RESPIRATION 103 



pressure, as determined in samples of alveolar air. Reference to 

 Figure 31 shows that at no time during the periodic breathing is 

 the CO 2 pressure in the respiratory center more than just suf- 

 ficient to excite the center by itself. 



It is very easy to see what has been happening. The oxygen 

 want caused by the partially reduced blood coming from the 

 lungs at the end of the apnoea has, along with the CO 2 present, 

 sufficed to excite the center; but this oxygen want is at once re- 

 lieved by the breaths which follow, since the oxygen pressure in 

 the lungs is raised beyond the exciting point. The result is a 

 prompt return of the apnoea, till the oxygen in the alveolar air 

 again returns to the stimulating point. The respiratory governor 

 is "hunting" just as the governor of a steam engine or turbine 

 hunts if there is no heavy flywheel or other steadying influence. 

 The chief flywheel of the respiratory center is the great storage 



Figure 36. 



Breathing through soda lime and long tube. Sample of alveolar 

 air at the end of a dyspnoeic period, Oz= 8.70 per cent, CO2= 5.48 

 per cent. 



capacity in the tissues for CO 2 . There is no such storage capacity 

 in connection with oxygen, so the flywheel has disappeared. 



When slight oxygen want, and not merely excess of CO 2 , is 

 exciting the center, the breathing very readily becomes periodic. 

 To realize this condition in a permanent manner we only had to 

 breathe in and out through a tin of soda lime with a piece of hose 

 pipe of variable length attached on the far side, so as to give a 

 suitable dead space. By this means the alveolar oxygen pressure 

 can be reduced to any required extent. Figure 36 shows the effect 

 of such an arrangement. This effect is at once knocked out if oxy- 

 gen is breathed. 



