together, held in place, and
their movements controlled, to a certain extent, by bands of various
forms, called ligaments. These are composed mainly of bundles of
white fibrous tissue placed parallel to, or closely interlaced with, one
another, and present a shining, silvery aspect. They extend from one of
the articulating bones to another, strongly supporting the joint, which
they sometimes completely envelope with a kind of cap (Fig. 28). This
prevents the bones from being easily dislocated. It is difficult, for
instance, to separate the two bones in a shoulder or leg of mutton, they
are so firmly held together by tough ligaments.

While ligaments are pliable and flexible, permitting free movement, they
are also wonderfully strong and inextensible. A bone may be broken, or its
end torn off, before its ligaments can be ruptured. The wrist end of the
radius, for instance, is often torn off by force exerted on its ligaments
without their rupture.

The ligaments are so numerous and various and are in some parts so
interwoven with each other, that space does not allow even mention of
those that are important. At the knee joint, for instance, there are no
less than fifteen distinct ligaments.

53. Imperfect Joints. It is only perfect joints that are fully
equipped with the structures just mentioned. Some joints lack one or more,
and are therefore called imperfect joints. Such joints allow little or no
motion and have no smooth cartilages at their edges. Thus, the bones of
the skull are dovetailed by joints called sutures, which are immovable.
The union between the vertebrae affords a good example of imperfect joints
which are partially movable.

[Illustration: Fig. 26.--Elastic Tissue from the Ligaments about Joints.
(Highly magnified.)]

54. Perfect Joints. There are various forms of perfect joints,
according to the nature and amount of movement permitted. They an divided
into hinge joints, ball-and-socket joints and pivot joints.

The hinge joints allow forward and backward movements like a hinge.
These joints are the most numerous in the body, as the elbow, the ankle,
and the knee joints.

In the ball-and-socket joints--a beautiful contrivance--the rounded
head of one bone fits into a socket in the other, as the hip joint and
shoulder joint. These joints permit free motion in almost every direction.

In the pivot joint a kind of peg in one bone fits into a notch in
another. The best example of this is the joint