218 PHYSIOLOGY FOB DENTAL STUDENTS. 



air. A consideration of these problems is included under the 

 subject of external respiration. 



The inspired or atmospheric air is a mixture of oxygen, car- 

 bon dioxide and nitrogen, and is relatively constant under ordi- 

 nary conditions. The expired air varies somewhat according to 

 the rate and depth of respiration. The following table gives the 

 average percentage composition of inspired and expired air: 



Nitrogen Oxygen CO o 



Inspired air 79 20.96 0.04 



Expired air 79+ 16.02 4.38 



