RESPIRATION . II5 



are greatly diminished. The most natural explanation of this is 

 that the oxygen, by increasing largely the amount of free oxygen 

 in the blood, diminishes the anoxaemia, since an oxygen supply 

 which is not dependent on the Bohr effect is added to the ordinary 

 oxygen supply from oxyhaemoglobin. Probably, therefore, the 

 symptoms referred to are mainly produced by anoxaemia caused 

 by the Bohr effect. The subject will be discussed further in Chap- 

 ter X. 



It is a very interesting fact that in many persons forced breath- 

 ing does not produce apnoea at all, although in such persons the 

 breathing is regulated in accordance with the alveolar CO2 pres- 



' sure, just as in other persons. This fact was investigated by Dr. 

 Boothby some years ago while he was working with meJ He 

 found that at the end of continuous forced breathing for one or 

 two minutes there was in himself not only no sign of apnoea, but, 

 on the contrary, increased natural breathing for a short time. This 

 soon passed away, but at no time was there any apnoea, though 

 the excretion of CO2 in the expired air was much diminished for 

 a considerable period. The cause of this absence of apnoea is not 

 yet clear. It seemed possible that the stimulus of anoxaemia from 

 the Bohr effect might, in persons who do not become apnoeic, 

 account for the absence of apnoea ; but even after forced breathing 



j of oxygen the apnoea was absent in one of these persons whom I 



! tested. His power of voluntarily holding a deep breath was 

 markedly increased by forced breathing of air, but natural apnoea 

 did not occur. 



Owing, apparently, to the existence of the Bohr effect, the in- 

 fluence of CO2 in relieving the general symptoms of anoxaemia 

 is not due merely to increased breathing and consequent rise in 

 the alveolar oxygen pressure. Lorrain Smith and I observed that 

 animals in a semi-comatose state from the anoxaemia of carbon 



I monoxide poisoning were revived by substituting expired air for 

 pure air without alteration of the percentage of carbon monoxide. 

 With the expired air mixture there could be no rise in the alveolar 

 oxygen pressure, and there was no alteration in the percentage 

 saturation of the blood with carbon monoxide. A still more strik- 

 ing effect is produced by simply adding CO2 to the air inspired 

 during CO poisoning. At the time we could not understand this 

 effect, as Bohr's discovery had not yet been made. But this dis- 

 covery furnishes an explanation of why a rise in the alveolar CO2 



I pressure, without alteration of the alveolar oxygen pressure, 



' Boothby, Journ. of Physiol., XLV, p. 328, 19 12. 



