202 Chapter XIII 



pressure in the blood really is after all, our theory must extend to 

 these facts. Beautiful, therefore, as Krogh's work is, its application 

 to man is beset Avith many difficulties. 



Whilst Krogh emerged complete master of the field which he had 

 so successfully circumvented, his victory was to some extent a 

 Pyrrhic one. For whilst he had been laying his plans the enemy had 

 left the position which he was about to capture and they had retired 

 to another, which they disclosed almost simultaneously with the news 

 of Krogh's conquest. Their position as put forward by Haldane and 

 Douglas was this : Oxygen secretion is a function of oxygen want. 

 Under normal circumstances there is abundant oxygen and therefore 

 there is no oxygen secretion diffusion is good enough but when 

 the body demands more oxygen than it can readily obtain, whether 

 by reason of diminished oxygen supply or increased oxygen demand, 

 oxygen secretion in the lung sets in. 



Krogh's investigations do not touch this new. situation, yet it is 

 one which cannot be neglected, if it is true that it affords one of the 

 most important and most beautiful forms of physiological adaptation. 

 That an essential consequence of exercise is an automatic means of 

 obtaining a forced draught for the necessary combustion is a physio- 

 logical conception which cannot be treated with anything but respect. 

 Moreover the conception is an extremely reasonable one, for were 

 there any possibility of secretion in the epithelium from which the 

 lung is derived, the process of natural selection would tend to 

 preserve it. 



We must give some account of the reasons which led the Oxford 

 school to abandon their former position in favour of their present 

 one. The validity of the experiments of Haldane and Lorrain Smith 

 depends upon the correctness of the following assumptions. 



1. That the curve given in Fig. 97 which was determined with ox 

 blood is correct for the blood of all the animals to which they sub- 

 sequently applied it. 



2. That this curve which was determined at the laboratory 

 temperature, correctly represents the facts at the body temperature. 



3. That the relative affinities of the blood of different species for 

 oxygen and carbon monoxide are the same. 



4. That the relation of oxygen and carbon monoxide to blood is 

 unaffected by dilution. 



5. That the same relation is unaffected by the presence of 

 carbonic or other acids. 



