RESPIRATION 173 



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 3J 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 ex- 

 changed 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 expira- 

 tion ; 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 (C0 2 ) occupies the same 

 volume as its oxygen (0 2 ) 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 differ- 



