THE CONTROL OF THE RESPIRATION 363 



same duration lie 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 2 want, such as cyanosis, before the desire to 

 breathe returns, which furnishes strong proof that 2 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 C0 2 from the blood by the forced respiration which 

 preceded the apnea washing out of the C0 2 , 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, on 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 C0 2 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 2 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 



