RESPIRATION 163 



arterial blood remains normal. That is to say, with a degree of 

 anoxaemia which would not seriously affect the heart in anoxae- 

 mia from imperfect oxygenation of the available haemoglobin 

 there will be marked response to anoxaemia in the respiratory 

 center, but not in CO poisoning. This points clearly to the very 

 important conclusion that it is practically speaking to the oxygen 

 pressure of the arterial blood that the respiratory center responds. 

 The blood which bathes the receptive end-organs (or whatever 

 else is sensitive to the respiratory chemical stimuli) of the respira- 

 tory center must therefore be blood which has lost very little of its 

 arterial charge of oxygen. 



There are other facts pointing in the same direction. Thus in 

 fainting or dizziness from fall of blood pressure there is no im- 

 mediate panting, although the anoxaemia which immediately 

 results in the cerebrum is sufficient to cause loss or impairment of 

 consciousness. The arterial blood, however, remains normal as 

 regards its pressures of oxygen and CO 2 during fainting; and in 

 accordance with the conclusion just reached, the breathing is not 

 stimulated till the stagnation of blood in the respiratory center is 

 very marked. 



It is to be kept in mind that at a moderate altitude the pressure 

 of oxygen in the arterial blood is diminished far more than the 

 mass of the oxygen, as expressed by the percentage saturation of 

 the haemoglobin. With CO it is the mass of oxygen which is 

 diminished in the blood, while the pressure may be normal. 



It also seems a priori probable that the respiratory center should 

 be continuously sampling and controlling the gas pressures of the 

 arterial blood. For it has to act for the whole body. Its function 

 is evidently, not to keep normal the gas pressures in the capillaries 

 of one particular part of the body, such as the medulla oblongata, 

 but to keep normal the arterial blood upon which every part of 

 the body draws in accordance with varying local requirements. 

 It keeps the gas pressures normal just as the heart keeps the blood 

 pressure normal, so that every part of the body can always indent 

 for arterial blood of standard quality and sufficient quantity. 



A further peculiarity of CO poisoning is that quite commonly 

 consciousness is lost for long periods in the poisonous atmosphere 

 without death occurring. Thus cases of CO poisoning afford strik- 

 ing opportunities of studying the effects of prolonged general 

 anoxaemia of the brain and every other organ in the body. The 

 reason why death does not occur more readily seems to be that, 

 although the amount of oxygen transported by the blood is dimin- 



