I would help if I knew whatever the book or subject your doing is. Sorry!
In Ancient times, sickness was thought to be caused by an excess of blood. Therefore, doctors would cut into their patients and let some of the blood out to help cure them. This theory advanced into thinking that the human body needed all of its "humors" to be balanced: blood was one of the humors so if the doctor thought you had an imbalance, they would 'let' some of the blood out.
