Galen’s understanding of anatomy and medicine was principally influenced by the then-current theory of the four humors: black bile, yellow bile, blood, and phlegm. His anatomical reports were based mainly on the dissection of Barbary apes, and later switched to other animals like pigs. Due to the prohibition of human dissections and vivisections, Galen used animals to study the human body. He encouraged his students to observe dead gladiators or bodies in order to better understand human anatomy.