a supply conduit;

a patient interface;

a branch swivel connector including a Y-piece and an elbow that swivels relative to the Y-piece, the branch swivel connector being adapted to be located to the rear of a patient's head in use and being connected to the supply conduit;

a pair of inlet tubes each having a first end positioned in use near a mouth of a patient and being connected to the patient interface, a middle portion arranged to pass across a cheek of a patient and an end portion being joined to the Y-piece of the branch connector, the inlet tubes having a cross-sectional configuration including a substantially flat exterior surface and an interior surface provided with a plurality of internal ribs to prevent the tubes being crushed; and

a strap secured to the patient interface and adapted to pass around the sides and rear of the head to hold the patient interface in position on the head,

wherein each inlet tube includes generally parallel, opposing upper and lower side walls, at least a pair of internal ribs extending from one of the upper and lower side walls, and at least one internal rib extending from the other of the upper and lower side walls, and wherein the at least a pair of internal ribs extending from one of the upper and lower side walls are offset from the at least one internal rib extending from the other of the upper and lower side walls to substantially prevent engagement between the internal ribs upon compression of the upper and lower side walls.