All the synovial membranes secrete a lubricating fluid, termed
_synovia_, which enables the surfaces of the bones and ligaments to move
freely upon one another. When this fluid is secreted in excessive
quantities, it produces a disease known as "dropsy of the joints." There
are numerous smaller sacs besides the synovial, called _bursæ mucosæ_,
which in structure are analogous to them, and secrete a similar fluid.
Some joints permit motion in every direction, as the shoulders, some in
two directions only, as the elbows, while others do not admit of any
movement. The bones, ligaments, cartilages, and synovial membrane, are
supplied with nerves, arteries, and veins.

When an animal is provided with an internal bony structure, it indicates
a high rank in the scale of organization. An elaborate texture of bone
is found in no class below the vertebrates. Even in the lower order of
this sub-kingdom, which is the highest of animals, bone does not exist,
as is the case in some tribes of fishes, such as sharks, etc., and in
all classes below that of the cartilaginous fishes, the inflexible
substance which sustains the soft parts is either shell or some
modification of bone, and is usually found on the outside of the body.
True bone, on the contrary, is found in the interior, and, therefore, in
higher animals, the skeleton is always internal, while the soft parts
are placed external to the bony frame. While many animals of the lowest
species, being composed of soft gelatinous matter, are buoyant in water,
the highest type of animals requires not only a bony skeleton, but also
a flexible, muscular system, for locomotion in the water or upon the
land. Each species of the animal kingdom is thus organically adapted to
its condition and sphere of life.

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