6o THE ACTIVE FORCES OF LIVING ORGANISMS 



relatively ample, or it may have been excessive, 

 amount of carbonic acid. When, however, at a given 

 height the increase of oxygen in proportion to the 

 carbonic acid in the blood exceeds a certain degree, 

 this condition is by no means favourable to health in 

 any save a few exceptional animals. This certainly 

 seems to prove that the relative proportions of these 

 gases in the blood affect the nervous system in a very 

 fundamental manner. 

 Effects of Although the inspiration of oxygen and the expira- 



diminished ^ \ . , - n j 



atmo- tion of carbonic acid have been said to be miluenced 

 pressure, by physiological factors, it seems probable that the 

 main, if not the most direct, cause of the relative 

 diminution of carbonic acid in the blood is to be found 

 in the decrease of atmospheric pressure. The general 

 diffusibility of gases and liquids is, we may suppose, 

 increased as you rise in altitude. Under these 

 circumstances both oxygen and carbonic acid would 

 find a more easy passage to and from the lungs, so 

 that if we consider the events from this point of view 

 there would be a gain in the amount of the former 

 and a loss in respect to the latter. This, indeed, 

 agrees with the results of analysis already mentioned. 

 The gain in oxygen is, however, only relative, for 

 the actual amount of that gas in the blood at a very 

 great height decreases. Nor is this compensated by 

 deeper breathing save in exceptional cases of ex- 

 haustion. The rule, indeed, seems to be rather that 

 as you rise in height the respiratory movements 

 become shallower, and this may be attributed both to 



