THE CONTROL OF THE RESPIRATION 373 



raised the C H sufficiently to excite the depressed center. Hyperpnea 

 follows, causing a washing out of the C0 2 and a resulting diminution of 

 the effective stimulus, so that again the center fails to be stimulated and 

 apnea supervenes, and so on. Support for this explanation would appear 

 to be furnished by the fact that, when patients exhibiting periodic breath- 

 ing are made to breathe an atmosphere containing a high percentage of 

 C0 2 , the periodicity of the breathing may give place to regular breath- 

 ing; a result which may also be obtained by making such patients 

 breathe in atmospheres rich in oxygen. In the former case, the stimulus is 

 raised to meet the depressed excitability of the center; in the latter, the 

 excitability of the center is increased because of better blood supply 

 so that it is enabled to react to the diminished stimulus. But even 

 granted that the excitability of the center is depressed, it is difficult to 

 see why this should occasion a periodic type of breathing unless w*e as- 

 sume that it is only when stimulus (i. e., C H of blood) and threshold of 

 excitability of the center are adjusted at a certain physiological level that 

 smooth and continuous action can go on. 



Haldane and his school aver that there is no permanent alteration in 

 the excitability of the center, but that the periodicity is due to several 

 causes, which do not always operate to the same degree in the different 

 conditions in which such periodicity exists. To study these causes the 

 exact conditions existing in the various types of periodic breathing that 

 can be produced experimentally in man have been investigated. 



The most simple to consider first is the periodic breathing that is 

 produced in a person susceptible to 2 want, by breathing through a tube 

 and bottle (of a total capacity of 1 liter), containing soda lime. 

 In such a case no outside air enters the lungs, for what we have really 

 done, besides providing for the absorption of C0 2 , is greatly to prolong 

 the dead space. The oxygen tension of the rebreathed air, therefore, 

 quickly 'falls, until at last a point is reached at which the respiratory cen- 

 ter is directly stimulated by 2 deprivation, as we have seen it to be 

 when this falls to a sufficiently low level (see page 350). The deep 

 breaths (hyperpnea) which follow, being of greater volume than 1000 

 c.c., cause outside air to be inspired so that the 2 want is made good 

 and the hyperpnea again disappears, possibly to the extent of apnea, for 

 now, in consequence of a coincident "washing out" of C0 2 , there has 

 been a lowering of the C H 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 effect of 2 

 want becomes very marked, as shown by the intense cyanosis which 

 develops. 



That breathing under these conditions should be periodic and not 



