One hundred years ago (2).
D000766:Anesthesia, Dental / Q000266:history; D000776:Anesthesiology / Q000266:history*; D049672:History, 19th Century; D049673:History, 20th Century; D006801:Humans; D009609:Nitrous Oxide; D010100:Oxygen
D009609:Nitrous Oxide; D010100:Oxygen