THE CHEMISTRY OF RESPIRATION 1199 



Bohr, as the result of experiments by himself and some of his pupils, is inclined 

 to endow the vagus nerves in the higher vertebrates, including mammals, with 

 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. 



