>Galen's theory of the physiology of the circulatory system remained unchallenged until c.1242, when Ibn al-Nafis published his book Sharh tashrih al-qanun li' Ibn Sina (Commentary on Anatomy in Avicenna's Canon), in which he reported his discovery of pulmonary circulation.[14] His anatomical reports remained uncontested until 1543, when printed descriptions and illustrations of human dissections were published in the seminal work De humani corporis fabrica by Andreas Vesalius,[15][16] where Galen's physiological theory was accommodated to these new observations.[17][18]
https://en.wikipedia.org/wiki/Galen

So no one bothered to check this shit for 1000+ years? lol