focus reaching the
surface of the bone at the site of reflection of the synovial membrane,
and the infection which begins at this point then spreads to the rest of
the membrane. Or it may take place in the central area, by the
projection of tuberculous granulation tissue into the joint following
upon erosion of the cartilage (Fig. 156).

[Illustration: FIG. 156.--Section of Upper End of Fibula, showing
caseating focus in marrow, erupting on articular surface and infecting
joint.]

_Changes in the Synovial Membrane._--In the majority of cases there is a
_diffuse thickening of the synovial membrane_, due to the formation of
granulation tissue, or of young connective tissue, in its substance.
This new tissue is arranged in two layers--the outer composed of fully
formed connective or fibrous tissue, the inner of embryonic tissue,
usually permeated with miliary tubercles. On opening the joint, these
tubercles may be seen on the surface of the membrane, or the surface may
be covered with a layer of fibrinous or caseating tissue. Where there is
greater resistance on the part of the tissues, there is active formation
of young connective tissue which circumscribes or encapsulates the
tubercles, so that they remain embedded in the substance of the
membrane, and are only seen on cutting into it.

The thickened synovial membrane is projected into the cavity of the
joint, filling up its pouches and recesses, and spreading over the
surface of the articular cartilage "like ivy growing on a wall."
Wherever the synovial tissue covers the cartilage it becomes adherent to
and fused with it. The morbid process may be arrested at this stage, and
fibrous adhesions form between the opposing articular surfaces, or it
may progress, in which case further changes occur, resulting in
destruction of the articular cartilage and exposure of the subjacent
bone.

In rare instances the synovial membrane presents nodular masses or
lumps, resembling the tuberculous tumours met with in the brain; they
project into the cavity of the joint, are often pedunculated, and may
give rise to the symptoms of loose body. The fringes of synovial
membrane may also undergo a remarkable development, like that observed
in arthritis deformans, and described as arborescent lipoma. Both these
types are almost exclusively met with in the knee.

_The Contents of Tuberculous Joints._--In a large proportion of cases of
synovial tuberculosis the joint is entirely filled up