If I'm not mistaken, your thumb is the only part of your body that has saddle joints. And with that knowledge understand that these bones in your saddles joint is responsible for the motion. Which is moving from back and forth and also side to side. These joints are free enough that they can make any motion in your body. They can also move 360 degrees and rotate with their own axis. These joints also are prone to being dislocated compared to other movable joints.
