DISEASES OF THE JOINTS 423 
Tn the early stages the contents of the joint are composed 
of thin pus and synovia. Later, as destruction of the synovial 
membrane proceeds, the flow of synovia is stopped, while 
the pus formation goes on until finally nothing but pus 
and dead tissue products fill the cavity. 
If the suppurative process has commenced from within, 
the pus that is formed is, as a rule, thick and creamy, 
comparatively unstained, and free from marked odour. If, 
on the other hand, air has gained access to the joint, or 
the suppurative process has started from the materials 
introduced by a foreign body, the joint contents are thin, 
blood-stained, and stinking. 
The inflammatory changes in the joint soon spread to the 
ligaments, and to the soft structures in contact with them. 
This means that the ligaments become infiltrated with 
inflammatory exudate, that the fibrous bundles composing 
them become separated, and that the ligaments are weakened 
and easily stretched. As a consequence, a certain amount 
of displacement or dislocation of the bones is allowed. 
In like manner the inflammatory changes keep spreading 
until we have the periosteum next the ends of the bones 
affected. The periostitis thus set up invariably takes the 
osteoplastic form, and as a result of this we have growths of 
new bone in the near neighbourhood of the joint. It is in 
the later stages of the disease—that is, when the pus has 
been evacuated and reparative changes commenced—that 
this osteoplastic periostitis is most marked, and it plays 
a large part in bringing about the condition of anchylosis, 
which we shall afterwards describe. 
Grave changes also occur in the articular cartilages. 
They quickly lose their peculiar glistening polish, their 
semitransparency is lost, and the natural tint of a pearl-like 
blue gives way to a dirty yellow. Later this is followed by 
erosion of the cartilages at such points as they happen to be 
in greatest contact. The ends of the bones are thus exposed, 
and their. medullary cavities exposed to infection. As a 
result we get in them the changes we have already described 
under Ostitis. 
