The chest is lined and the lungs covered by a serous membrane—the pleura. Normally there is only just sufficient lymph in the space between the visceral layer of the pleura which invests the lungs and the parietal layer which lines the chest-wall to prevent friction during respiration. When the pleura is inflamed, one layer of the membrane rubs against the other. In the early or “dry” stage of pleurisy, the physician recognizes this condition by the friction-sound which he hears on placing his stethoscope against the chest. In a later stage lymph (pleuritic fluid) is poured out. It accumulates in the lower part of the chest, and is recognized by the absence of the resonant note which, under normal conditions, is given out by the chest when percussed. The lungs are not compressed during expiration; they are not squeezed, as a pair of bellows or a sponge may be squeezed, emptying it of its contents. At the end of tranquil expiration the lungs still contain about 3½ litres of air. At the top of tranquil inspiration the volume of their contents does not exceed 4 litres. It is evident, therefore, that air is not drawn into and driven out from the air-chambers by the movements of respiration. The tide of air does not extend far beyond the ends of the bronchi. The gases in the air-chambers are exchanged with the fresh air drawn into the infundibula by diffusion. The composition of the air which is in contact with the bloodvessels is constant. It is about 4 per cent. poorer in oxygen and 3 per cent. richer in carbonic acid than the outside air. Of the air drawn into the windpipe during an inspiration, about one-third returns to the open with the following expiration; two-thirds remains in the lungs. If, therefore, the air taken in at each tide equals one-seventh of the quantity already in the lungs, and if of this one-seventh two-thirds remains, each alveolus renews about one-tenth of its air. Its contents are completely changed in ten respirations. Fresh air is composed of 21 per cent. oxygen, 79 per cent. nitrogen, and a trace (0·04 per cent.) of carbonic acid. Forced by a syringe through lime-water, fresh air does not produce any appreciable milkiness, whereas air breathed through a tube into lime-water renders it turbid owing to the formation of carbonate of lime. Carbonic acid (CO₂) occupies the same volume as its oxygen (O₂) would occupy if free. The oxygen which breathed air has lost slightly exceeds in amount the carbonic acid which it has gained in exchange. The difference is due to the retention of some of the oxygen for the purpose of uniting with hydrogen to form water, and of forming urea. The proportion between carbonic acid gained and oxygen lost, CO₂/O₂ is termed the “respiratory quotient.” Its value varies, of course, with diet. In a herbivorous animal, whose food consists of carbohydrates, it departs but little from unity; in a carnivore, which eats fat and nitrogen-containing food, it is about 0·8.