g
simultaneously with it.

2. OSTITIS.

We shall next deal with the inflammatory changes occurring 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 without 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 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 bon