nsion of ligaments and of the muscles surrounding the joint.
The articular capsule (capsular ligament) is directly continuous with
the periosteum, and is lined by a synovial layer, which at the line of
attachment of the capsule is reflected on to the bone as far as the
articular cartilage. The synovial layer invests intra-articular
ligaments, and is projected into the interior of the joint in the form
of loose folds wherever the articulating surfaces are not in immediate
contact. The surface of the synovial layer is covered with minute
processes or villi, which in diseased conditions may become
hypertrophied. The synovia owes its lubricating property to mucin,
derived from the solution of the endothelial cells on the free surface
of the synovial layer. The opposing surfaces of a joint being always
in accurate contact, the so-called cavity is only a potential one. If
fluid is poured out into the joint, the synovial layer and the capsule
are put upon the stretch, causing discomfort or actual pain, which is
partly relieved by slightly flexing the joint. If the distension
persists, the ligaments become elongated and the joint unstable.

The common origin of bone, cartilage, periosteum, and synovial layer
from one parent tissue of the embryo, accords with the readiness with
which any one of these tissues may be converted into another under
traumatic or pathological influences; and how in ligaments and in
synovial membrane foci of hyaline cartilage may form and, after
increasing in size, undergo ossification.

Joints derive an abundant blood supply through the articular arteries.
The lymphatics, which take origin in the synovial layer, pass to
efferent vessels which run in the intermuscular and other
connective-tissue planes of the limb. The nerve supply is derived
chiefly from the nerves distributed to the muscles acting on the joint
and to the skin over it.

#Sources of Joint Strength.#--The capacity of a joint to resist
dislocation depends upon (1) the shape of its osseous elements; (2)
the strength and arrangement of its ligaments; (3) the support it
receives from muscles or tendons placed in relation to it; and (4) the
relative stability of adjacent structures. While all these factors
contribute to the strength of a given joint, one or other of them
usually predominates, so that certain joints are osseously strong,
others are ligamentously strong, while a few depend chiefly upon
adjacent muscles for their stability.