of the more serious and progressive forms of joint
disease, especially those of bacterial origin. The destruction of
cartilage may be secondary to disease of the synovial membrane or of the
subjacent bone. When the disease begins as a synovitis, the synovial
membrane spreads over the articular surface, fuses with the cartilage
and eats into it, causing defects or holes which are spoken of as
ulcers. When the disease begins in the bone, the marrow is converted
into granulation tissue, which eats into the cartilage and separates it
from the bone. Following on the destruction of the cartilage, the
articular surface of the bone undergoes disintegration, a condition
spoken of as _caries of the articular surface_. The occurrence of
ulceration of cartilage and of articular caries is attended with the
clinical signs of fixation of the joint from involuntary muscular
contraction, wasting of muscles, and starting pains. These _starting
pains_ are the result of sudden involuntary movements of the joint. They
occur most frequently as the patient is dropping off to sleep; the
muscles becoming relaxed, the sensitive ulcerated surfaces jar on one
another, which causes sudden reflex contraction of the muscles, and the
resulting movement being attended with severe pain, wakens the patient
with a start. Advanced articular caries is usually associated with some
abnormal attitude and with shortening of the limb. It may be possible to
feel the bony surfaces grate upon one another. When all its constituent
elements are damaged or destroyed, a joint is said to be _disorganised_.
Should recovery take place, repair is usually attended with union of the
opposing articular surfaces either by fibrous tissue or by bone.

#Conditions of Impaired Mobility of Joints.#--There are four conditions
of impaired mobility in joints: rigidity, contracture, ankylosis, and
locking. _Rigidity_ is the fixation of a joint by involuntary
contraction of muscles, and is of value as a sign of disease in
deep-seated joints, such as the hip. It disappears under anaesthesia.

_Contracture_ is the term applied when the fixation is due to permanent
shortening of the soft parts around a joint--muscles, tendons,
ligaments, fasciae, or skin. As the structures on the flexor aspect are
more liable to undergo such shortening, contracture is nearly always
associated with flexion. Contracture may result from disease of the
joint, or from conditions outside it--for example, diseas