Joints are distinguished as movable and immovable ; wherever two or more bones of the skeleton meet a joint is formed. Movable joints are those in which the bones forming them, move on each other ; they are divided into perfect and imperfect. There are four kinds of perfect joints :

1.  Gliding joints, consisting of bones which glide on each other, as the wrist.

2.   Ball and socket joints, consisting of a round head, which rotates in a socket, as the shoulder joint.

3.   Hinge joints, capable of only backward and forward movement, as the elbow.

4.  Pivot joints, where one bone rotates round a projection of another bone, as the radius on the humerus.

In each of these joints the surfaces which move on each other, called articulating surfaces, are covered with a layer of cartilage, which is again covered with a membrane called synovial membrane ; it forms a closed sack, called the synovial

cavity, which contains synovia, a fluid resembling white of egg, which serves to keep the joints moist.

An imperfect joint is one in which a plate of cartilage intervenes between the bones, and the movement in the joint is due solely to the flexibility of the layer of cartilage ; the vertebrae form such joints.

Immovable joints are those in which the bones are in contact without any cartilage between ; each bone has teeth-like edges which dovetail, as in the skull.