No one is glad to hear that his body has to be cut open by a surgeon (外科医生) and part of it taken out . Today , however , we needn't worry about feeling pain during the operation . The sick person falls into a kind of sleep , and when he awakes , the operation is finished . But these happy conditions are fairly new . It is not many years since a man who had to have operation felt all its pain .Long ago , operation had usually to be done while the sick man could feel everything . The sick man had to be held down on a table by force while the doctors did their best for him . He could feel all the pain if his leg or arm was being cut off , and his fearful cries filled the room and the hearts of those who watched . Soon after 1770 , Josept Priestley discovered a gas which is now called “laughing gas” . Laughing gas became known in America . Young men and women went to parties to try it . Most of them spent their time laughing , but one man at a party , Horace Wells , noticed that people didn't seem to feel pain when they were using this gas . He decided to make an experiment on himself . He asked a friend to help him . Wells took some of the gas , and his friend pulled out one of Well’s teeth . Wells felt no pain at all . As he didn't know enough about laughing gas , he gave a man less gas than he should have . The man cried out with pain when his tooth was being pulled out . Wells tried again , but this time he gave too much of the gas , and the man died . Wells never forgot this terrible event .