RESPIRATION 187 



tion of the height to which it is possible for a human being to 

 ascend. An animal placed in a chamber from which the air is 

 pumped dies when the pressure falls to 250 millimetres of 

 mercury. It has been ascertained that a man under the same 

 circumstances can bear with impunity a reduction to 300 milli- 

 metres. How much lower must the pressure fall before it 

 proves fatal ? Of three aeronauts who ascended in the balloon 

 Zenith to a height of 8,600 metres (26,500 feet), two died. The 

 third, Tissandier, became unconscious, but recovered during 

 the descent. The pressure of the atmosphere at such a height 

 is 260 millimetres. The greatest mountain heights yet at- 

 tained are 23,100 feet (Aconcagua, in the Southern Andes), 

 reached by Fitzgerald, and 23,400 feet (Trisul, in the Garhwal 

 Himalayas), reached by Dr. Longstaff and his companions. 

 The pressure at this height was 320 millimetres. From these 

 facts it is clear that mountaineers have just about reached the 

 limit ; but since they have not as yet mounted to a height at 

 which the barometric pressure is less than 300 millimetres, it 

 is possible that slightly higher mountains are still waiting to 

 be conquered. At 23,000 feet the oxygen contained in arterial 

 blood does not exceed 10 volumes per cent. (cf. p. 190). It is 

 therefore about half the normal amount. Hence the breath- 

 lessness and sense of feebleness experienced by climbers. The 

 least exertion leads to the consumption of all the circulating 

 oxygen. But since the effects of want of oxygen are felt at 

 altitudes much lower than those to which reference has been 

 made, it is clear that the question cannot be regarded as simply 

 ore of physics. The nervous system suffers when an attempt 

 is made to do work with a deficient oxygen-supply. Violent 

 headache and nausea attack most persons long before a level is 

 reached at which the combination of haemoglobin with oxygen 

 ceases to be possible. The occurrence of this " mountain sick- 

 ness " reminds us that we must not take for granted that the 

 nervous system will continue to do its work right up to the 

 altitude at which oxy-hsemoglobin is dissociated. Still, the 

 figures show that, apart from these nervous symptoms, which 

 disappear after a time, no serious disturbance occurs even though 

 the atmospheric pressure be but little higher than the absolute 

 minimum at which haemoglobin combines with oxygen. 

 The capacity of the blood for rapidly absorbing oxygen in 



