Bloodletting

Also known as phlebotomy or venesection, is a medical procedure that involves removing blood from a patient to treat or prevent disease. The practice was used from ancient times until the 19th century, when modern medicine emerged. 

Bloodletting was based on the idea that an imbalance of bodily fluids, known as "humors", could lead to illness. The procedure was performed using a variety of instruments, including lancets, leeches, and fleams. The amount of blood removed could range from small amounts to liters over several days