RESPIRATION 149 



the pressure gradually to one-half and one-third of an atmosphere 

 (760 mm. Hg). Under normal conditions oxygen exerts a pressure of 

 20.94 per cent, of an atmosphere or about 153 mm. Hg. At 10 per 

 cent, (one-half of an atmosphere) the animal will become restless and 

 dyspneic, and at 7 per cent, (one-third of an atmosphere) lose conscious- 

 ness and die. This pressure corresponds to the pressure prevailing at 

 an altitude of 30,000 feet. 



