In an experiment that reprised his investigation of the brain a few years earlier, Leonardo injected molten wax into the left ventricle and aorta of an ox’s heart, to determine the shape of the sinus (widening) at the root of the aorta, just beyond the aortic valve. He then made a glass model of the sinus, shown in cross-section at top right, and pumped water mixed with grass seeds through the model. Leonardo observed swirling eddies within the sinus of his model, and concluded that such eddies were crucial in closing the aortic valve when blood flow ceased after each beat of the heart. This brilliant observation was confirmed by medical science only in recent years. Displayed alongside is a false-colour MRI scan of a beating heart, obtained using a 10-tonne magnet 30,000 times stronger than the earth’s magnetic field. The left ventricle is seen pumping blood through the aortic valve into the aorta. After each beat of the heart, swirling eddies of blood can be made out in the sinus, exactly as Leonardo postulated.