=Mouthpiece.= The design of the mouthpiece should consider the size and shape of the flange which goes between the lips and teeth; this should be such as to prevent leakage of gas into the mouth and should reduce to a minimum any chafing of the gums. The opening through the mouthpiece is held distended at its inner end by a metallic bushing to prevent its collapse, if, under stress of excitement, the jaws are forced over the flange and closed. Rubber has proved a very satisfactory material for this part of the facepiece. =Flexible Hose.= The flexible hose leads from the angle tube to the canister. This should combine flexibility, freedom from collapse, and extreme physical ruggedness. These specifications are met successfully by the stockinette-covered corrugated rubber hose. The angular corrugations not only give a high degree of flexibility but are extremely effective in preventing collapse. The flexibility gained by this construction is not only lateral but also longitudinal; a hose having a nominal length of 10 inches functions successfully between lengths of 8 and 12 inches. The covering of stockinette, which is vulcanized to the rubber in the manufacturing process, adds materially to the mechanical strength by preventing incipient tears and breaks. =Exhalation Valve.= The exhalation valve allows the exhaled air to pass directly to the outside atmosphere. (This valve is not found on the German mask.) This valve has the following advantages: (1) It tends to reduce very materially the dead air space in the mask. (2) It prevents deterioration of the absorbent on account of moisture and carbon dioxide of the expired air. (3) It reduces the back pressure against expiration, since it is unnecessary to breathe out against the resistance of the canister. The disadvantage, which may under certain conditions be very serious, is that, if for any reason the valve fails to function properly, inspiration will take place through the valve. It can be readily seen that any failure of this nature will allow the poisonous atmosphere to be drawn directly into the lungs of the wearer. The type of valve generally used is shown in Fig. 55, which shows one of these valves mounted and unmounted. While it is rather difficult to give a clear description of its construction, the valve may be considered as a flattened triangular sack of rubber, whose altitude is two or three times the base and from which all three corners have been clipped, each giving openings into the interior of the sack. The opening at the top is slipped over the exhalation passage of the angle tube, and the air passes out through the other two corners. Closure is obtained by the combination of two factors,—first, the difference in atmospheric pressure, and second, the tension due to mounting a section which has been cured in the flat over an elliptical opening. [Illustration: FIG. 55.—American Type Exhale Valve, Mounted and Unmounted.]