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lungs are overdistended, however, as in emphysema, 

 it is greatly increased. The volume of air which we 

 ordinarily use for breathing, and which passes out and 

 in at each breath, is about 20 cubic inches. This is 

 called the 'tidal air.' For the most part, however, it 

 does not go straight into the alveoli, but only into the 

 upper air passages, the renewal of the air in the lungs 

 themselves taking place by a process of diffusion, whilst 

 the composition of the alveolar air remains fairly con- 

 stant. 



The tidal air is sufficient for the ordinary purposes of 

 life, but, when an emergency arises, we are able to get 

 an increased supply of oxygen by taking into the lungs 

 by a full breath another 100 cubic inches of air which is 

 termed complemental By a forced expiration an equal 

 quantity can be expelled in the wake of the tidal air, 

 and this is termed supplemental. Were it not for these 

 reserves of capacity every extraordinary effort would 

 entail great dyspnoea and the risk of premature death. 

 ' Every man,' says Sibson, ' has much more lung than 

 he requires in the quiet pursuits of life ; he requires less 

 when he lies down, sleeps, or is depressed; but more 

 when he walks, runs, wrestles, or is roused by passion. 

 The lung that is used is an ever-varying quantity. . . . 

 The more the lungs are used, the more is their capacity 

 nursed. The man that under one training is the feeble, 

 narrow-chested, sickly mechanic, is under another the 

 active, full-chested, and healthy sailor.' 



The tidal air supplies only enough oxygen to last us 

 a few seconds ; if the breath is suddenly held, discomfort 

 begins to be experienced after the lapse of that time. 



