RESPIRATION 117 



tion air is passing in or out of them. At first sight it would appear 

 that air ought to enter the sinuses during inspiration, but the 

 reverse is the case ; it is only during expiration that they are 

 filled, whilst during inspiration air is sucked out. Considering 

 the position of the common inlet to these sinuses, it is difficult 

 to understand why they should fill during expiration, though the 

 advantage of hot instead of cold air entering is evident. 



Respiratory Changes in the Air and Blood. — The changes which 

 the air undergoes on passing into the lungs must now be con- 

 sidered. 



Atmospheric Air contains in 100 Parts at o° C. and 760 mm. 



(30 Inches) — 



By Volume. 



Oxygen - - - - - -20-96 



Nitrogen - - - - - - 78-09 



Carbon dioxide - - 0003 



Argon - 0094 



Helium - - Traces 



Hydrogen ------ Traces 



The proportion of carbon dioxide is small ; it is a natural 

 impurity in the air, though essential to plant life. The atmo- 

 sphere also contains moisture the amount of which depends 

 upon the temperature ; the higher the temperature the greater 

 the amount of water which the air can contain as vapour, and 

 the lower the temperature the less the amount. Air may be 

 dry or saturated, the latter term implying that it contains as 

 much vapour as it can hold at the observed temperature ; it 

 generally contains about 1 per cent, of moisture, and is spoken 

 of as dry if it contains £ per cent. The air which passes 

 from the lungs is always saturated with moisture. 



When air is taken into the lungs it alters in composition : it 

 loses a proportion of its oxygen and gains in carbon dioxide, 

 as may be seen in the following table : 



The volume of oxygen absorbed is slightly greater than that 

 of the carbon dioxide which takes its place, so that if dried and 

 reduced to standard barometric pressure and temperature, the 

 volume of dry air expired is slightly less than that of the air 



