1230 PHYSIOLOGY 



at a height over 5000 metres, and though for a certain length of time 

 by bringing into play the reserve mechanisms already described, 

 they may raise themselves to a height considerably above 5000 metres, 

 it seems questionable whether without artificial means, such as the 

 inhalation of oxygen, it will be possible for any man to attain the 

 highest points on the earth's surface, or at any rate to arrive there by his 

 own unaided efforts. The highest summits in the Himalayas have a 

 height approaching that attained by Tissandier with his two com- 

 panions in his famous balloon ascent, namely, 8600 metres. In this 

 ascent, although oxygen inhalation was used (somewhat ineffectively), 

 two of the party succumbed. 



The stimulating effect of oxygen lack on the blood-forming organs 

 extends also to the muscular system, so that one of the effects of a 

 residence in high altitudes is increased assimilation of nitrogen. For 

 a time the nitrogen output is less than the nitrogen intake, and there 

 is an actual building up of new tissue. The condition of the indi- 

 vidual is similar to that of a growing animal, a fact which may explain 

 the admirable results of a mountain holiday. We can hardly im- 

 agine that the power of the organism to react in this way was evolved 

 through generations of mountain climbing. We are probably here 

 making use of an adaptation which has been evolved for the purpose 

 of retrieving loss of blood by haemorrhage, such as must have been of 

 continual occurrence in the struggle of individual against individual, 

 which has resulted in the survival of the animals of to-day. 



ALTERATIONS IN THE NITROGEN TENSION. The nitrogen 

 of the atmosphere plays no part in the metabolism of the body, and 

 must be regarded as a purely inert gas. It is a matter of indifference 

 whether under normal atmospheric pressure we breathe an atmosphere 

 of pure oxygen or one containing one-fifth part of this gas diluted 

 with four-fifths of nitrogen. The very inertness of nitrogen may be 

 of danger to the body under certain conditions. If a man or an 

 animal be exposed, as in a diving-bell, to a pressure of three, four, or 

 six atmospheres, the respiratory functions are unaffected, but the 

 amount of nitrogen dissolved in the ftuids of the body is increased in 

 direct proportion to the pressure. If the pressure be now suddenly 

 released, the nitrogen, which cannot be used up by the tissues, is given 

 off from the body-fluids in the form of bubbles, just as carbonic acid 

 gas rises in bubbles from soda-water when the pressure is removed by 

 withdrawing the cork from the bottle. These bubbles occurring 

 in all the capillaries obstruct the flow of blood, and therefore, if the 

 evolution of gas is sufficiently large, the animal dies in convulsions. 

 A similar evolution of gas may occur in the spinal cord, giving rise to 

 destruction of the cord and paralysis (' divers' palsy '). In order to 

 prevent this sudden evolution of gas it is necessary that the change 



