If you have the luck to dissect a human body, you will be able to readily lay bare each of the parts. Even the greatest experts in anatomy, examining the body at leisure, have made many mistakes. But one who has practised beforehand on animals, and especially on apes, lays bare with the utmost of ease each of the parts for dissection. The four humours. Blood is formed in the heart, phlegm in the brain, yellow bile in the liver, and black bile in the spleen.
Health is seen as a proper balance of the four humours.
All disease is an imbalance of these humours. Treatment of disease is an attempt to rebalance these humours - this might be done by diet, exercise, administering purgatives, diuretics or emetics, or by blood letting. This is the basis for the ancient and medieval practice of blood letting. Some patients were considered to be suffering from an excess of blood, thus upsetting the balance of the four humours. They might have some of the symptoms of high blood pressure - red face, protruding veins, etc.