Doctors in ancient times and in the Middle Ages thought the human body contained a mixture of four substances, called humors, that determined a person's health and character. The humors were fluids (humor means fluid in Latin), and they differed from each other in being either warm or cold and moist or dry. Each humor was also associated with one of the four elements, the basic substances that made up the universe in ancient schemes of thought. Blood was the warm, moist humor associated with the element fire, and phlegm was the cold, moist humor associated with water. Black bile was the cold, dry humor associated with the earth, and yellow bile was the warm, dry humor associated with the air. Illnesses were thought to be caused by an imbalance in the humors within the body, as were defects in personality, and some medical terminology in English still reflects these outmoded concepts. For example, too much black bile was thought to make a person gloomy, and nowadays symptoms of depression such as insomnia and lack of pleasure in enjoyable activities are described as melancholic symptoms, ultimately from the Greek word melancholia, excess of black bile, formed from melan-, black, and khole, bile. The old term for the cold, clammy humor, phlegm, lives on today as the word for abnormally large accumulations of mucus in the upper respiratory tract. Another early name of yellow bile in English, choler, is related to the name of the disease cholera, which in earlier times denoted stomach disorders thought to be due to an imbalance of yellow bile. Both words are ultimately from the Greek word chole, bile.

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