xii MECHANICS OF RESPIRATION 435 



exaggerated. The respiratory oscillations of arterial pressure 

 are not obtained with open thorax, unless the rhythmical 

 pulmonary dilatation with the bellows is grossly exaggerated. 

 The same authors constantly observed that the respiratory waves 

 of arterial pressure obtained with closed thorax in the curarised 

 rabbit, with a medium degree of rhythmical insufflation, became 

 notably weaker, or even disappeared altogether, when the pleural 

 cavity was scarcely yet open, and it was only on increasing the 

 insufflations that they could be made to reappear, or resume their 

 former level. 



This fact shows the predominating importance of the oscilla- 

 tions of intrathoracic and intra-abdominal pressure, as causal 

 factors in the respiratory arterial undulations. 



Heinricius and Kronecker (1888), taking up Einbrodt's experi- 

 ments, showed that whatever impeded the cardiac diastole lowered 



FIG. 200. Tracings of intrathoracic pressure (To), pressure in crural artery (Acr), and in carotid 

 artery (Aca) in anaesthetised dog with cut phrenics. (Luciani.) 



arterial pressure, and whatever facilitated and aided the former 

 increased the latter. The influence of the respiratory movements 

 of the filling and emptying of the heart would thus be the funda- 

 mental 'condition of the respiratory waves of arterial pressure. 

 "Regular respiration," according to these authors, "produces a 

 salutary massage of the heart." 



In order adequately to interpret the arterial respiratory wave, 

 the influence which respiratory rhythm, when sufficiently pro- 

 nounced, can exert on cardiac rhythm, must also be taken into 

 account. When the vagi are highly excitable, cardiac accelera- 

 tion may frequently be observed in inspiration, and a delay in 

 expiration. The tracings in Fig. 200 give a striking example of 

 this phenomenon. Since this effect disappears after section of 

 the vagi, Einbrodt correctly takes it to be the effect of a reflex 

 rhythmical excitation of the bulbar centre of the cardiac vagi 

 during the expiratory acts. 



