If they get out of balance, illness results. Doctors bleed their patients to restore this balance, because blood is considered to have pre-eminence over the other humours. In fact, leeching is a separate type of operation. Blood is usually drawn from the arm or the foot. Someone with a natural abundance of choler is said to be choleric , or naturally angry and quick-tempered.
Black bile is considered to be the foam off the top of the blood.
Whatever that is. Andrew Boorde's Breviary of Health is a popular text around many households for advice on staying healthy. The liver , not the heart, is considered the source of the emotions, although the heart is the source of love. The stomach is the seat of courage. The spleen is the source of anger.