this brief introduction we will now describe such pathological changes
as occur in the separate structures, and which we are likely to encounter
in the various diseases of the foot. While so doing, we shall draw
attention to such diseases as we have previously described in which the
pathological conditions we are considering may be met with.

1. PERIOSTITIS.

This we shall consider under _(a)_ Simple Acute Periostitis, _(b)_
Suppurative Periostitis, _(c)_ Osteoplastic Periostitis.

_(a) Simple Acute Periostitis_.--This is the periostitis that follows on
the infliction of a slight injury to the membrane--an injury without an
actual wound and free from infective material. It is one, therefore, which
we always judge as existing in those cases where we have distinct evidence
or history of injury, but in which the injury has not been severe enough to
lead to fracture or to the infliction of an actual wound.

Such cases may be those of lamenesses persisting after violent blows upon
the foot--cases where the animal has been kicking against the stable
fittings, or where the foot has been partially passed over by the wheel of
a waggon. It may be, too, that in a case of 'nail-bound' a great deal of
the pain and lameness is due to a simple periostitis caused by pressure of
the bulged inner-layer of horn upon the sensitive structures.

Simple acute periostitis may also occur in cases where an actual wound is
in existence, but where such wound, fortunately, remains aseptic. We may
thus have this condition accompanying ordinary cases of pricked foot, of
treads in the anterior region of the coronet, and of accidental injuries of
other kinds.

In simple acute periostitis the membrane is thicker and redder than normal,
and is easily stripped from the bone. As it is pulled off it is noticed
that there are numerous fibril-like processes hanging to its inner surface,
and which draw out from the substance of the bone. These are simply the
vessels (bloodvessels and nerves) which, loosened by the inflammatory
exudate, are readily detached and drawn from the Haversian canals into
which they normally run. In addition to its increased redness, the membrane
has a swollen and gelatinous appearance owing to its infiltration with the
inflammatory discharges. Simple acute periostitis may and often does end
in resolution. On the other hand, it may end in suppuration or may become
chronic. If the latter, then the osteoblasts of the innermost layer