It was supposed to cure illness and make a patient healthier. The human body was relieved of juices and blood, which became unbalanced and caused sickness . Blood was taken with lancets or by applying leeches. Late 18th and early 19th century tools. Leeches were used through history.
Leeches are parasites.
When attached to the skin, they fasten themselves with two suckers, one at the tail and one atthe head. Then they drink for days or weeks. In 1850, Frenchmen bought about 100 million leeches. In Paris alone, leeches drank about 200,000 pounds of human blood. Nineteenth century leech jars Woodcut dated 1639.