550 THE RESPIRATORY EXCHANGE 



Bohr maintains that the excretion of carbon dioxide is, in part 

 at least, an active process ; Haldane and Priestley's results for 

 the regulation of the ventilation of the lungs support, it would 

 seem strongly, the theory of diffusion ; " the respiratory centre," 

 they state, " is exquisitely sensitive to any rise in the alveolar 

 C0 2 pressure ; a rise of 0-2 per cent, of an atmosphere in the 

 alveolar C0 2 pressure being, for instance, sufficient to double the 

 amount of alveolar ventilation during rest." Why, it may be 

 asked, should the excretory activity of the lungs not be effective 

 against this O2 per cent, of carbon dioxide ; why should the 

 animal respond by increased muscular work to double the alveolar 

 ventilation if it were not to reduce by diffusion the pressure of 

 carbon dioxide ? 



On the other side, research and the collection of evidence have 

 not been wanting. Loewy and Zuntz ( 24 ) have recently published a 

 long series of experiments upon the respiratory exchange of man 

 at high altitudes and the conditions of the absorption of oxygen 

 by the blood, and the diffusion of gases through the lung of the 

 frog. An account of some of these experiments has been given 

 in other parts of this work, and it is only necessary here to state 

 their conclusion : the conditions for the diffusion of oxygen fronT 

 the alveolar air into the blood and from the blood into the tissues 

 are so favourable that a sufficiency of oxygen is maintained thereby 

 even in the most rarefied atmospheres which can be tolerated 

 by man. 



Although so much space has been given to this question, 

 justice has not been done to its fundamental importance. Further 

 experiments, further discussions are needed, for the only con- 

 clusion which appears to be justified by the conflicting evidence 

 is that there is truth on both sides, that the gaseous exchange in 

 the lungs is due both to diffusion and secretion. Such a verdict 

 may be unsatisfactory to those who hold extreme views on either 

 side, by both it may be considered as a confession of weakness, 

 but it is consonant with the evidence and with the wider views of 

 those who do not limit vitalism to the phenomena of living 

 things alone. 



Internal Respiration. It has already been mentioned that the 

 real seat of respiration is the cell. This is proved by the condi- 

 tion in the simplest forms of life, in which the single cell is the 

 organism, and even the most complex organisms, man included, 



