From da Vinci's manuscripts we learn that while dissecting human hearts, he discovered that the blood which enters the heart is different from the blood which leaves the heart. He wrote: "The heart has tides like the sea."
從手稿文字中可發現，在解剖心臟時，他發現湧進和退出心臟的血液並不相同，於是寫下：「心臟像海洋潮汐的漲退」。