RESPIRATORY UNDULATIONS. 



391 



free escape of the venous blood. They evidently arise from the expiratory 

 movements in some way hindering and the inspiratory movements assisting 

 the return of blood from the brain. We have already ( 105) 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 punc- 

 ture 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. 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 expira- 

 tion and diminish again during inspiration. And indeed a little considera- 

 tion will show 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 respiratory movements, are evidently in 

 some way connected with respiration. Similar undulations may be observed 

 in the pulse-tracings taken from man. 



FIG. 107. 



Comparison of Blood-pressure Curve with Curve of Intra-thoracic Pressure. (Dog.) a is the 

 blood-pressure curve taken by means of a mercury manometer ; it shows the respiratory undula- 

 tion, the slower beats on the descent being very marked. 6 is the curve of intra-thoracic pressure 

 obtained by connecting one limb of a manometer with the pleural cavity. Inspiration begins at 

 i, expiration at e. With the beginning of inspiration (i) the expansion of the chest causes a 

 marked fall of the mercury in the intra-thoracic manometer; but the effect soon diminishes, since 

 the lessening of intra-thoracic pressure does not bear on the manometer alone, but on the lungs also ; 

 and as the lungs expand more and more the fall in the mercury becomes less and less until toward 

 the end of inspiration the curve becomes very nearly a straight line. Conversely, the return of 

 the chest at the beginning of expiration ] produces at first a marked rise of the mercury in the 

 manometer ; but this soon ceases as the air leaves the chest and the lungs shrink, whereupon the 

 mercury falls slowly. 



When these undulations of the blood-pressure curve are compared care- 

 fully with the respiratory movements or with the variations of intra-thoracic 

 pressure, what is most commonly observed is that while the blood -pressure, 

 on the whole, rises during inspiration and falls during expiration, neither the 

 rise nor the fall is exactly synchronous with either inspiration or expiration. 

 Fig. 107 shows two tracings from a dog taken at the same time, one, a, being 

 the ordinary blood-pressure curve from the carotid, and the other, b, repre- 

 senting the condition of the intra-thoracic pressure as obtained by carefully 

 bringing a manometer into connection with the pleural cavity. On com- 

 paring the two curves it is evident that neither the rise nor the fall of arterial 

 pressure coincides exactly either with inspiration or with expiration. At 



