40 



PHYSIOLOGY 



inspiration and expiration for the special process that 

 brings the gases in contact with the membrane occurring 

 only in higher animals. 



Hygiene of Respiration. — There is not so much dif- 

 ference as is ordinarily supposed between fresh air that 

 is taken into the lungs and impure air that is given off ; 



Deep 

 breathing 



Moderate 

 breathing 



Fresh air 



. Air left 



in lungs 



Fig. 27.— Diagram showing the relative amounts of fresh and stale air in the lungs 

 at different depths of breathing. (From Fitz.) 



the one contains about 20 per cent, of oxygen and but 

 a trace of carbon dioxide, the other about 16 per cent, 

 of oxygen and 4 per cent, of carbon dioxide. Four parts 

 out of every hundred do not seem a large proportion of 

 carbon dioxide, and yet this air is utterly unfit to be 

 breathed in again, for the body is unable to get from it 

 sufficient oxygen for its nourishment. In addition, this 

 air is directly poisonous, for it contains poisonous sub- 

 stances from the body and in many cases disease germs. 



