Springing from the “true to nature” ethos of his paintings, Leonardo’ da Vinci's scination with the human body took him to the morgues and hospitals of Florence, where he performed dissections of corpses, often of executed criminals. His greatest feat was understanding the workings of the heart. After discovering a bulb-shaped swelling at the root of the aorta, he came strikingly close to uncovering the mechanisms of blood circulation more than a century before formal science arrived at it. In fact, he injected melted wax into the heart of an ox, then a glass model of the cast and pumped it with water with a suspension of grass seeds in order to observe the vortexes at work. He then concluded that the swelling made the aortic valve close after each heartbeat, a proposition which cardiologists didn’t arrive at until the early 20th century and didn’t fully confirm until the 1980s.

https://www.brainpickings.org/2012/07/17/leonardo-da-vinci-anatomist/?fbclid=IwAR2HZSM2ctLAV23B3-1ZoM7WphtOMupwUL3COogfdxcVlwm_VgEHpRl7oIM