framework in action

Your skeleton must be able to support you as you move urself and various parts of your body.That's a lot harder to do than supporting something that doesn't move.The problem is solved by the joints that are located where bones come together.Different kinds of joins determine the kind of movement you can make.The ball-and-socket is one kind of joint.Your arm and shoulder are joined in this way.The top of the arm bone is shaped like a ball.The ball fits into a shallow cuplike socket in the shoulder blade.,much the way the ball in a ballpoint pen is held in place.This arrangement,which is also found in the hip joint,let's you swing your arms and legs freely in a full circle.Another kind of joints is the hinge joints,which is like the hinges on a room door.These joints,such as the ones in your knee or elbow,allow motion mainly in one direction.The joints btw the bones of your spine allow the bones to move over one another with a gliding motion.There are even joints that prevent motion,as in the joints bt the flat bones that from your skull.Joints are surrounded by a case called a capsule.A special fluid in the capsule lets the bones move smoothly over one another,like oil on the moving parts of machine