Today, there are billions of people living on this Earth. Despite this huge number, everybody is unique. We differ in size, height and shape. The colours of our skin, hair and eyes varies between different individuals and different races. Despite these differences, the human body is always built to a set a pattern and works in the same way. Everyone's body contains the same kinds of cells, the same set of bones, the same group of muscles and the same control systems. Without them the body would not function properly.