18 ANESTHETICS; MEDICAL GASES AND CONVULSIVE AGENT FOR CONVULSIVE SHOCK THERAPY, INCLUDING CYCLOPROPANE; ETHYLENE; ETHYL CHLORIDE; ETHYL VINYL ETHER; TRIFLUOROETHYL VINYL ETHER; TRICHLOROETHYLENE; NITROUS OXIDE; OXYGEN; CARBON DIOXIDE; OXYGEN-CARBON DIOXIDE MIXTURES; HELIUM; HELIUM-OXYGEN MIXTURES AND HEXAFLUORODIETHYL ETHER