Looking at a persons forearm its pretty obvious that something is in them that is not the normal skin color. So what did humans think of these before the idea of veins and arteries came about. It's also obvious that when a hand extends something is moving on top of the hand so what did humans think of that? Now we know its muscles, tendons, arteries, and veins but what did they think?
Or was the idea that something (muscles) controlled there movement and tubes (veins and arteries) circulated stuff so far beyond them that they didn't think about it.
I feel like if I lived in the medieval ages and something that was not my normal skin color was inside my arm I would wonder what it was. I know the church could just ignore it or say that its part of the human body, but what did others not influenced by the church think?
People understood these basic structures. They butchered animals and ate meat. They were injured and treated wounds. So -- they understood that veins and arteries carried blood, even if they didn't get the whole nature of the circulatory system and its biochemistry. They knew that bigger muscles meant greater strength, even if they did not necessarily understand what caused muscles to contract. As for the formal study of anatomy, this begins perhaps with the Egyptians but picks up in classification and sophistication with the ancient Greeks. They dissected animals and wrote treatises about what they found, including the first known identification of the difference between the function of veins and arteries.