THE CHEMISTRY OF RESPIRATION 1075 



of his pupils, is inclined to endow the vagus nerves in the higher vertebrates, including 

 mammals, with an analogous regulatory influence on the gaseous exchanges in the lungs. 

 As regards the evolution of carbon dioxide the facts elucidated by Haldane himself 

 would make one hesitate in ascribing any special secretory activity to the pulmonary 

 epithelium. We find, namely, that the respiratory centre reacts immediately to the 

 slightest increase in the tension of the carbon dioxide in the alveolar air. Since this 

 behaviour of the respiratory centre is independent of any nervous connections between 

 the lungs and the brain, it seems to indicate, as indeed Krogh has found, that the tension 

 of the carbon dioxide in the blood follows closely the tension of the carbon dioxide in 

 the alveolar air. If the carbon dioxide were secreted by the pulmonary epithelium we 

 should expect the lungs to react to increased carbon dioxide in the alveoli by simply 

 increasing their work so as to maintain the tension of carbon dioxide in the blood at a 

 constant level. This at any rate is the way in which the kidney would behave under 

 analogous circumstances. Moreover there is no likeness between the thick typical 

 secreting cells of the ' red gland,' which is the gas -secreting part of the swim bladder, 

 and the thin structureless plates which separate the capillaries of the lungs from the 

 alveolar air. 



