138 RESPIRATION 



naturally explained on the theory that the proportioning is only 

 4 approximate, and there are various other facts which point in the 

 same direction. 



One of these facts is as follows. When the breathing is suddenly 

 interrupted voluntarily the breath can be held for a certain time — 

 usually about 40 seconds if only an ordinary breath is inspired 

 before the interruption. Leonard Hill and Flack^ discovered, how- 

 ever, that if the lungs are filled with oxygen first the breath can 

 be held for two or three times longer ; also that the alveolar CO2 

 percentage is considerably higher at the breaking point. On the 

 other hand, when the same air was rebreathed continuously from 

 a small bag filled at the start with a breath of alveolar air, the 

 alveolar CO2 percentage went as high as when the breath was 

 held with oxygen, though not so high as when oxygen was re- 

 breathed from the bag. The following table, illustrating these 

 results, is taken from Hill and Flack's paper. 



It was difficult, at the time, to interpret these results satisfac- 

 torily, since the alveolar oxygen percentages, when the breath was 

 held after breathing ordinary air, did not seem to be low enough to 

 stimulate the breathing appreciably. In order to obtain still more 

 definite information Douglas and I repeated the observations, 

 but in such a way as to have great variations in the alveolar oxy- 

 gen percentage.* We then found that the beneficial effects of in- 

 creasing the alveolar oxygen percentage were still evident, though 

 to a diminishing extent, till 1 7 per cent of oxygen was present in 

 the alveolar air. Oxygen in excess of this made no difference. But 

 1 7 per cent is 3 per cent more than what is present in normal al- 

 veolar air; and, as we have already seen, there are no effects on 

 the breathing from want of oxygen when ordinary air is breathed 

 by normal persons, or even when the oxygen percentage of the 

 alveolar runs down to 10 or even 8 per cent. The results were 

 therefore very mysterious at the time, and we were compelled to 

 . adopt the improbable hypothesis that holding the breath has some 

 considerable effect on the circulation in the brain, leading to 

 anoxaemia of the respiratory center. There is, however, no reason 

 whatever to expect such an effect. 



The experiments on shallow breathing have furnished the 

 solution to this mystery. It is evident that the relation between 

 blood supply and ventilation in individual groups of alveoli is 

 not an even one. In some alveoli the oxygen runs down and CO2 



'Leonard Hill and Flack, Journ. of Physiol.. XXXVII, p. t^, 1908. 

 * Douglas and Haldane, Journ. of Physiol., XXXVIII, p. 435, 1909. 



