BREATHING 287 



deepest inspiration. The maximum volume of a single 

 breath, commonly about 250 cubic inches, is called the 

 vital capacity. It is often spoken of as the capacity of 

 the thorax, but this is evidently inaccurate. After the 

 most intense expiratory effort there is still air in the lungs 

 which we cannot crowd out. This is the so-called residual 

 air and it is said to amount to some 60 cubic inches. 

 It is our habit to breathe with the chest in an intermediate 

 position, about half way between extreme distention and 

 severest compression. 



The Changes in the Respired Air. These are partly 

 physical and partly chemical. In the former class we 

 note the warming and saturation of the air already 

 mentioned. As much as a pint of water may be re- 

 moved from the body by the breath in the course of 

 twenty-four hours. We ought not to say that this is 

 from the lungs for it comes chiefly from the nasal lining. 

 The air is warmed and moistened on its way in and the 

 water from the nose is carried into the lungs to be brought 

 back again to the exterior. 



The chemical changes call for a fuller statement. 

 We anticipate that expired air will contain less oxygen 

 than standard atmospheric air and more carbon dioxid. 

 The actual alteration is less extensive than might be ex- 

 pected. The outside air contains nearly 21 per cent, 

 of oxygen; the percentage in expired air may be reduced 

 to 16. The quantity of carbon dioxid in the atmosphere 

 is insignificant about 0.04 per cent. The amount in 

 expired air may be 4 per cent, or a little more. All such 

 figures are naturally subject to variation. But we have 

 learned to anticipate that a disappearance of 5 volumes 

 of oxygen will be simultaneous with the discharge of 4 

 or 4.5 volumes of carbon dioxid. 



The fact that the carbon dioxid discharged is normally 

 a little less than the oxygen consumed has an interesting 

 significance. If all the oxygen taken into the blood- 

 stream went to form carbon dioxid the volume of this 

 gas would just equal that of the oxygen used to generate 



