The human body is the most complicated machine in the world. We see with it, hear with it, breathe with it, walk and run with it, and sense pleasure with it. Its bones, muscles, arteries, veins and internal organs are organized with marvellous design, and when we examine this design in detail we find even more amazing facts. Every part of the body, though each may seem to be so different from another, is made up of the same material: cells.
