RESPIRATION 45 



If, on the other hand, the inflation or deflation was made during 

 the hyperpnoea caused by breathing air containing an excess of 

 CO 2 the expiratory or inspiratory pressures mount up at onre. 

 The mounting up of the initial pressure is thus dependent on the 

 accumulating chemical stimulus to the respiratory center. If the 

 breathing is interrupted, not just after, but before the completion 

 of inspiration or expiration, the inspiratory phase is continued if 

 inspiration has been interrupted, and the expiratory if expiration 

 has been interrupted, as shown in Figure II. 



If, instead of interrupting the breathing by means of a tap or 

 other obstacle which cannot be overcome, the only interruption 

 is by a limited adverse pressure capable of being overcome by 

 the breathing, the apparent "apnoea" is terminated by an expira- 

 tion if the pressure is positive, or an inspiration if the pressure is 

 negative. This simply means that with a positive pressure the 

 expiration occurs at the moment when the expiratory effort has 

 increased sufficiently to overcome the adverse positive pressure, 

 and similarly with a negative pressure. This is illustrated by 

 Figures 12 and 13, which reproduce stethographic tracings ob- 

 tained in man. 2 The subject at first breathed quietly through the 

 limb of a wide-bore three-way tap open to the air. At the end of 

 an inspiration the tap was suddenly turned so that the mouth of 

 the subject was connected with the air of a bag under a pressure 

 of about 3 inches of water. The consequence of this was that the 



Figure 12. 



Figure 13. 



Effects of prolonged distention of the lungs. To be read from left to right. Time 

 marker = seconds. Distention continued between the two crosses. In Fig. 12 pure 

 air was used for distention; in Fig. 13 air containing 7.3 per cent of COa and 

 8.2 per cent of oxygen. 



lungs were suddenly distended with a large volume of air. It 

 will be seen that after about half a minute the apparent pause in 



'Christiansen and Haldane, Journ. of Physiol., XLVIII, p. 272, 1914. 



