The bones at movable joints are held together by thick cords of tough, stringy tissue called ligaments. To aid movement, at least one of the two adjoining bones has a small hollow that contains a lubricating fluid. This fluid helps the bones move smoothly over one another, just as oil helps the parts of an engine move over one another.
All the bones of the body and their connecting cartilage and ligaments make up the body's skeletal system.
