THE CONTROL OF THE RESPIRATION 363 



same duration he becomes apneic. "When the desire to breathe re- 

 turns, the breathing is at first very shallow, but gradually becomes more 

 marked, until at last normal respiration'is reestablished. If a sample of 

 alveolar air is removed at the time when the desire to breathe returns, 

 it will be found to contain a very small percentage of 2 indicating 

 that for some time previous to the onset of breathing there had been in 

 the alveolar air, and therefore in the blood, so low a percentage of 2 

 that if 2 deficiency could stimulate breathing, this would have started 

 much earlier than it actually did. A curve showing the results of such 

 an experiment by Haldane is given in Fig. 131. The person may begin 

 to show symptoms of 0, want, such as cyanosis, before the desire to 

 breathe returns, which furnishes strong proof that 0, want itself can 

 not serve as a stimulus to the respiratory center. The failure of the 

 center to act must rather be due to the lowering of the C H consequent 

 upon the removal of CO, from the blood by the forced respiration which 

 preceded the apnea washing out of the CO,, as it is called. That this 

 has really occurred can readily be shown by estimating the C0 2 con- 

 tent of a sample of alveolar air collected by having the subject make a 

 forced expiration early in apnea. Extremely low values along with a 

 respiratory quotient (page 547) of about 0.2 are often found, whereas, 

 during the preceding forced breathing while the C0 2 is being washed 

 out, the quotient is often ten times as great viz., 2.0. 



As would be expected, the low 2 percentage present in the alveolar 

 air toward the end of the apneic pause is not without some effect, indi- 

 rect though it may be, 011 the excitability of the respiratory center. 

 This accounts for the fact that the alveolar air, at the moment the de- 

 sire to breathe returns, usually- contains a lower percentage of C0 2 than 

 the normal, indicating that some nonvolatile acid must have accumulated 

 in the organism so as to raise the C H of the blood, and thus require a 

 lower tension of CO, to overstep the threshold of excitability of the re- 

 spiratory center. In agreement with this explanation it has been found 

 that, if the last two or three forced respirations preceding the apnea 

 are made in an atmosphere of 0, instead of air, so as to fill the alveoli 

 with 2 , the apnea can be maintained for a very much longer period; 

 and when the natural desire to breathe returns, the C0 2 tension of the 

 alveolar air, instead of being below the normal, is above it. The effect 

 of 2 in prolonging apnea must, therefore, be dependent on the fact that 

 it prevents the accumulation in the organism of the unoxidized acids, 

 leaving to C0 2 alone the function of raising the C H in the blood to the 

 level required to excite the respiratory center. By this means the period 

 during which the breath can be held after breathing 2 is sometimes 



