into two branches, which diverge as they pass backward
into the horny bulbs of the heel. In front of the middle cleft the two
branches unite to form the body of the frog, which ends in the point of
the frog. The bar of a bar shoe should rest on the branches of the frog.
In unshod hoofs the bearing edge of the wall, the sole, frog, and bars
are all on a level; that is, the under surface of the hoof is perfectly
flat, and each of these structures assists in bearing the body weight.

With respect to solidity, the different parts of the hoof vary widely.
The middle layer of the wall is harder and more tenacious than the sole,
for the latter crumbles away or passes off in larger or smaller flakes
on its under surface, while no such spontaneous shortening of the wall
occurs. The white line and the frog are soft-horn structures, and differ
from hard horn in that their horn cells do not under natural conditions
become hard and hornlike. They are very elastic, absorb moisture
rapidly, and as readily dry out and become hard, brittle, and easily
fissured. Horn of good quality is fine grained and tough, while bad horn
is coarse grained and either mellow and friable or hard and brittle. All
horn is a poor conductor of heat, and the harder (drier) the horn the
more slowly does it transmit extremes of temperature.

THE PHYSIOLOGICAL MOVEMENTS OF THE HOOF.

A hoof while supporting the body weight has a different form, and the
structures inclosed within the hoof have a different position than when
not bearing weight. Since the amount of weight borne by a foot is
continually changing, and the relations of internal pressure are
continuously varying, a foot is, from a physiological viewpoint, never
at rest. The most marked changes of form of the hoof occur when the foot
bears the greatest weight, namely, at the time of the greatest descent
of the fetlock. Briefly, these changes of form are: (1) An expansion or
widening of the whole back half of the foot from the coronet to the
lower edge of the quarters. This expansion varies between one-fiftieth
and one-twelfth of an inch. (2) A narrowing of the front half of the
foot, measured at the coronet. (3) A sinking of the heels and a
flattening of the wings of the sole. These changes are more marked in
the half of the foot that bears the greater weight.

The changes of form occur in the following order. When the foot is set
to the ground the body weight is transmitted through the bones and