DISEASES OF BONES 189 



tTie rarefying osteitis, -wMcli destroys it here and there, mak- 

 ing openings into it tlirougli whicli the pus and sequestrum 

 may be gotten rid of. The openings in the involucrum, the 

 lesult of the rarefying osteitis, are termed cloacae. In time 

 the soft tissues surrounding the sequestrum may undergo 

 gangrenous changes and the pus and bony fragments reach 

 thb external world at the point of least resistance, at which 

 moment a fistula is established. 



What, is an exfoliation f 



A superficially located sequestrum, the result of a super- 

 ficial necrosis. 



Mow do you treat hone necrosis ? 



The essential feature consists in the removal of the 

 necrotic fragment. For this purpose the chisel, the curette 

 or the trephine may have to be employed. As a rule, caustic 

 injections and even the actual cautery are less desirable. 



Atrophy of Bone. 



What is atrophy of hone ? 



A decrease in size of the bone without change in its 

 •osseous structure. 



What forms of hone atrophy are recognized f 



1. Concentric : The thinning progresses from without to 

 within, as a result of which the diameter of the shaft becomes 

 lessened. 



3. Excentric : The thinning progresses from within to 

 "without. 



Give examples of atrophy of hones. 



In sheep the cranial bones may become thin as paper as 

 the result of the pressure upon the bone by the coenurus cere- 



