g of blood, phlegm, yellow
bile, and black bile, were in balance within the body, good health ensued.
Conversely, when one or more of these humors was overabundant or in less
than adequate supply, disease resulted. The humors were paired off with
specific qualities representing each season of the year and the four
elements according to the well-accepted doctrine of Empedocles, in which
all things were composed of earth, air, fire, and water. Thus, yellow
bile, fire, and summer were contrasted to phlegm, water, and winter, while
blood, air, and spring were contrasted to black bile, earth, and autumn.
When arranged diagrammatically, the system incorporating the humors,
elements, seasons, and qualities appears as shown in Figure 1. The
earliest formulation of humoralism was to be found in the physiological
and pathological theory of the Hippocratic treatise, _On the Nature of
Man_.[7]

Plethora, an overabundance of body humors, including blood, which
characterized fevers and inflammations, was properly treated by
encouraging evacuation. This could be done through drugs that purged or
brought on vomiting, by starvation, or by letting blood. During starvation
the veins became empty of food and then readily absorbed blood that
escaped into the arteries. As this occurred, inflammation decreased. Galen
suggested that instead of starvation, which required some time and
evacuated the system with much discomfort to the patient, venesection
should be substituted to remove the blood directly.[8]

Peter Niebyl, who has traced the rationale for bloodletting from the time
of Hippocrates to the seventeenth century, concluded that bloodletting was
practiced more to remove excess good blood rather than to eliminate
inherently bad blood or foreign matter. Generally, venesection was
regarded as an equivalent to a reduction of food, since according to
ancient physiological theory, food was converted to blood.[9]

[Illustration: FIGURE 1.--Chart of elements, seasons, and humors.]

Galen defined the criteria for bloodletting in terms of extent, intensity,
and severity of the disease, whether the disease was "incipient,"
"present," or "prospective," and on the maturity and strength of the
patient.[10] Only a skilled physician would thus know when it was proper
to bleed a patient. Venesection could be extremely dangerous if not
correctly administered, but in the hands of a good physician, venesection
was regarded by Galen as a more accura