98 RESPIRATION 



for the cumulative action of the stoppage of breathing. After the 

 long stoppage of 130 seconds the breathing and alveolar CO 2 

 pressure had not nearly returned to normal, even after the fortieth 

 breath following the stoppage. 



Figure 30 shows the converse experiment. Forced breathing 

 was continued two minutes so as to wash out CO 2 from the lungs, 

 arterial blood, and respiratory center; and oxygen had been taken 

 into the lungs, so as to cut out the effects of want of oxygen. The 

 apnoea lasted 4% minutes, and an alveolar sample (the taking of 

 which is recorded on the tracing and somewhat disturbs it) was 

 obtained as soon as the slightest inclination to breathe was noticed. 

 It will be seen that the CO 2 percentage in this sample was 7.12 

 per cent (51.5 mm. of CO 2 pressure) a value far above the normal 

 40 mm. required to excite the center under normal conditions. 

 Separate experiments showed that by the end of two minutes of 

 forced breathing the alveolar CO 2 pressure had fallen to about 

 13 mm. and during the apnoea rose to normal again at the end of 

 2j^ minutes. During the last 2 minutes the alveolar CO 2 pressure 

 was above normal ; but sufficient CO 2 had not accumulated in the 

 tissues of the respiratory center to stimulate it, till the alveolar 

 CO 2 pressure had gradually risen to 51.5 mm. At this point the 

 center, which had now just reached its normal CO 2 pressure, began 

 to work quietly and smoothly, reducing the alveolar CO 2 pressure 

 to normal, and picking up the normal regulating activity. The 

 breathing cannot indicate a gradual return of the CO 2 pressure 

 in the center to normal, corresponding to the gradual return in 

 Figure 29, since, as is shown by the experiments described in 

 Chapter II, complete apnoea results from a fall of 0.2 per cent or 

 1.5 mm. of the CO 2 pressure in the respiratory center. 



The apnoea following forced breathing can be temporarily 

 interrupted by sending a block of blood highly charged with CO 2 

 to the respiratory center. The effect of this is shown in Figure 31. 

 As soon as the breathing and the "apnoeic" venous blood return- 

 ing to the lungs have removed the extra CO 2 introduced into the 

 lungs the apnoea returns again. 



The washing out of CO 2 from the body during forced breath- 

 ing, and its gradual reaccumulation during the next ten or twenty 

 minutes, were strikingly illustrated in some experiments carried 

 out by Boothby. 14 Thus in an experiment on myself he found that 

 during i*/2 minutes of forced breathing I had removed about 

 1,400 cc. extra of CO 2 from the body. During the subsequent ap- 



14 Boothby, Journ. of Physwl., XLV, p. 328, 1912. 



