Leonardo’s last and greatest anatomical campaign was an investigation of the heart. (A bovine heart was used for most of his dissections.) To the left are two views of an ox’s heart, displaying the aortic valve and coronary arteries. At lower right are detailed drawings of the aortic valve shown both open (labelled ‘apertaserrata’). Leonardo regarded the three-cusped valves of the heart as a perfect example of mathematical necessity in the workings of nature. Despite his understanding of the valves, Leonardo never grasped the idea of circulation, instead adhering to the ancient theory of the flux and reflux of the blood between the two ventricles.