386 DISEASES OF THE HORSE'S FOOT 



2. Ostitis. 



We shall next deal with the inflammatory changes occur- 

 ring in the bones themselves, and shall consider them 

 under (a) Rarefying or Rarefactive Ostitis, (h) 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 

 inflammatory 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. 



