1134 PHYSIOLOGY 



allowed to accumulate, 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 dioxide 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 frequently 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. 

 In these experiments the muscular contractions were produced by tetanis- 

 ing, through the spinal cord, the lower limbs of an animal. Under these 

 circumstances the activity of the muscle would be associated with a 

 diminished blood flow, so that the contractions would be carried out in the 

 absence of a sufficient supply of oxygen. In the absence of sufficient 

 oxygen, muscular contractions result in the production, not of carbon 

 dioxide but of lactic acid ; and it is highly probable that in the experiments 

 in question there was a discharge of acid substances into the blood, diminish- 



