Blood-letting instruments. 17th-century illustration of instruments (fleam, thumb lancet, cups) and leeches (bottom) used for blood-letting. Also shown (left) are a scarification pattern and incision patterns on veins. Blood-letting, the withdrawal of often large amounts of blood from a patient, was a common practice in medicine for thousands of years. It was based on the theory that the bodily 'humors' needed to be kept in balance. In most cases, the blood-letting would not have been beneficial, and would often have been harmful. Artwork from a 1671 work published in Naples by 17th-century Italian barber-surgeon Cintio D'Amato.