ty, of a slight bending of
the column in different directions. These movements are caused, not by one
bone gliding over another, but by compressions and extensions of the
cartilage. Between the vertebrae in the back of the spinal column, however,
there is a slight movement of the bone surfaces upon one another.

*Structure of the Movable Joints.*--By far the most numerous and important
of the joints are those that are freely movable. Such joints are strongly
constructed and endure great strain without dislocation, and yet their
parts move over each other easily and without friction. The ends of the
bones are usually enlarged and have specially formed projections or
depressions which fit into corresponding depressions or elevations on the
bones with which they articulate. In addition to this the articular
surfaces are quite smooth and dense, having no Haversian canals, and they
are covered with a layer of cartilage. Strong ligaments pass from one bone
to the other to hold each in its place (_A, _Fig. 103). Some of these
consist simply of bands, connecting the joint on its different sides,
while others form continuous sheaths around the joint.

[Fig. 103]

Fig. 103--*Outside and inside view of knee joint.* 1. Tendons. 2.
Ligaments. 3. Cartilage. 4. Space containing synovial fluid. This space is
lined, except upon the articular surfaces, by the synovial membrane.

The interior of the joint, except where the bone surfaces rub upon each
other, is covered with a serous lining, called the _synovial membrane_
(_B_, Fig. 103). This secretes a thick, viscid liquid, the _synovial
fluid_, which prevents friction. The synovial membrane does not cover the
ends of the bones, but passes around the joint and connects with the bones
at their edges so as to form a closed sac in which the fluid is retained.

*Kinds of Movable Joints.--*The different kinds of movable joints are the
ball and socket joint, the hinge joint, the pivot joint, the condyloid
joint, and the gliding joint. These are constructed and admit of motion,
as follows:

1. In the _ball and socket_ joint the ball-shaped end of one bone fits
into a cup-shaped cavity in another bone, called the socket. The best
examples of such joints are found at the hips and shoulders. The ball and
socket joint admits of motion in all directions.

2. In the _hinge_ joint the bones are grooved and fit together after the
manner of a hinge. Hinge