h erodes
the surrounding bone and so causes the abscess to increase in size. If
the erosion proceeds uniformly, the cavity is spherical or oval; if it
is more active at some points than others, diverticula or tunnels are
formed, and one of these may finally erupt through the shell of the bone
or into an adjacent joint. Small irregular sequestra are occasionally
found within the abscess cavity. In long-standing cases it is common to
find extensive obliteration of the medullary canal, and a considerable
increase in the girth of the bone.

[7] Alexis Thomson, _Edin. Med. Journ._, 1906.

[Illustration: FIG. 120.--Segment of Tibia resected for Brodie's
Abscess. The specimen shows two separate abscesses in the centre of the
shaft, the lower one quiescent, the upper one active and increasing in
size.]

The size of the abscess ranges from that of a cherry to that of a
walnut, but specimens in museums show that, if left to Nature, the
abscess may attain much greater dimensions.

The affected bone is not only thicker and heavier than normal, but may
also be curved or otherwise deformed as a result of the original attack
of osteomyelitis.

The _clinical features_ are almost exclusively local. Pain, due to
tension within the abscess, is the dominant symptom. At first it is
vague and difficult to localise, later it is referred to the interior of
the bone, and is described as "boring." It is aggravated by use of the
limb, and there are often, especially during the night, exacerbations in
which the pain becomes excruciating. In the early stages there are
periods of days or weeks during which the symptoms abate, but as the
abscess increases these become shorter, until the patient is hardly ever
free from pain. Localised tenderness can almost always be elicited by
percussion, or by compressing the bone between the fingers and thumb.
The pain induced by the traction of muscles attached to the bone, or by
the weight of the body, may interfere with the function of the limb, and
in the lower extremity cause a limp in walking. The limb may be disabled
from _involvement of the adjacent joint_, in which there may be an
intermittent hydrops which comes and goes coincidently with
exacerbations of pain; or the abscess may perforate the joint and set up
an acute arthritis.

The _diagnosis_ of Brodie's abscess from other affections met with at
the ends of long bones, and particularly from tuberculosis, syphilis,
and new growths, is made by