882 DISEASES OF THE HORSE’S FOOT 
cavity is extremely small—almost nil, in fact—which explains 
in some measure how easy it is when suppuration exists to 
get necrosis and exfoliation of, say, portions of the os pedis. 
Necrosis and sloughing of the periosteum itself may also 
happen, but as the extreme vascularity of the membrane is 
a fairly strong safeguard against that it is of only rare 
occurrence. 
In connection with the deep layer of the periosteum, and 
forming part of it, are found numerous bone-forming cells 
(osteoblasts). These, under ordinary conditions, are relatively 
quiescent. Under the slightest irritation or stimulation, 
however, their bone-forming functions are stirred into 
abnormal activity, thus explaining how easy it is (especially 
with bones so open to receive slight injuries as are those of 
the foot) to get ossific deposits, the starting-point of which 
we are quite unable to account for. 
With 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. Periostitts. 
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 
