These joints are characterized by the presence of a joint cavity and by
their mobility. They are often called movable or true joints. The
structures which enter into their formation are:
1. The =joint surfaces= (Facies articulares), which are usually more or
less expanded. They are in most cases smooth, and vary much in form.
They are formed of specially dense bone, which differs histologically
from ordinary compact substance. In certain cases (_vide_ Osteology) the
surface is interrupted by non-articular cavities known as =synovial
fossæ=.
2. The =articular cartilages= (Cartilagines articulares), usually
hyaline in type, form a covering over the articular surfaces of the
bones. They vary in thickness in different joints; they are thickest on
the areas of the greatest pressure, and usually tend to accentuate the
curvature of the bone, _i. e._, on a concave surface the peripheral part
is the thickest, while on a convex surface the central part is the
thickest. The articular cartilages are non-vascular, very smooth, and
have a bluish tinge in the fresh state. They diminish the effects of
concussion and greatly reduce friction.
[Illustration: