Galen was a great scientist and physician. He was born in 129 AD in a city called Pergamon, now in Turkey. He influenced parts of Western medicine for over 1500 years. He was a Greek that became a leading doctor to many Roman emperors.
He started as a doctor for gladiators so they could come out again and fight. That also helped him with discovering things about the human body when he was treating wounds. Sadly, he could not dissect a human body so he would dissect animals, most commonly monkeys or pigs.
He wrote many books, some that still survive today. He took a lot a inspiration from Hippocrates, even though he had been dead for over 400 years.
He did use bloodletting for diseases, even though it does not help (he didn’t know that). He also believed that diseases were caused by bad air. However, he did know that feeling a pulse is useful. He discovered that urine was made by the kidney, not the bladder. He died in 210 AD.