THE CONTROL OF THE RESPIRATION 387 



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 oxygen 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 we 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. 



The fact that alterations in the excitability of the respiratory center 

 in clinical conditions are very commonly associated with periodic breath- 

 ing, suggests that similar alterations must be responsible for the experi- 

 mental forms. Support for this view is found in the fact that most of 

 these latter are produced under conditions where there must be a cer- 

 tain degree of anoxemia. Apparently when the oxygen tension of the 

 blood falls to a certain degree the excitability of the center becomes 

 altered and when it is so, the respiratory hormone, afforded by the ten- 

 sion of C0 2 , causes an irregular stimulation, but why it should do so 

 is impossible to explain. A further factor that may come into play 

 is dependent on the time taken for blood to travel between the pulmo- 

 nary alveoli and the medulla. This may explain the gradual rather 

 than sudden development of the apneic and hyperpne ; ic phases. At 

 present all we can do in attempting to explain this mysterious phenom- 

 enon is to examine the exact conditions under which it occurs. 



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 to prolong the 

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

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

 center is directly stimulated by anoxemia (see page 374). The deep 

 breaths (hyperpnea) which follow, being of greater volume, cause some 

 outside air to be inspired so that the 2 want is made good and the hy- 

 perpnea again disappears, possibly to the extent of apnea, for now in 

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



