488 



PHYSIOLOGY 



CHAP. 



The analysis of the different cases of respiratory rhythm which 

 we have so far been examining rather lead us, on the other hand, 

 to the conclusion that no apnoea is exclusively determined by the 

 diminished venosity of the blood. Even the apnoea of birds 

 produced by continuous ventilation, which Miescher considered 

 the most typical case of true apnoea, is, according to our results, 

 a complex effect, essentially determined by a vagus reflex. 

 Indeed, when the pressure by which pulmonary ventilation is 

 effected is strong enough, it is instantaneously produced, without 

 any gradual diminution of the respiratory acts, i.e. before any 

 decarbonisation of blood can have taken place. Accordingly, 

 it must depend essentially on reflex excitation by the centripetal 



FIG. 225. Effect of continuous ventilation after section of vagi in birds. (Luciani and Bordoni.) 

 A, In pigeon ; B, in turkey ; C, in fowl. In all three tracings the insufflation of air commences 



at V. 



paths from the lungs and air-sacs, in which the air is normally 

 but little regenerated, and which, accordingly, are highly sensitive 

 to the passage of air, even at low pressures. 



In fowls apnoea is hard to obtain, perhaps because their air- 

 sacs are less sensitive to the mechanical action of air. Striking 

 evidence of this theory is afforded by the fact that after section of 

 the vagi it was no longer possible to obtain complete apnoea in any 

 of the birds experimented on. The decarbonisation of the blood 

 reduces the respiratory processes considerably, but it does not 

 suppress them, which upsets the usually accepted theory that 

 respiratory rhythm is maintained by the external stimuli of the 

 blood or interstitial lymph, circulating in the centres. 



No less interesting (although of doubtful significance) is the 



