The practice of bleeding, or bloodletting, was intended to drain 'poisons' or exces blood from the body in order to restore the balance of the humours. It was believed that the human body contained four humours (blood, phlegm, yellow bile, black bile) which mirrored the four elements (air, water, fire, earth). If these humours lost their natural balance, illnes would result. Different illneses 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.
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