222 APPLIED PHYSIOLOGY 



382. Rarefied air. Every square inch of surface, in- 

 cluding that of the body, sustains a weight of fifteen 

 pounds of air, but it is balanced by an equal pressure of 

 air inside the body, in the lungs and stomach and other 

 cavities, and so it is not felt. At high elevations there is less 

 atmosphere pressing from above, and so the air expands 

 and becomes lighter. Then a lung full of air will contain 

 less oxygen. At a height of three and a half miles the air 

 is only one half as dense as at the surface of the earth, 

 and at the height of five miles it is almost impossible to 

 breathe enough oxygen to sustain life. The lessened 

 pressure upon the body disturbs the flow of blood, espe- 

 cially in the brain, and produces dizziness and fainting. 



In mountainous regions the air is lighter and holds less 

 moisture than in lower regions. It is also purer, for it is 

 removed from the contamination of cities which crowd the 

 lower waterways. So those regions are favorable for those 

 suffering with lung diseases such as consumption. Proba- 

 bly a still greater benefit is derived from the respiratory 

 exercises and the full expansion of the lungs which are 

 necessary in order to obtain sufficient oxygen. 



383. Effect of increased pressure of air. In working 

 under water in laying deep foundations for buildings, a 

 large box called a caisson is sunk to the bottom, and into it 

 air is forced so as to keep out the water. Men work within 

 the caisson subjected to double or triple the natural pres- 

 sure of air. Although more air is inspired with each breath, 

 the blood does not seem to take up more oxygen than usual ; 

 but the increased pressure of air upon the arteries and veins 

 produces great disturbances of the circulation. It is impos- 

 sible to remain in the caisson longer than an hour or two 

 at a time. In leaving the caisson, the air pressure must 

 be diminished as slowly as on entering, so as to permit 



