nded by a sheath or case of new bone, known as the
_involucrum_ (Fig. 118). Where the periosteum has been perforated by pus
making its way to the surface, there are defects or holes in the
involucrum, called _cloacae_. As these correspond more or less in
position to the sinuses in the skin, in passing a probe down one of the
sinuses it usually passes through a cloaca and strikes the dead bone
lying in the interior. If the periosteum has been extensively
destroyed, new bone may only be formed in patches, or not at all. The
dead bone is separated from the living by the agency of granulation
tissue with its usual complements of phagocytes and osteoclasts, so that
the sequestrum presents along its margins and on its deep surface a
pitted, grooved, and worm-eaten appearance, except on the periosteal
aspect, which is unaltered. Ultimately the dead bone becomes loose and
lies in a cavity a little larger than itself; the wall of the cavity is
formed by the new case, lined with granulation tissue. The separation of
the sequestrum takes place more rapidly in the spongy bone of the
ossifying junction than in the compact bone of the shaft.

When foci of suppuration have been scattered up and down the medullary
cavity, and the bone has died in patches, several sequestra may be
included by the new case; each portion of dead bone is slowly separated,
and comes to lie in a cavity lined by granulations.

Even at a distance from the actual necrosis there is formation of new
bone by the marrow; the medullary canal is often obliterated, and the
bone becomes heavier and denser--sclerosis; and the new bone which is
deposited on the original shaft results in an increase in the girth of
the bone--hyperostosis.

[Illustration: FIG. 118.--Shaft of Femur after Acute Osteomyelitis. The
shaft has undergone extensive necrosis, and a shell of new bone has been
formed by the periosteum.]

_Pathological fracture_ of the shaft may occur at the site of necrosis,
when the new case is incapable of resisting the strain put upon it, and
is most frequently met with in the shaft of the femur. Short of
fracture, there may be bending or curving of the new case, and this
results in deformity and shortening of the limb (Fig. 119).

The _extrusion of a sequestrum_ may occur, provided there is a cloaca
large enough to allow of its escape, but the surgeon has usually to
interfere by performing the operation of sequestrectomy. Displacement or
partial extrusion of