What did Galen base his drawings of the human body on?

Explanation:

Galen's understanding of anatomy and medicine was principally influenced by the then-current theory of humorism (also known as the theory of the four humors: black bile, yellow bile, blood, and phlegm), as advanced by ancient Greek physicians such as Hippocrates. Galen's views dominated and influenced Western medical science for more than 1,300 years. His anatomical reports were based mainly on the dissection of monkeys. However, while dissecting them he discovered that their facial expressions were too much like those of humans; thus, he switched to other animals, especially pigs. The reason for using animals to discover the human body was due to the fact that dissections and vivisections on humans were strictly prohibited at the time. Galen would encourage his students to go look at dead gladiators or bodies that washed up in order to get better acquainted with the human body. Galen’s most famous experiment that he recreated in public was the squealing pig. The squealing pig experiment was when Galen would cut open a pig, and while it was squealing he would cut the nerve, or vocal cords, showing they controlled the making of sound. 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 where Galen's physiological theory was accommodated to these new observations.[11] Galen's theory of the physiology of the circulatory system remained unchallenged until ca. 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.