Galen of Pergamon

Aelius Galenus or Claudius Galenus ( Greek: Κλαύδιος Γαληνός; September 129 AD – c. 200/c. 216), and better known as Galen of Pergamon  was a prominent Greek physician, surgeon and philosopher in the Roman Empire. Arguably the most accomplished of all medical researchers of antiquity, Galen influenced the development of various scientific disciplines, including anatomy, physiology, pathology, pharmacology, and neurology, as well as philosophy and logic.
 Galen's understanding of anatomy and medicine was principally influenced by the then-current theory of humorism (also known as the four humors – black bile, yellow bile, blood, and phlegm), as advanced by ancient Greek physicians such as Hippocrates. His theories dominated and influenced Western medical science for more than 1,300 years. His anatomical reports, based mainly on dissection of monkeys, especially the Barbary macaque, and pigs, 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. Galen's theory of the physiology of the circulatory system endured until 1628, when William Harvey published his treatise entitled De motu cordis, in which he established that blood circulates, with the heart acting as a pump. Medical students continued to study Galen's writings until well into the 19th century.

…Galen's influence was so great that when dissections discovered anomalies compared with Galen's anatomy, the physicians often tried to fit these into the Galenic system. An example of this is Mondino de Liuzzi, who describes rudimentary blood circulation in his writings but still asserts that the left ventricle should contain air. Some cited these changes as proof that human anatomy had changed since the time of Galen…

Galen