362 THE RESPIRATION 



acids, of which lactic acid may be taken as the representative, or inor- 

 ganic substances, like the acid phosphates. That it is not lactic acid is 

 shown by both direct and indirect evidence. The direct evidence has 

 been furnished by Ryffel, who was unable to find any increased per- 

 centage of this substance either in the mine or in the blood of persons 

 who had been living for some time in the famous Regina Margherita 

 hut on Monte Rosa. 27 The indirect evidence has been furnished by ob- 

 serving the time that it takes after the individual has started breathing 

 the rarefied air for the alveolar C0 2 tension to fall, as well as that re- 

 quired to bring about the recovery to the normal when he descends to 

 sea level. The following curve, which is self-explanatory, will illustrate 

 these points. 



Thus, on Pike's Peak, where the barometric pressure is 459 mm. Hg, 

 the C0 2 tension after an initial fall took about seven days before it 

 came to its permanent level for that barometric pressure, and fourteen 

 days elapsed after descending from the mountain before the sea-level 

 tension had been regained. The slow nature of these changes, when com- 

 pared with the rapid changes observed in the experiment with the bags 

 already alluded to (page 358), shows clearly tliat lactic acid can not be 

 responsible for the increase in H-ion concentration in mountain sickness. 

 By exclusion it would appear that the increase in C H is the result of an 

 excess of fixed inorganic acid (H 3 P0 4 ) in the blood dependent on a dis- 

 proportionate excretion of bases by the kidneys during the period of 

 acclimatization to the rarefied air. 



Other observers aver that the acidosis does not really exist, but that 

 the excitability of the respiratory center itself becomes raised (its 

 threshold lowered), so that it responds more readily to the normal C H 

 of the blood. It has been stated that the increase in excitability of the 

 center is dep'endent upon the action of the intense light rays at high 

 altitudes — the erythema of the skin, etc., being evidence of this excit- 

 ing action of light. The constant irritation of the skin, these authors 

 say. serves by stimulation of afferent nerves to maintain a hyperexcit- 

 ability of the respiratory center. Others believe that the hyperexcit- 

 ability of the center is a direct result of the maintained 2 deficiency. 

 The balance of evidence, however, stands in favor of the view that the 

 phenomena of mountain sickness depend on changes occurring in the in- 

 organic nonvolatile acids of the blood. The other phenomena of this 

 interesting condition will be discussed elsewhere (page 399). 



APNEA 



If a man breathes forcibly and quickly for about two minutes, he 

 will experience no desire to breathe for a further period of about the 



