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 vertebræ 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 between the first and
second vertebræ (see sec. 38). The radius moves around on the ulna by
means of a pivot joint. The radius, as well as the bones of the wrist and