by the diffuse
thickening of the synovial membrane. In a small number there is an
abundant serous exudate, and with this there may be a considerable
formation of fibrin, covering the surface of the membrane and floating
in the fluid as flakes or masses; under the influence of movement it may
assume the shape of melon-seed bodies. More rarely the joint contains
pus, and the surface of the synovial membrane resembles the wall of a
cold abscess.

_Ulceration and Necrosis of Cartilage._--The synovial tissue covering
the cartilage causes pitting and perforation of the cartilage and makes
its way through it, and often spreads widely between it and the
subjacent bone; the cartilage may be detached in portions of
considerable size. It may be similarly ulcerated or detached as a result
of disease in the bone.

_Caries of Articular Surfaces._--Tuberculous infiltration of the marrow
in the surface cancelli breaks up the spongy framework of the bone into
minute irregular fragments, so that it disintegrates or crumbles
away--caries. When there is an absence of caseation and suppuration, the
condition is called _caries sicca_.

The pressure of the articular surfaces against one another favours the
progress of ulceration of cartilage and of articular caries. These
processes are usually more advanced in the areas most exposed to
pressure--for example, in the hip-joint, on the superior aspect of the
head of the femur, and on the posterior and upper segment of the
acetabulum.

The occurrence of _pathological dislocation_ is due to softening and
stretching of the ligaments which normally retain the bones in position,
and to some factor causing displacement, which may be the accumulation
of fluid or of granulations in the joint, the involuntary contraction of
muscles, or some movement or twist of the limb. The occurrence of
dislocation is also favoured by destructive changes in the bones.

_Peri-articular tubercle and abscess_ may result from the spread of
disease from the bone or joint into the surrounding tissues, either
directly or by way of the lymphatics. A peri-articular abscess may
spread in several directions, sometimes invading tendon sheaths or
bursae, and finally reaching the skin surface by tortuous sinuses.

Reactive changes in the vicinity of tuberculous joints are of common
occurrence, and play a considerable part in the production of what is
clinically known as _white swelling_. New connective tissue forms in the
p