196 BIOLOGICAL PROBLEMS 



For the observer the scope of activities has also 

 become very much greater. 



In addition to such nervous excitement, the in- 

 creased height to which machines fly has imposed a 

 great physical strain, especially upon the mechanisms 

 of respiration and circulation all this in addition to 

 the physical and nervous stress encountered during 

 the period of training. In short, in a relatively brief 

 space of time the young pilot is called upon to break 

 the ancient bonds by which man's progress was tied 

 to the earth, and to rise from ground-level to great 

 altitudes, from comparative warmth to intense cold, 

 from relative quiet to continuous rush and roar, from 

 a state of equilibrium to one of instability, from mus- 

 cular and mental rest to highly skilled and nerve- 

 trying evolutions, from comparative security to pos- 

 sible disaster, all this in a space of relatively a few 

 minutes with a return to earth at least as sudden 

 thereby subjecting himself to intensive, intermittent, 

 and cumulative stimuli of a degree to which man has 

 probably never before been exposed. It is obvious, 

 therefore, that for such work the aviator must be the 

 fittest of the fit. 



From the historical point of view the literature in 

 regard to early aeronautics and to life at high alti- 

 tudes affords valuable information concerning certain 

 of the present-day physiological aspects of flying. 

 In other respects, however, it will be seen later that 

 such information is of little value. 



In regard to the effect of altitude, the essential 

 problem is that dealing with the diminishing amount 

 of oxygen in the atmosphere. It has long been known 

 that with 17 per cent of oxygen, although a match 

 will not burn, a man feels little or no discomfort. 

 With 14 per cent the depth of breathing is augmented, 

 the blood pressure slightly raised, and the pulse rate 



