The circulation of the blood
William Harvey annouced his discovery that blood circulated around the body in 1616. He had successfully challenged Galen's view, popular for 1400 years, that blood was continually being made and used up. Instead, Harvey proved that there was a fixed amount of blood which was pumped around the body by the heart. He also showed that blood flowed in one direction only. By the use of experiments and demonstrations using ligatures, Harvey set about proving his theory. In 1628 Harvey published his book, 'An Anatomical Exercise Concerning the Motion of the Heart and Blood in Animals', which explained these findings. This book made him famous throughout Europe and also generated much criticism from those who were unwilling to accept new ideas.
However, Harvey was not the first to believe in the circulation of blood. Ibn an-Nafis, an Arab doctor in the 12th century, had disagreed with Galen, but his ideas had not been followed up. In fact, nobody realised what he had said until his book was rediscovered in 1924. The Egyptians had also believed that blood flowed through the body and had used leeches and bleeding to unblock the passages carrying blood.
Harvey's theory on the heart and circulation of the blood met with much resistance as by implication it threw doubt on the value of blood letting. Given it was now known that, there could not be too much blood in the system the practice of bleeding seemed unecessary.