The theory that dominated Western medical thinking during the Medieval and Renaissance periods. Galen was a second-century Roman physician who was best known for his role in establishing the science of physiology through dissection. Although Roman laws prohibited the dissection of human beings, Galen was able to learn about the internal structure of African apes and theorized that the two species were much alike. Galen established the fact that arteries carry blood (not air, as was previously believed), determined the difference in function between veins and arteries, and accurately described the valves of the heart. His reputation during the medieval period was so high that he was regarded as one of the foremost medical authorities.
Galen’s theory about the causes of sickness, however, has been disproved by modern science. Galen suggested that the human body contains four major substances, or humors, that were excreted by various organs. He believed that, for a body to be healthy, these humors had to be in balance. Overabundance or lack of a certain humor could be diagnosed by analyzing a persons temperament. Black bile was supposed to be secreted by the kidneys or spleen, and too much of it in a person’s system caused gloominess. An irritable person was believed to have a liver that produced too much yellow bile. An overabundance of phlegm caused a person to be cold, moist, and sluggish. Blood was the fourth humor and caused a person to be choleric and angry. Galen’s ideas continued to be popular well into the 18th century when they were displaced by more modern theories of anatomy, physiology, infection, and diagnosis.