I open and shut my hand, I bend and straighten my arm or my leg, and it is easy to compare these movements with the backward and forward movements of a door on its hinge. Such joints are known as hinge-joints. They are the most numerous in the body. We find them atthe elbow, the knee, the wrist, and the ankle, and in the fingers and toes. We have already had occasion to notice the admirable arrangement of the thumb. It has not onlya backward and forward movement, such as the fingers have, it can move also from side to side in a direction at right angles to that. It should be carefully noted that the remarkable freedom of movement in the thumb is not brought about altogether by the structure of the thumb itself. The thumb works upon a long bone, which forms the link to connect it with the wrist. It is the joint between this bone and one of the wrist bones which provides for the double movement.
