SEC. 9. THE RELATIONS OF THE RESPIRATORY 

 SYSTEM TO THE VASCULAR AND OTHER SYSTEMS. 



381. 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 arteries 

 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 ( 116) 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 movement. W T hen 

 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 vascular system. 



This is well illustrated by the effects of respiration on arterial 

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

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

 which, since their rhythm is synchronous with that of the respi- 

 ratory movements, are evidently in some way connected with 



