exudate in the joint and in the
substance of the synovial membrane. The mode of infection is the same as
in the pyogenic diseases of bone, the metastasis occurring most
frequently from the mucous membrane of the pharynx (J. B. Murphy). The
localisation of the infection in a particular joint is determined by
injury, exposure to cold, antecedent disease of the joint, or other
factors, the nature of which is not always apparent.

The effects on the joint vary in severity. In the milder forms, there is
engorgement and infiltration of the synovial membrane, and an effusion
into the cavity of the joint of serous fluid mixed with flakes of
fibrin--_serous synovitis_. In more severe infections the exudate
consists of pus mixed with fibrin, and, it may be, red blood
corpuscles--_purulent_ or _suppurative synovitis_; the synovial membrane
and the ligaments are softened, and the surface of the membrane presents
granulations resembling those on an ulcer; foci of suppuration may
develop in the peri-articular cellular tissue and result in abscesses.
In _acute arthritis_, all the structures of the joint are involved; the
articular cartilage is invaded by granulation tissue derived from the
synovial membrane, and from the marrow of the subjacent bone; it
presents a worm-eaten or ulcerated appearance, or it may undergo
necrosis and separate, exposing the subjacent bone and leading to
disintegration of the osseous trabeculae--_caries_. With the destruction
of the ligaments, the stability of the joint is lost, and it becomes
disorganised.

The _clinical features_ vary with the extent of the infection. When
this is confined to the synovial and peri-synovial tissues--_acute
serous_ and _purulent synovitis_--there is the usual general reaction,
associated with pyrexia and great pain in the joint. The part is hot and
swollen, the swelling assuming the shape of the distended synovial sac,
fluctuation can usually be elicited, and the joint is held in the flexed
position.

When the joint is infected by extension from the surrounding cellular
tissue, the joint lesion may not be recognised at an early stage because
of the swollen condition of the limb, and because there are already
symptoms of toxaemia. We have observed a case in which both the hip and
knee joints were infected from the cellular tissue.

If the infection involves all the joint structures--_acute
arthritis_--the general and local phenomena are intensified, the
temperature ris