330 THE EQUILIBRIUM OF COLLOID AND 



As the percentage of oxygen decreases the will and energy to do 

 work diminish, and at a partial pressure of about half that present 

 in the atmosphere any attempt at muscular work has to be aban- 

 doned, and the mental processes also become most sluggish. 



The earlier experiments on the absolute minimum amounts of 

 oxygen required to support life in animals in a state of quiescence 

 were vitiated by the simultaneous accumulation of carbon dioxide 

 from the respiration of the animals. 



When means are taken to exclude this source of error by ab- 

 sorbing the carbon dioxide with soda-lime as rapidly as it is formed, 

 it is found that animals (rabbits) can be kept alive for as long as 

 forty hours on a respiratory mixture containing as little as 5 to 6 

 per cent, of oxygen. With slightly less than 5 per cent, of oxygen 

 death occurs very rapidly. 



These results, considered together, show that there is a mini- 

 mum concentration of oxygen necessary for sufficient oxidation to 

 support life; that as the pressure rises the degree of combination 

 or union between bioplasm and oxygen increases, quickening the 

 oxidation processes in the tissues; that an optimum of activity 

 exists somewhere above the normal amount present in atmo- 

 spheric air; and that still higher up embarrassment occurs from 

 too high pressure, causing firmer union between the oxygen and 

 bioplasm. 



A very parallel set of results are obtained in the case of carbon 

 dioxide. Here it might be thought that since carbon dioxide is 

 a waste product of the oxidation process, the best possible con- 

 dition would be its complete removal; but it has been clearly 

 shown by Haldane that a definite minimal percentage of carbon 

 dioxide is required for the regulation of the respiratory exchange, 

 and that when the percentage is reduced by artificial ventilation, 

 the subject passes into apnoea or suspension of breathing until 

 the amount is brought back towards normal in the lungs and tissues. 

 The normal amount of carbon dioxide in the alveolar spaces lies 

 between 4 and 5 per cent., and if it rises or falls but slightly from the 

 normal corresponding changes take place in the respiratory rhythm 

 and depth which tend to restore the balance once more. 



It has further been shown by Henderson that excessive and 

 prolonged ventilation of the lungs by artificial means leads by 

 lowering of the carbon dioxide concentration to irregularity of the 

 heart-beat, and finally, if pushed, to delirium cordis and death of 



