This link refers to the Knight's Tale.
Galen's theory of the circulation of the blood involves the formation of spirits. Nutritive elements went from the intestines to the liver, where, mixed with spirits, they were transformed into blood. Some of this blood went directly to the extremities of the body; the rest went to the heart (right ventricle), from which some went to the lung and the rest went to the left ventricle. The heart's work was to endow the blood with "vital spirits" (they came from the lung) and to send the blood to the periphery. Some blood went to the brain, where "animal spirits" were incorporated into it, and then dispersed through the nerves to the rest of the body. (Source for this summary: Ackerknecht 74-75).
How could this system become the basis for a theory of personality? For starters, consider the role of "spirits" and the various points at which spirits entered the blood system.
Chaucer understood the circulation along this model, as we see in the Knight's Tale, when Arcite has fallen from his horse and is badly injured. His blood has become corrupted--that is, "bad blood," infected by bad spirits--and has to be drained; his lungs are infected and he cannot purify the blood himself; laxatives won't help either, and thus he dies (KnT, lines 2747-55). So the Knight says, "Fare wel phisik! Go ber the man to chirche!"
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