The doctrine of humors (from the Latin for liquid or fluid) refers to the ancient Greek theory of the four bodily fluids: blood, phlegm, choler (yellow bile) and melancholy (black bile) that determined health and temperament. Humoral theory formed the basis of western medicine and had tremendous influence up to the 19th century. The common practice of blood letting, for example, wasintended to rid the body of excess humors. Even such innovators as Franciscus Sylvius (1614-1672), who examined the chemical basis of disease, still kepttheir systems within the general framework of a humoral explanation. WilliamHarvey (1578-1657), who proved that blood circulated in the body, was attacked by many of his contemporaries for trying to demolish established ideas.

Humoral theory was an extension of the earlier writings of Empedocles (504 to443 B.C.), who proposed the universe and everything in it wascomposed of four cosmic elements, fire, air, earth, and water. These elements in turn were each associated with warm, cold, dry, and moist qualities. Itwas Hippocrates of Cos (c.460-c.377 B.C.) who proposed that an imbalance of the bodily humors resulted in pain and disease, and that, conversely, maintaining a balance of the fluids was the key to good health. For example, according to humoral theory, epilepsy was caused by phlegm blocking the airways, andseizures were the result of the body trying to free itself. Humors could be influenced by many factors in the environment: congenital (present from birth), accidental, or the result of natural phenomena. Hippocrates also believed the humors were essentially glandular secretions, that is, they originated inthe heart, brain, liver, and spleen. The trick was to keep the bodily fluidsin balance both with one another and with influences from outside.

Galen (c.130-c.200), the Greek-born court physician to Marcus Aurelius, enhanced the doctrine of the humors with another aspect, that of the four temperaments. These were features of personality that reflected the influence of particular humors: sanguine (from blood), or buoyant; phlegmatic, or sluggish; choleric, or quick-tempered, and melancholic, or dejected. Thus, not only general health but emotional stability depended on an appropriate balance among the four bodily humors. An excess of any one could produce illness or an exaggerated personality trait. Over time, "humor" came to mean any personality quirk. Elizabethen and Renaissance literature often featured humor characters whose monomaniacal passions served to point out human weaknesses.

Even in its heyday, humoral theory was not universally accepted. In 300 B.C., the Greek anatomist and physiologist Erasistratus held that health and disease were intimately connected with the pneuma, a subtle vapor permeating the air people breathe. Asclepiades of Bithynia (124 B.C.-c.40 B.C.) taught that disease results from physical states of the solid particles he believed made up the body, a concept derived from the atomic theory of the 5th century philosopher Democritus.

Other medical traditions also embraced a kind of humoral theory. The ancientChinese believed the body to be composed of five elements (earth, fire, water, wood, and metal) and they understood health to be a balance between these various elements. In India, the theory of tridosha (three humors) was the basis of the Ayurvedic system of medicine. Wind, bile, and phlegm were substances held to be present in all living creatures. Imbalance or derangementof these humors, or doshas, owing to diet, wrong conduct, or environmental conditions resulted in ill health and disease. Correcting the imbalance broughtabout a cure. The Hindu concept of doshas as a fundamental principle in Ayurveda shares many similar features with Galen's concept of humors in Greek medicine. Nevertheless, each system defines the terms by referring to their ownphilosophies and histories, and with that in mind some scholars of Ayurveda emphasize the differences rather than the similarities.

In 1858 with publication of Rudolf Virchow's (1821-1902) work showing that the basis of human disease is to be found in the cells, humoral theory fell outof favor. But humoral ideas remain pervasive as folk traditions among many people in the way they experience and describe their illness. Moreover, traditional medical systems such as Ayurveda and Unani continue to emphasize the role of humors in the course of their routine clinical practice.

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