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The practice of bleeding, or bloodletting, was intended to drain 'poisons' or excess blood from the body in order to restore the balance of the humours of the body. It was believed that If these humours lost their natural balance, illness would result. Different illnesses required treatment by bleeding from different parts of the body, and complex charts were made describing where the bleeding knife or leeches should be applied. Bleeding was a popular therapy in the Middle Ages, and continued to be practised as late as the 19th century. Placed on the skin, leeches can drink five times their own weight in blood, chemicals in their mucus stopping the blood from clotting. In 1833, France imported over 40 million medicinal leeches.