y when the animal is at rest, and that the faster he moves the
greater is the shock which the fore feet must receive as the body is
thrown forward by the propelling force of the hind legs. This shock
could not be withstood by the tissues of the fore feet and legs were it
not that it is largely dissipated by the elastic muscles which bind the
shoulder to the body, the ease with which the arm closes on the shoulder
blade, and the spring of the fetlock joint. Even these means, however,
are not sufficient within themselves to protect the foot from injury; so
nature has further supplemented them by placing the coffin joint on the
hind part of the coffin bone instead of directly on top of it, whereby a
large part of the shock of locomotion is dispersed before it can reach
the vertical column represented by the cannon, knee, and arm bones. A
still further provision is made by placing a soft, elastic pad--the frog
and plantar cushion--at the heels to receive the sesamoid expansion of
the flexor tendon as it is forced downward by the pressure of the
coronet bone against the navicular. Extraordinary as these means may
appear for the destruction of shock, and ample as they are when the
animal is at a slow pace or unweighted by rider or load, they fail to
relieve the parts completely from concussion and excessive pressure
whenever the opposite conditions are present. The result, then, is that
the coronet bone forces the navicular hard against the flexor tendon,
which, in turn, presses firmly against the navicular as the force of the
contracting muscles lifts the tendon into place. It is self-evident,
then, that the more rapid the pace and the greater the load, the greater
must these contending forces be, and the greater the liability to
injury. For the same reason horses with excessive knee action are more
liable to suffer from this disease than others, concussion of the foot
and intense pressure on the tendon being common among such horses.

Besides the above-mentioned exciting causes must be considered those
which predispose to the disease. Most prominent among these is heredity.
It may be claimed, however, that an inherited predisposition to
navicular disease consists not so much in a special susceptibility of
the tissues which are involved in the process as in a vice of
conformation which, as is well known, is liable to be transmitted from
parent to offspring. The faults of conformation most likely to be
followed by the developm