a dispensing valve for regulating the amount dispensed the additive gas can be oxygen or some other gas such as no the dispensing unit 38 can also hold medication or some liquid additive which is dispensed through the dispensing channel 36 into the gas flow channel 18 the additive can be vaporized in this channel or dispersed in small droplets before being delivered to the patient 2 an expiratory valve 40 is also connected to the inspiratory line 6 in the illustrated example the expiratory valve 40 is built into the wall of the inspiratory line 6 so it occupies as little space as possible a tube or the like can also be connected between the inspiratory line 6 and the expiratory valve 40 without affecting its function as described below the expiratory valve 40 is connected to the gas flow channel 18 so that the constriction 28 and the openings of the channels 24 26 32 36 lie between the expiratory valve 40 and the distal end 6a in this embodiment the expiratory valve 40 is a pneumatically controlled on off valve e g a mushroom valve it is connected to the pressure tank 12 via a control channel 42 and a switching valve 44 the control channel 42 proceeds in the wall of the inspiratory line 6 parallel to the gas.