366 EFFECTS ON CIRCULATION. [BOOK n. 



at work which in each case delay the effect. Extended observa- 

 tions however shew that such a relation as that shewn in 

 the figure though frequent is not constant. In fact the 

 effects of the respiratory movements on blood-pressure ' are 

 found to vary very widely according as the respiration is 

 quick or slow, easy and shallow or laboured and deep, and 

 especially as the air enters into the chest readily or with 

 difficulty. A similar variety of effect is seen in sphygmographic 

 tracings", and these further shew that the respiratory movements 

 bring about changes not only in the mean blood-pressure but in 

 the form and characters of each pulse-wave. In Fig. 60, which 

 represents a state of things frequently occurring, but which must 

 not be considered as illustrating what always takes place, it will 

 be seen that during the 'greater part of expiration the pulse-wave 

 is high and the dicrotic wave is not very prominent, while during 

 inspiration the height of the wave is diminished, and dicrotism 

 becomes much more marked ; the former indicates a higher and 

 the latter a lower blood-pressure. 



These variations in the exact relations of the respiratory 

 undulations to the respiratory movements themselves will prepare 

 the reader for the statement that the causation of the undulations 

 is complex. Apparently the respiratory act affects the vascular 

 system in several different ways, and the general effect varies 

 according as one or other influence is predominant. These several 

 actions are sufficiently interesting and important to deserve dis- 

 cussion. 



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 ex- 

 piration 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 ex- 

 piratory movements in some way hindering and the inspiratory 

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

 already (p. 127) stated that during inspiration the pressure of blood 

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

 of the atmosphere ; and a puncture of one of these veins may 

 cause immediate death by air being actually drawn into the vein 

 and thus into the heart during an inspiratory movement. 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 



