Heart. Diagram showing the heart and its blood vessels as drawn by the Italian artist and scientist Leonardo da Vinci (1452-1519). As well as the illustrations is Leonardo's mirror writing. By dissecting 30 cadavers he studied the structure and working of the heart and its valves. He speculated on the possibility of blood circulation. Leonardo wrote thousands of pages of illustrated notes on a variety of scientific topics. Leonardo wrote backwards, from right to left; his writing can be read normally if viewed in a mirror. He also used his own abbreviations and no punctuation. This illegibility ensured that his work remained obscure until the late 19th century.