406 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1945 



ing that the bubbles are largely composed of nitrogen — that being 

 the most plentiful source of gas within the body fluids — it was sug- 

 gested that the supply of nitrogen for bubble formation could be 

 reduced by breathing pure oxygen for some time before a flight. 

 Nitrogen in the tissues is thus replaced by oxygen. Because the oxygen 

 is consumed by cellular metabolism, and because its tension in the 

 blood rapidly falls when a little is removed, it is less potent as a 

 source of bubbles. This has been proved experimentally, and the 

 practice of breathing oxygen before a flight or on the climb to altitude 

 is now a proved means for preventing decompression pains. 



Ultimately these symptoms will be unknown, for, in the sealed-cabin 

 aircraft of the future, compressors will maintain the pressure constant 

 throughout a flight to any altitude. In the meantime, engineering 

 science has given fliers machines which overtax their physiological 

 powers of adjustment. To make possible the tactical employment of 

 these aircraft in long-time, extremely high-altitude photographic 

 reconnaissance or in rapid-climb bombing flights, the discovery of the 

 preoxygenation procedure has been a great boon. 



There is another notable instance in which the accomplishments of 

 the engineers have exceeded the capabilities of the flier. Two of the 

 primary requirements for good fighter craft are high speed and great 

 maneuverability. These are the characteristics which enable them 

 to excel in plane-to-plane combat, to evade the heavier fire-power 

 of larger craft, and to give effective protection to our bomber missions. 

 Engineers and metallurgists worked for years to develop these planes 

 that would withstand the centrifugal forces of high-speed turns, but 

 during that time there were no corresponding improvements in the 

 physiological mechanisms of the men who were to utilize these new 

 machines. 



A normal heart and circulation will deliver enough blood to the 

 brain when the body is erect or recumbent, and will meet the needs 

 during sudden changes in posture. Nerve messages, from pressure- 

 sensitive nerve endings in the walls of the carotid artery and in the 

 arch of the aorta, promptly report to the nerve centers regulating 

 the heart and blood vessels a drop in blood pressure in the vessels 

 supplying the brain. The effect of this is an accelerated heart rate 

 and a constriction of peripheral vessels. Thus the circulation of the 

 brain is again increased. But the cardiovascular system and this reflex 

 control were not evolved for pumping blood made five to ten times as 

 heavy by a suddenly applied centrifugal force. 



Under these conditions, blood accumulates in the lower extremities 

 and in the viscera. The result is an inadequate supply of oxygen to the 

 nerve cells in the brain. "Gray-out," then "black-out," of vision are 

 the first effects. If the centrifugal force be sufficiently great, and 



