I believe that most of our early understanding of human anatomy comes from scientists and doctors who liked to dissect dead bodies. It was quite a popular activity at one point – they often used to get hold of the bodies of prisoners who had been hanged. Sometimes—though this was illegal—they would pay crooks to dig up freshly buried corpses of innocent people from the graveyard!
But it’s one thing to cut up a dead body and see what’s in there. It’s quite another to figure out what are the most important bits. It was probably fairly obvious that the brain was for thinking but it did take quite a while to figure out that the heart is a pump to keep blood flowing around the body. I’m not sure when people discovered what all the other bits were for e.g. liver, kidneys, pancreas.
One of the motivations of early dissectors was to try to find the location of the human soul – which some people thought might exist in part of the brain. But they never found one.
If you like this topic and are not too squeamish, you should go to visit the Hunterian Museum in London. It’s in the Royal College of Surgeons and contains lots of information about anatomy, surgery and disease. There are jars and jars of specimens – mostly dissected animals but plenty of human bits and pieces as well (including skeletons!). Some of it is pretty gruesome but my daughter and I had a great day there last autumn.
A lot of early experiments involved animals and some of them were pretty cruel by today’s standards. For example, the first blood transfusion was done with a dog in the 1600s. With the nervous system some experiments were done with squid and sea cucumbers. Some important discoveries about the brain came from studying patients with different brain injuries. These usually lead to a pattern of problems where some abilities are lost, but others are unaffected. By looking carefully at these patterns early researchers like Broca, Wernicke learned about the organization of the brain.