Equally important to the amount of blood to be taken was the timing of the bloodletting. During the Middle Ages astrology dominated diagnostic and therapeutic thinking. One of the earliest printed documents relating to medicine was a calendar for bloodletting printed in 1457. Determining the best time for bloodletting reached a high degree of perfection during the 14th and 15 centuries. Calculating devices were developed which were adopted from astronomy and navigation. These were attached to a belt which was worn around the waist. The barber-surgeon used this device in conjunction with a calendar plus a drawing of a man showing suitable veins, to determine the accuracy, time, amount, and site to bleed for an illness. Laws were even enacted that required barber-surgeons and physicians to consult these devices before bloodletting to minimize improper and unnecessary bleeding.
A calculator, also known as a volvella, was used in conjunction with the vein-man drawing and calendar table, seen on the right, to determine the best time for bloodletting.
First image courtesy of Wikimedia Commons
Second image courtesy of Wellcome Library