192 PRACTICAL PHYSIOLOGY 



CHAPTER LI. (Demonstration). 

 THE EFFECTS OF CHANGES IN ATMOSPHERIC PRESSURE. 



Decreased Atmospheric Pressure. DEMONSTRATION. A mouse and 

 a frog are placed under the bell glass of the air pump (Fig. 189). A 

 side tube is connected with a mercury manometer. The latter must 

 be long enough to indicate the pressure of the atmosphere. On 

 lowering the pressure ^-i of the atmospheric pressure the mouse 



Fio. 188. Geryk air-puuip. The piston is covered with oil and opens a spring 

 valve during its ascent. 



becomes unconscious and asphyxiated, while the frog is unaffected 

 The effect of lessening the atmospheric pressure depends entirely 

 on the partial pressure of oxygen. The normal pressure of oxygen 

 is 20-94 per cent, of an atmosphere. At 10 per cent, of an atmosphere 

 there arise restlessness and dyspnoaa, and at about 4 per cent , death. 

 A partial pressure of oxygen equal to 7 per cent, of an atmosphere cor- 

 responds to an altitude of 30 000 feet. Death from want of oxygen is 

 common in foul wells, mines, etc., where "choke-damp" collects, and has 

 occurred in balloon ascents. 



