374 RESPIRATION 



the origin of the common and quite erroneous opinion that 

 mountain sickness is due simply to exhaustion and has nothing to 

 do with barometric pressure. It now seems probable that in so 

 far as acclimatization is due simply to increased power of oxygen 

 secretion good physical training in heavy exertion will do as 

 much as continued exposure to the high altitude. As we have 

 already seen, however, acclimatization consists not merely in 

 increased power of oxygen secretion, but also in increased haemo- 

 globin percentage and diminution in the available alkali in the 

 blood and tissues so as to permit of increased breathing without 

 the development of alkalosis. It takes time to bring about these 

 changes, and they are not brought about by training for muscular 

 work. The increased haemoglobin, though it was the first acclima- 

 tization change to be discovered, is probably of relatively minor 

 importance, inasmuch as recovery from mountain sickness and 

 related conditions commonly occur before there is any noticeable 

 change in the haemoglobin percentage. The diminution in avail- 

 able alkali seems to be much more important, but the process is 

 evidently a rather slow one. This is readily intelligible when one 

 considers the amount of alkali that has, apparently, to be got rid 

 of, partly by excretion through the kidneys, and partly through 

 suspension of formation of ammonia inside the body. Possibly 

 this part of acclimatization might be greatly hastened by the 

 administration of ammonium chloride, the striking effects of 

 which on the blood reaction were described in Chapter VIII. 



The question of acclimatization has assumed new interest, owing 

 to the recent great extension of the use of aeroplanes at high 

 altitudes. The great advantage of good physical training seems 

 evident in this connection. At the same time it also seems evident 

 that only a limited amount of acclimatization can be produced 

 either by physical training or by intermittent exposures in aero- 

 planes to low atmospheric pressure. The limitation was distinctly 

 evident in the experiments, mentioned in Chapter IX, on the 

 degree of acclimatization produced by intermittent exposures at 

 low pressures. 



We must now discuss the symptoms of balloonists and other 

 airmen at very great altitudes, and the means of averting these 

 symptoms. Enormous heights can easily be reached by balloons; 

 and quite recently, in consequence of the great improvements 

 during the war in the construction of aeroplanes and their engines, 

 a height nearly as great as those reached in balloons has been 

 reached in aeroplanes. The limitation in the heights to which men 



