How many hinge-joints can you find?
Think how many hinges must be used by the boy who takes off his hat and makes a polite bow to his teacher, when she meets him on the street.
How many hinges do you use in running up-stairs, opening the door, buttoning your coat or your boots, playing ball or digging in your garden?
You see that we use these hinges nearly all the time. We could not do without them.
BALL AND SOCKET JOINTS.
All our joints are not hinge-joints.
Your shoulder has a joint that lets your arm swing round and round, as well as move up and down.
Your hip has another that lets your leg move in much the same way.
The hip-joint.