In 20 drawings and notes running to over 1,500 words, Leonardo describes in minute detail the eddies of blood that help to close the aortic valve. He had an almost perfect understanding of the action of the valves and the motion of the heart. The discovery of the circulation of the blood was within his grasp. But Leonardo was unable to reconcile what he observed – the one-way flow of blood through the valves – with what he had always believed to be true – that the arterial and venous systems were separate. And there, it seems, Leonardo’s studies of the heart came to a halt.