题目

Until a century ago, bloodletting was used to treat many ailments. Dating back to before the time of Christ, the treatment involved letting a type of worm, called a leech, such blood from the patient. People believed that there were liquids called humors in the body and that these determined a person’s personality and heath. Bloodletting, they thought, restored a balance to these humors.     At the time, little was known of the working of the human body, but people did know that the same liquid, blood, flowed throughout everyone’s body. They knew it was a vital substance, for loss of an已知点与点关于轴对称，则 .