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similar to that which constantly occurs when the abdominal wall 

 is compressed by the hand along the course of the aorta. 



When the respiratory rhythm begins to assume its normal 

 form, in respect of frequency and intensity, its influence on 

 arterial blood pressure diminishes proportionally till it entirely or 

 almost disappears, as seen in the tracings of Fig. 198. A similar 

 result was obtained by Marey, who explained it by the antagonistic 

 influence exerted by the movements of the diaphragm 'on the 



FIG. 198. Respiratory oscillations of pressure in vena cava superior (Cs) compared with tracing 

 of pressure in carotid artery (A<-) in chloroformed dog. (Luciani.) 



pressure of the thoracic and abdominal cavities. This interpreta- 

 tion does not seem to us correct, when we consider on the one 

 hand the secondary part played by the diaphragm in respiratory 

 mechanics, and on the other the strong and constant expiratory 

 activity of the abdominal muscles as observed in the dog. It 

 suffices, in order to explain the small or negative effect of ordinary 

 respiratory rhythm on arterial pressure, to admit that normally 



FIG. 199. Tracing of intrathoracic pressure (To) and carotid (CVi) in non-anaesthetised dog of 



medium size, showing slight trembling, particularly in expiration. (Luciani.) 



the respiratory movements are accomplished slowly and quite 

 gradually, and that the abdominal muscles either act moderately 

 during expiration (dog), or remain completely inactive (man). 



When the respiratory rhythm becomes very slow and deep, 

 a marked interference is perceived between the respiratory waves 

 of intrathoracic pressure and the respiratory curves of arterial 

 pressure. This phenomenon was first illustrated by Einbrodt 

 (1860) in an excellent publication from Ludwig's laboratory. 

 The tracings of Fig. 199 show the phenomenon in the most 



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