388 THE RESPIRATION 



lowering of the C0 2 -tension of the blood below the threshold value. 

 During the apnea the 2 is rapidly used up, till a point is reached at 

 which the center again becomes excited. In such an experiment the 

 effects of anoxemia such as cyanosis may show themselves. That breath- 

 ing under these conditions should be periodic and not merely show a 

 steadily increasing hyperpnea is probably due as we have seen, to the un- 

 equal rates at which the 2 and C0 2 tensions change in the blood. Be- 

 cause of a " buffer action" the latter fluctuates much less than the for- 

 mer. Another cause for the periodicity is no doubt the delay between 

 the gas exchange in the lungs and the arrival of the arterialized blood in 

 the brain. When the 2 tension of the blood supplying the respiratory cen- 

 ter falls to so low a level that excitation of the center occurs, the resulting 

 increased breathing aspirates outside 2 into the alveoli. After a moment 

 or so, the 2 is carried by the blood to the center, so that its stimula- 

 tion by 2 deficiency is removed, and it is left in a condition in which 



Fig. 133. Quantitative record of breathing air through a tube 260 cm. long and 2 cm. in diameter. 



(From Douglas and Haldane.) 



it fails to discharge any impulses, since there is a subnormal C H of the 

 blood as a consequence of the lowering of the C0 2 tension produced by 

 the hyperpnea. A little time must now elapse before the C0 2 again 

 rises or the 2 falls sufficiently to excite the center. 



Periodic breathing can. be brought about by this method in decerebrate 

 cats, the animals being caused to respire through a tube and small flask 

 containing soda lime, the total capacity being about 80-100 c.c. That 2 

 deficiency is responsible for the result is indicated by the fact that adminis- 

 tration of 2 through a catheter inserted in the trachea brings about normal 

 breathing. 



A similar although less marked degree of periodic breathing can 

 sometimes be induced by merely respiring through a long tube without 

 any provision for the absorption of C0 2 . In this case it is more difficult 

 to explain the cause of the periodic breathing, but that the main factor 

 concerned is one of 2 deprivation is evidenced by the fact that in this, 



