386 DISEASES OF THE HORSE’S FOOT 
2. OsTirtis. 
We shall next deal with the inflammatory changes occur- 
ring in the bones themselves, and shall consider them 
under (a) Rarefying or Rarefactive Ostitis, (b) Osteoplastic 
Ostitis, and (c) Caries and Necrosis. 
Inflammatory changes occurring in the medulla we may 
pass without consideration, for in the bones of the foot the 
medullary cavity is so small, and the changes taking place 
in it of such minor importance, that we may do this with- 
out in any way seriously prejudicing our work. 
(a) Rarefying or Rarefactive Ostitis—By this term is 
indicated an inflammation of the bone attended by its 
absorption, the absorption being due to the action of 
certain cells, termed osteoclasts. This condition may be 
due to the pressure of tumours, may occur as the result of 
injury when a piece of bone is stripped of periosteum, or 
may be the result of an inflammation occurring in the 
periosteum elsewhere. 
A piece of bone undergoing rarefactive ostitis is redder 
than normal, and the openings of the Haversian canals 
are distinctly increased in size. As a result a greater 
number of them become visible. Their increase in size 
is due to the inflammatory absorption of the bony tissue 
forming them, and in the larger of them may be seen 
infammatory granulation tissue surrounding the blood- 
vessels. This enlargement of the Haversian canals is well 
seen when the bone is macerated, the whole then giving 
the appearance of a piece of very rough pumice-stone. 
This process of rarefaction or absorption of bone tissue 
may be confined to quite a small portion, or it may be 
spread over the whole of the bone, rendering it more 
porous than is normal, but stopping short of complete 
destruction of the bone tissue (a condition which is some- 
times known as inflammatory osteoporosis (see Fig. 118). 
In this latter case the condition is a chronic one, and the 
bone tissue remaining often appears to be strengthened by 
a compensatory process of condensation. 
