain, is best protected by
being floated upon the cerebro-spinal fluid and bloodvessels (which fluids
transmit waves of concussion or pressure _through_ the organ without injury
to the delicate cells forming it), so, in like manner, does the extreme
vascularity of the foot protect the cells of its softer structures from the
effects of pressure and concussion.

[Footnote A: The _Veterinary Record_, vol. iii., p. 518.]

That this law of hydraulics may operate in the horse's foot to the best
advantage, the veins must be provided with valves, and valves of no
mean strength. These we know to be absent. It is here that the lateral
cartilages and the elastic substances of the coronary and plantar cushions
step in to supply the deficiency.

At the time when weight is placed upon the foot (with, of course, a
tendency to drive the blood upwards in the limb), and, therefore, the time
when a valvular apparatus is needed to retain the fluid in the foot, we
find the wanting conditions supplied by the pressure outwards of the
plantar cushion compressing the large plexuses of veins on each side of the
lateral cartilages, to which plexuses, it will be remembered, the bulk
of the venous blood from the foot was directed. A more perfect valvular