THE CONTROL OF THE RESPIRATION 375 



The explanation for other types of experimental periodic breathing is 

 much less satisfactory. Important evidence that changes occurring in 

 the tensions of 2 and C0 2 in the alveolar air and therefore in the 

 arterial blood of the respiratory center are largely responsible for periodic 

 breathing has been secured by studying the condition that develops after 

 a period of apnea produced by voluntary forced breathing. The results 

 of such observations are given in the curve shown in Fig. 131. 



The thin line represents the 2 tension of the alveolar air, the thick 

 line the CO, tension. The double line running across the chart repre- 

 sents the average tension of C0 2 during quiet normal breathing. The 

 respiratory movements are represented by the tracing at the foot of 

 the curve along the abscissa. It will be observed that the oxygen ten- 

 sion falls very rapidly during the apneic period, until just before breath- 

 ing recommences it may be as low as 30-35 mm. Hg instead of the nor- 

 mal of about 95. Meanwhile the C0 2 tension rises from the very low 

 level of 12 mm., at first very rapidly, then more gradually, although, 

 M'hen breathing recommences, it has not yet gained the normal level. 

 As a result of the first periods of breathing, the 2 tension suddenly 

 shoots up, but the C0 2 falls only slightly. During the next apneic stage 

 the 2 quickly comes down again, and the CO, rises so as almost to at- 

 tain normal tension before breathing again supervenes. As the apneic 

 periods subsequently become less pronounced, the C0 2 tension comes to 

 stand almost at its normal level, whereas considerable variations in the 

 0, tension continue to occur. 



Several interesting features of these results demand attention. In 

 the first place, it is plain that the body is possessed of some mechanism 

 by which it can prevent great fluctuations in the C0 2 tension of the 

 blood, whereas towards 2 no such "buffer action" is displayed. It will 

 further be observed that the C0 2 tension of the alveolar air rises very 

 rapidly during the first part of the apneic period, and then more grad- 

 ually, the explanation being that during the forced breathing the C0 2 

 has been washed out from the blood but not from the body as a whole. 



At first sight one might attribute the periodicity to the same cause 

 as that operating during breathing through a long tube with soda lime 

 namely, to oxygen deficiency. But this explanation is untenable, be- 

 cause the periodicity remains evident for some time after all possibility 

 of direct stimulation of the center of 2 deficiency is over. A possible 

 clue is furnished by the fact that breathing returns while the C0 2 ten- 

 sion is still considerably below its normal level. The return, as we have 

 seen, is accounted for by the appearance of lactic acid, and if we assume 

 that this has occurred particularly in the respiratory center itself, a 

 slight degree of hyperpnea will be excited, which by supplying 2 will 



