Leonardo’s last known anatomical campaign, an analysis of the heart, was perhaps the most brilliant of his many scientific investigations. After 1511 he had little or no access to human material, and his dissections were therefore of an ox’s heart. Leonardo analysed the structure of the heart, the arrangement of the vessels and the action of the valves. But he had no knowledge of circulation of the blood, and there is a mounting dissatisfaction in his notes as he tried to reconcile his understanding with ancient beliefs about the physiology of the heart.