RESPIRATION 1 1 1 



veolar oxygen percentage. In some persons a lowering by as little 

 as 5 per cent in the oxygen percentage of the inspired air will 

 sensibly increase the breathing, but in most persons a lowering of 

 at least 7 per cent (i.e., from 20.94 to 14) is needed to produce a 

 measurable effect, while in others very little effect is produced 

 before consciousness is lost from want of oxygen. It is thus for 

 many persons peculiarly dangerous to pass into an atmosphere in 

 which the oxygen percentage is very low, or to ascend to a very 

 great height in a balloon, since increased breathing may give 

 very little warning, particularly if the change is gradual, so that 

 the extra CO2 is blown off gradually. 



It was discovered in 1908 by Yandell Henderson^ that when 

 effective artificial respiration in an animal has been pushed to 

 excess for some time, so that the pressure of CO2 in the blood and 

 tissues is very greatly reduced, there is not only a prolonged 

 succeeding apnoea, but the animal dies of want of oxygen without 

 attempting to draw a single breath. The artificial respiration must 

 be performed somewhat forcibly, by means of a suction and ex- 

 haust pump ; and the reason for this will be evident from what has 

 already been said in Chapter III as to the control of the chest- 

 movements by the Hering-Breuer refl^ during artificial respira- 

 tion produced by ordinary means. 



This important experiment shows that when the CO2 pressure \ 

 is reduced below a certain point in the respiratory center the latter ' 

 ureases to respond to even the extremest stimulus of want of oxy- I 

 /gen. The apnoea produced in the ordinary way by voluntary 

 ri forced breathing is terminated, as shown in Chapter V, by the 

 I combined stimulus of CO2 and want of oxygen, and in some 

 persons the oxyhaemoglobin in the arterial blood runs down so 

 low that the lips and face become alarmingly blue before breath- 

 ing begins. In the case of Poulton, for instance, his face presented 

 such an alarming appearance when he demonstrated our experi- 

 ments at a meeting of the Physiological Society that one or two 

 members of the Society could hardly be restrained from applying 

 artificial respiration on the spot. In my own case, and that of many 

 others, the blueness is much less marked, although, as already 

 shown, the termination of the apnoea is quite clearly due to want 

 of oxygen, and not merely to accumulation of CO2. 



It is evident from the foregoing account that the respiratory / 

 response to the stimulus of uncomplicated oxygen want is a com- v 

 plex one. The anoxaemia tends to increase the breathing, but the 



'Yandell Henderson, Amer. Journ. of Physiol., XXI, p. 142, 1908. 



