REGULATION OF RESPIRATORY MOVEMENTS 1209 

 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 accumulate 

 it was found that extreme and intolerable hyperpnoea was produced 

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

 dioxide with 14- 8 per cent, oxygen. When the carbon dioxide was 

 absorbed it was possible to breathe in and out of the bag for a much 

 longer period. No hyperpncea was produced, 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 sensibility for carbon dioxide, which deter- 

 mines the normal depth and rhythm of the respiratory movements. 

 Although the respiratory centre, in common 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 circum- 

 stances 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 main- 

 tained 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 



