The ball and socket joint is one that can move freely in all directions. We see it illustrated in the joints of the shoulder and hip. In these joints one of the bones has a deep depression in it, and this depression forms the socket. The other bone has a rounded head that fits into the depression. We call this rounded head the ball.
Fig. 78. The structure of a bone.
The hinge joint
The hinge joint is illustrated in the knee and elbow joints. These joints can move backward and forward in one plane like a hinge, but they cannot move in a circle like the ball and socket joints. You cannot swing your forearm about on a pivot at the elbow as you can your whole arm, nor will the knee joint bend in every direction as does the joint at the hip.
Serrated joints
Serrated joints do not move. The bones having serrated joints are fitted tightly together so that they form practically one bone. We find this kind of joint illustrated in the way the bones of the skull are put together.
Ligaments
The joints of the body are not held together by rivets, pins or bolts as are the joints of a machine, but by bands of very tough tissue placed about a joint in such a way as to allow it to move freely, although the bones are all the time held firmly together. These bands are called ligaments. Ligaments are much better than bolts or pegs would be, because they stretch a little, and thus prevent the breaking of the bones when the joint is put under a severe strain.