202 BIOLOGICAL PROBLEMS 



idea of the power of a candidate to withstand the 

 strain of altitude is obtained by directing him to hold 

 his breath for as long as possible. It has been shown 

 that the longer the breath can be held the higher, 

 generally speaking, one can go without discomfort. 

 The subject breathes out deeply to get rid of the ex- 

 pired gases, and then fills the chest well and holds 

 the breath with the nose clipped. During the time 

 that the breath is being held the oxygen is gradually 

 being taken from the lungs, and the subject is ex- 

 posing himself to a gradually rarefying atmosphere. 

 Although expired gases are accumulating during this 

 time, research has shown that the point of breakdown 

 is due to discomfort arising from lack of oxygen, and 

 it has also been shown that the longer the breath is 

 held the lower, generally speaking, is the tension of 

 oxygen reached within the lungs. 



It has been shown by careful correlation with more 

 complicated tests, such as the analysis of the lung 

 gases before and after holding the breath, that the 

 airman who can fly at high altitudes can hold the 

 breath a relatively long time (eighty to ninety seconds), 

 and does not experience discomfort until the oxygen 

 tension within the lungs is relatively low (about 10 

 per cent), whereas the subject who always feels dis- 

 comfort, even at relatively low altitudes, can hold the 

 breath but a short time, and experiences discomfort 

 even with a relatively good supply of oxygen remain- 

 ing in the lungs (e.g. 14 to 13 per cent). The stand- 

 ards of this test have been set by the examination of 

 successful pilots, and at the entrance examination the 

 capacity of the subject to fly high is estimated by the 

 length of time the breath is held, and also by the 

 nature of the sensations experienced during the hold- 

 ing of the breath. 



Valuable confirmatory information may also be 



