The mucous membrane which lines the nose and windpipe is kept moist in order that it may catch particles of dust drawn in with the air. At the same time the nasal chambers serve to warm the air, and to add moisture to it if it be too dry; for the lining epithelium of the lungs would suffer if dry air came in contact with it. The wall-surface of the nasal chambers is increased by the projection of folded and chambered “turbinate bones.” The importance of warming the air before it is admitted to the lungs is remarkably illustrated in the case of certain sea-birds. The nasal chambers of the frigate-bird, and of some other birds which resemble it, are exceptionally complicated. Since the animal is devoid of any sense of smell, and the air which it breathes must be nearly saturated with moisture, the only function which can be assigned to these convoluted passages is that of warming inspired air. The larynx will be more minutely described when it is considered as the organ of voice. In connection with respiration, it must be regarded as primarily a valve which closes the entrance to the windpipe during swallowing. It is overhung by a leaf-like appendage—the epiglottis—formed of exceedingly elastic tissue. It was thought until lately that the epiglottis drops over the aperture of the larynx when food is passing down the gullet, and springs up again as soon as the act of deglutition is over; but recent observations have shown that during deglutition the epiglottis is pressed against the back of the tongue, and that the closure of the larynx is effected by its own sphincter muscles. The mucous membrane of the larynx is extremely sensitive to stimulation by anything which would be prejudicial to the tissue of the lungs. When its sensory nerve—the superior laryngeal—is stimulated, the larynx closes. It is the agent in carrying out many reflex actions, in which not the larynx only, but also the muscles of the chest and diaphragm, take part. For example, it immediately stops inspiration if an irritating vapour is present in the air. It stops respiration if any foreign body, such as a crumb of bread or a drop of water, touches the mucous membrane. When the trunk of the nerve is stimulated by an electric current, respiration is inhibited. Further, under suitable stimulation the nerve brings about respiratory movements in which inspiration is gentle and expiration sudden, violent, convulsive. Rib-muscles and diaphragm combine to produce a cough, which ejects the noxious body. Again, its stimulation in a different way probably helps to produce constriction of the smaller bronchi which regulate the amount of air supplied to the air-cells of the lungs; although this constriction may be largely due to a reflex which starts in the air-cells. The epithelium of the air-cells has an immensely rich supply of sensory nerves. In some persons this protective mechanism is very prone to overact its part. A little dust or foul gas in the