SEC. 8. THE RELATIONS OE THE RESPIRATORY 

 SYSTEM TO THE VASCULAR AND OTHER SYSTEMS. 



§ 310. Many events in the body shew the influence which 

 the respiratory movements exert on the circulation. When the 

 brain of a living mammal is exposed by the removal of the skull, 

 a rhythmic rise and fall of the cerebral mass, a pulsation of the 

 brain, quite distinct from the movements caused by the pulse in 

 the arteries of the brain, is observed ; and upon examination it 

 will be found that these movements are synchronous with the 

 respiratory movements, the brain rising up during expiration 

 and sinking during inspiration. They disappear when the arte- 

 ries going to the brain are ligatured, or when the venous sinuses 

 of the dura mater are laid open so as to admit of a free escape 

 of the venous blood. They evidently arise from the expiratory 

 movements in some way hindering and the inspiratory move- 

 ments assisting the return of blood from the brain. We have 

 already (§ 98) stated that during inspiration the pressure of 

 blood in the great veins may become negative, i.e. may sink 

 below the pressure of the atmosphere ; and a puncture of one 

 of these veins may cause death by air being actually drawn into 

 the vein and thus into the heart during an inspiratory move- 

 ment. When the veins of an animal are laid bare in the neck 

 and watched, the so-called pulsus venosus may be observed in 

 them, that is, they swell up during expiration and diminish again 

 during inspiration. And indeed a little consideration will shew 

 that the expansion and contraction of the chest must have a 

 decided effect on the flow of blood through the thoracic portion 

 of, and thus indirectly on that through the whole of, the vas- 

 cular system. 



This is well illustrated by the effects of respiration on arte- 

 rial blood-pressure. We have seen, while treating of the circu- 

 lation, that the arterial blood-pressure curves are marked by 

 undulations, which, since their rhythm is synchronous with that 

 of the respiratory movements, are evidently in some way con- 

 nected with respiration. Similar undulations may be observed 

 in the pulse tracings taken from man. 

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