xiii RESPIRATORY RHYTHM 489 



i'act that with intact vagi it is not possible to obtain complete 



apuoea, when continuous ventilation is made 



with pure oxygen instead of air. Is it possible ^^^^^^ 



that oxygen lowers the tone or paralyses the Hfl 



afferent fibres of the vagi, so that the respiratory 



centres resume partial or total independence of 



their rhythmical and alternate messages ? Or 



does oxygen perhaps excite the peripheral 



extremities of these nerves to such an extent 



as to throw into reflex activity other centres, 



which in functioning counteract the inhibitory 



messages to the centres of respiration ? The 



general movements and restlessness of the 



animal on ventilation with oxygen, rather tends 



to support this second hypothesis. At all 



events this curious phenomenon is a fresh 



argument towards showing that the decarbon- 



isation of blood and its marked arterialisation 



is not enough to check the rhythmical and 



alternate activity of the respiratory centres. 



Rhythmical and alternate activity! That 

 is to say, rhythmical activity of the inspira- 

 tory centres, alternating with rhythmical 

 activity of the expiratory centres, even in 

 apuoea, even during the quiet expiration of 

 sleep ! This fact which, as we have seen, was ^^^^ 

 clearly demonstrated by Aducco excludes the 

 assumption that the mechanical activity of 

 respiration depends upon any kind of external 

 stimulus. So that it does not appear to us 

 possible, in explaining the alternate activity of 

 two centres that are antagonistic in action, to 

 invoke as a causal factor an external stimulus 

 of any kind whatsoever, acting continuously 

 upon these centres. 



We can only conclude that the activity of 

 the said respiratory centres depends essentially 

 upon the special intrinsic organisation of the 

 elements of which they are composed. 



These elements are not merely endowed 

 with reflex excitability, i.e. are thrown into 

 excitation by simple external stimuli, coming 

 to them in the form of nervous vibrations 

 from the periphery of the centripetal nerves, 

 or as the chemical products of tissue con- 

 sumption acting directly upon those tissues : but they also possess 

 automatic excitability properly so-called, i.e. they are capable of 





