Good question. From 100s AD, https://www.researchgate.net/publication/277408882_Galen's_130-201_AD_Conceptions_of_the_Heart https://www.researchgate.net/publica

Good question. From 100s AD, https://www.researchgate.net/publication/277408882_Galen's_130-201_AD_Conceptions_of_the_Heart https://www.researchgate.net/publication/277408882_Galen's_1... > Galen, like other physicians of his time, knew that the heart had four cavities, but like Erasistratus, he regarded only the ventricles as forming the heart proper and the atria as being a sort of enlargement or bulge of the vessels leading into the ventricles. He points out the presence of four openings in the ventricles, two in each cavity: one to introduce the blood, the other to expel it. The difference between the thick structure of the left ventricle in contrast to the thin of the right impresses him. At the base of the left ventricle, he writes, there is a large hole that comprises three sigmoid membranes and opens into the great artery (aorta), giving rise to all the arteries. He believed wrongly that there was communication between the right and left ventricles because of perforations in the septum, a notion that misled the medical community for centuries Looking up Erasistratus (200s BC), https://oajournals.fupress.net/index.php/ijae/article/download/11661/11091/13056 https://oajournals.fupress.net/index.php/ijae/article/downlo... says > Erasistratus was the first physician who recognized the heart’s activity as a “pump” ... [Ctesibius's] pump consists of two identical cylinders, just like the two heart chambers ... Another great innovation of Erasistratus, was the meticulous description of all four heart valves. It sounds like they were on a good track of research, then progress pretty much stalled until the Turks you mention?