Blood MattersIn late medieval and early modern Europe, definitions of blood in medical writing were slippery and changeable: blood was at once the red fluid in human veins, a humor, a substance governing crucial Galenic models of bodily change, a waste product, a cause of corruption, a source of life, a medical cure, a serum appearing under the guise of all other bodily secretions, andafter William Harvey's discovery of its circulationthe cause of one of the