is maintained above lateral reservoir openings 150 as well as above the bottoms of feeder tubes 134 when the patient exhales the exhalation travels upwards in central air column 138 to one way exhalation valve 152 this valve releases easily under this pressure permitting the exhaled gas to travel directly into the mixing space in the inhalator and thence as described above out to the atmosphere through the side ports in the demand valves valve 152 closes automatically when the patient inhales instead of the illustrated spring loaded stop 156 one way exhalation valve 152 may simply comprise a rubber flapper which is seated against cylindrical aperture in the end face which closes off central column 138 the general procedure for operation of the inhalator of fig1 9 is as follows the inhalator is connected to the sources of gas and the demand valves are all adjusted to supply gas at the same demand pressure the indicator dial is first set to deliver 67 nitrous oxide 33 oxygen and 0 carbon dioxide to the patient the inhalator can then be placed on its suspension from movable boom 140 over the patient 39 s head as shown in fig3 the patient is reclining and the therapist seated near the patient 39 s head lowers the inhalator and places the face cushion over the patient 39 s nose and mouth with sufficient force to make a pneumatic seal it will be understood that depending upon the patient the face adapter with its cushion may be attached to a.