er.

The changes observed are those of intense engorgement of the marrow,
going on to greenish-yellow purulent infiltration. Where the process is
most advanced--that is, at the ossifying junction--there are evidences
of absorption of the framework of the bone; the marrow spaces and
Haversian canals undergo enlargement and become filled with
greenish-yellow pus. This rarefaction of the spongy bone is the earliest
change seen with the X-rays.

The process may remain localised to the ossifying junction, but usually
spreads along the medullary canal for a varying distance, and also
extends to the periosteum by way of the enlarged Haversian canals. The
pus accumulates under the periosteum and lifts it up from the bone. The
extent of spread in the medullary canal and beneath the periosteum is in
close correspondence. The periosteum of the diaphysis is easily
separated--hence the facility with which the pus spreads along the
shaft; but in the region of the ossifying junction it is raised with
difficulty because of its intimate connection with the epiphysial
cartilage. Less frequently there is more than one collection of pus
under the periosteum, each being derived from a focus of suppuration in
the subjacent marrow. The pus perforates the periosteum, and makes its
way to the surface by the easiest anatomical route, and discharges
externally, forming one or more sinuses through which fresh infection
may take place. The infection may spread to the adjacent joint, either
directly through the epiphysis and articular cartilage, or along the
deep layer of the periosteum and its continuation--the capsular
ligament. When the epiphysis is intra-articular, as, for example, in the
head of the femur, the pus when it reaches the surface of the bone
necessarily erupts directly into the joint.

While the occurrence of purely periosteal suppuration is regarded as
possible, we are of opinion that the embolic form of staphylococcal
osteomyelitis always originates in the marrow.

The portion of the diaphysis which has sustained the action of the
concentrated toxins has its vitality further impaired as a result of the
stripping of the periosteum and thrombosis of the blood vessels of the
marrow, so that _necrosis_ of bone is one of the most striking results
of the disease, and as this takes place rapidly, that is, in a day or
two, the term _acute necrosis_, formerly applied to the disease, was
amply justified.

When there is marked rarefact