Mechanical leech in a case, c 1850.
© Science Museum / Science & Society Picture Library
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. Bleeding was a popular therapy in the Middle Ages, and continued to be practised as late as the 19th century. In 1833, France imported over 40 million medicinal leeches.