The underlying principle of medieval medicine was the theory of 'humours'. This was derived from the ancient medical works, and dominated all western medicine up until the 19th century. The theory stated that within every individual there were four humours or principal fluids - black bile, yellow bile, phlegm, and blood. These were produced by various organs in the body and they had to be in balance for a person to remain healthy. The balance of humours in humans could be achieved by diet, medicines, and by blood-letting using leeches. The four humours were also associated with the seasons and the astrological signs of the zodiac.

The Humours:
Sanguine - blood; head; hot, moist; air
Choleric - yellow bile; gall bladder; hot, dry; fire
Phlegmatic - phlegm; lungs; cold, moist; water
Melancholic - black bile; spleen; cold, dry; earth

For more information about medieval medicine click on the following link: Medieval Medicine