1084 PHYSIOLOGY 



being caused. In fact in many cases the subject of such an experiment may 

 lose consciousness suddenly before he has been aware of any serious deficiency 

 in his aeration. 



The difference in the sensitiveness of the centre to increase of carbon dioxide and 

 lack of oxygen respectively is well shown by an experiment of Haldane's, in which the 

 same person breathed in and out of a bag, in the first place allowing the carbon dioxide 

 produced in respiration to accumulate, and in the second removing the carbon dioxide 

 by means of soda lime, so that the sole effect of respiration was to produce a continual 

 diminution in the percentage of oxygen. In the first case, when the carbon dioxide was 

 allowed to accumu'ate it was found that extreme and intolerable hyperpncea was pro- 

 duced when the gaseous content of the bag consisted of 5 '6 per cent, carbon dioxide 

 with 14-8 per cent, oxygen. When the carbon diox : de was absorbed it was possible 

 to breathe in and out of the bag for a much longer period. No hyperpncea was pro- 

 duced, and the experiment was stopped as soon as the subject was becoming blue in 

 the face and experienced slight throbbing in the head. The pulse frequency had gone 

 up from 80 to 108. The bag was found to contain no carbonic acid and only 8-7 per 

 cent, oxygen. In another similar experiment the oxygen had been reduced to 6 7 per 

 cent, before it was necessary to stop the experiment. 



We must conclude that the respiratory centre possesses a specific sensi- 

 bility for carbon dioxide, which determines the normal depth and rhythm 

 of the respiratory movements. Although the respiratory centre, in com- 

 mon with the rest of the central nervous system, is sensitive to and can be 

 excited by lack of oxygen, this quality is rarely brought into play. Under 

 all ordinary circumstances an increased need for oxygen is associated with 

 an increased production of carbon dioxide in the oxidative processes of the 

 body, and the augmentation of respiration, produced by the excitatory 

 effect of a small excess of carbon dioxide tension in the blood, suffices to 

 provide fully for the increased needs of the organism for oxygen. The 

 reactions of the organism have not been evolved in order to adapt it to 

 balloon ascents or experiments in respiratory chambers. As an example of 

 a normal adaptation we may take the changes in respiration which occur 

 in an animal as the result of muscular exercise. During their activity a 

 large amount of carbon dioxide is produced in the muscles. The blood 

 passing from the muscles to the heart will not be able to get rid of the excess 

 of the carbon dioxide in passing through the lungs, and will reach the 

 respiratory centre more highly charged with this gas, the tension of which 

 will be raised. The respiratory centre is thus stimulated, and the increased 

 pulmonary ventilation thereby produced lowers the alveolar carbon dioxide 

 pressure, until a point is reached at which an equilibrium is maintained 

 between the effect of the increased production of carbon dioxide in raising 

 the arterial carbon dioxide tension and that of the increased respiratory 

 activity in lowering it. Under these circumstances it is found that the 

 increased consumption of oxygen in the contracting muscles is more than 

 compensated, so that the oxygen tension in the alveoli and in the arterial 

 blood is rather above than below normal. 



In certain experiments Zuntz and Geppert found that during muscular 

 exercise the respiratory movements were increased to such an extent as 

 to bring the tension of carbon dioxide in the arterial blood below normal. 



