THE PHYSIOLOGY OF THE AVIATOR 109 



become complete. In fact there are many perfectly 

 healthy persons who, if caused to breathe progressively 

 lowered tensions of oxygen down to 6 or 7 per cent, in 

 the course of half an hour, feel nothing. Their breathing 

 shows no considerable augmentation. They simply lose 

 consciousness, and if left alone they would die, without 

 any apparent effort on the part of respiration to compen- 

 sate for the deficiency of oxygen. In such persons the 

 stimulant of oxygen deficiency exerts only a slowly de- 

 veloping influence upon the sensitiveness of the respira- 

 tory center to the stimulus of CO 2 . They can become ac- 

 climatized to great altitude only at the cost of prolonged 

 mountain sickness. Evidently they are not suited to be 

 aviators. 



In very sensitive subjects, on the contrary, the period 

 of readjustment is much shorter. It is a matter not of 

 days but of hours, and the functional alterations begin 

 to develop almost immediately even under slight oxygen 

 deficiency. The upper air is for those men whose organi- 

 zation readily responds with vigorous compensatory re- 

 action. 



With this inadequate sketch of present scientific 

 knowledge regarding life at great altitudes as a back- 

 ground, we may turn to the application of this knowledge 

 to the problems of human engineering in the aviation 

 service of our army during the war. In September, 1917, 

 I was appointed chairman of the Medical Research Board 

 of the Air Service and was asked to lay out a plan for 

 the development of a method of testing the ability of 

 aviators to withstand altitude. 



You will readily guess the line along which one would 

 attack such a problem. It consisted in the development 

 of an apparatus from which the man under test breathes 

 air of a progressively falling tension of oxygen. The 

 particular form which we use is called a rebreathing ap- 

 paratus. It consists of a steel tank holding about 100 

 liters of air, connected with a small spirometer to record 



