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 bloodvessels.  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 sometimes 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.  For an example of rarefactive ostitis as met with in cases of disease of the feet, we refer the reader to laminitis (see Fig. 118).  The osteoplastic or condensing process that appears to exist simultaneously with it explains, no doubt, how it is that bones so affected do not more commonly fracture.

A further example of this process is illustrated in Fig. 133.  The pressure of a tumour (in this case a keraphyllocele) has led to rarefactive changes in the bone, forming a neat indentation in the normal contour of the bone which serves to accommodate the tumour.

(b) Osteoplastic Ostitis, Osteosclerosis, or Condensation of Bone.—­This, too, is essentially a chronic process.  It may occur as a result of, or, as we have just shown, exist simultaneously with the condition of, diffuse rarefactive ostitis.  In this case there is a formation of new bone in the connective tissue surrounding the vessels in the Haversian canals.  As a consequence the bone affected is greatly increased in density, and many of the Haversian canals by this means obliterated.  The end result is an increase in size of the bones in such positions as the horny box admits of it, and a peculiar ivory-like change in their consistence.

For an example of this, we again refer the reader to the changes occurring in chronic laminitis.

(c) Caries and Necrosis.—­Caries is a word which appears to be used with a considerable amount of looseness.  In addition to the meaning implied by necrosis (namely, ‘death’ of the part), caries is generally used to indicate that there is also a condition of rottenness, decay, and stench.  It is particularly applied, in fact, when the death of the bone is slowly progressive, and is due to the inroads made upon it by putrefactive or septic matter.