142 J- Lindhard. 



from the first-mentioned observations of Haldane, that such an 

 influence has been at work. If so, it would also contribute to the 

 reduction of the alveolar carbonic acid tension. 



In the work of Haldane and Fitzgerald cited above, the au- 

 thors have sought to find a possible daily period in the alveolar 

 carbonic acid tension, but without being able to find it or make it 

 seem probable. Nor is there any indication in this direction in my 

 day experiments. As the table shows, the results are very uniform 

 throughout the day and in general a little higher than in the morn- 

 ing, at any rate in the November experiments. In the May experi- 

 ments this feature is more difficult to detect, as the cold made its 

 influence felt distinctly in the morning experiments. That the num- 

 bers for May are a little higher than for November, is possibly con- 

 nected with the generally lower air-temperature in the period first- 

 mentioned. For the 14 November experiments the average temper- 

 ature was 14-7°, for the 11 May experiments 11-8°. For the No- 

 vember experiments the carbonic acid tension was on an average 

 30-4 + 0-24, n = 1-35 or 44 ^/o of the average. For May the cor- 

 responding values were 319 + 017, /л == 084 or 26 ^/o of the' 

 average. For the 4 experiments in December 1906 the average 

 was 33-2. 



Total volume of air respired ("Ventilation"). 



It may be regarded as certain, especially after the frequently 

 mentioned English experiments, that it is the carbonic acid tension 

 in the respiratory centre and this alone, which stimulates the centre 

 and thus in so far determines its activity. The receptivity of the 

 centre may change; but the stimulation is always the same, other 

 vicarious forces do not seem present. 



On the other hand, Haldane and Priestley's supposition, that 

 the alveolar carbonic acid tension is a constant magnitude for the 

 same individual and that the total volume of air respired is so 

 regulated as to maintain this constancy, does not find confirmation 

 in later investigations. 



It will have been seen from the foregoing, that increased car- 

 bonic acid tension as the result of a feeling of cold is accompanied 

 by reduced total volume of air respired, that the reduced alveolar 

 carbonic acid tension in the light occurs together with a greatly 

 increased total volume of air respired and that the changes in the 

 carbonic acid tension due to variations in the total pressure do not 

 seem to have any effect on the total volume of air respired. 



We are unable to reason, therefore, from changes in the alveolar 



