RESPIRATORY UNDULATIONS. 393 



lungs, and the ordinary pressure of the atmosphere. Now, we have seen 

 that the pressure necessary to counterbalance the elasticity of the lungs, 

 when they are completely at rest (in the pause between expiration and in- 

 spiration), is in man about 5 to 7 mm. of mercury, and that when the lungs 

 are fully distended, as at the end of a forcible inspiration, the pressure rises 

 to as much as 30 mm. of mercury. Hence, at the height of a forcible in- 

 spiration the pressure exerted on the heart and great vessels within the 

 thorax is 30 mm. less than the ordinary atmospheric pressure of 760 mm., 

 and even when the chest is completely at rest, at the end of an inspiration, 

 the pressure on the heart and great vessels is slightly (by about 5 mm. of 

 mercury) below that of the atmosphere. We may add that any obstacle to 

 the free ingress of the inspired air, any difficulty in the full expansion of 

 the pulmonary alveoli, of course increases the negative pressure to which 

 the thoracic structures outside the lungs are subjected by the expansion of 

 the chest. Hence, when the trachea is closed a very large part of the thoracic 

 expansion is directed to increasing the negative pressure around the heart 

 and great bloodvessels. 



During an inspiration, then, the pressure around the heart and great 

 bloodvessels becomes considerably less than that of the atmosphere on the 

 vessels outside the thorax. During expiration this pressure returns toward 

 that of the atmosphere, but in ordinary breathing never quite reaches it. 

 It is only in forcible expiration that the pressure on the thoracic vascular 

 organs reaches or exceeds that of the atmosphere. But- if during inspira- 

 tion the pressure bearing on the right auricle and the venae cavse become 

 less than the pressure which is bearing on the jugular, subclavian, and other 

 veins outside the thorax, this must result in an increased flow from the lat- 

 ter into the former. Hence, during each inspiration a larger quantity of 

 blood enters the right side of the heart. This probably leads to a stronger 

 stroke of the heart, and at all events causes a larger quantity to be ejected 

 by the right ventricle ; this causes a larger quantity to escape from the left 

 ventricle, and thus more blood is thrown into the aorta, and the arterial 

 pressure proportionately increased. During expiration the converse takes 

 place. The pressure on the intra-thoracic bloodvessels returns to the 

 normal, the flow of blood from the veins outside the thorax into the venae 

 cava3 and right auricle is no longer assisted, and in consequence less blood 

 passes through the heart into the aorta, and arterial pressure falls again. 

 During forced expiration the intra-thoracic pressure may be so great as to 

 afford a distinct obstacle to the flow from the veins into the heart. 



The effect of the respiratory movements on the arteries is naturally differ- 

 ent from that on the veins. During inspiration the diminution of pressure 

 in the thorax around the aortic arch tends to expand the aortic arch and 

 thus to check the onward flow of blood and to diminish the pressure of blood 

 within the aorta. During expiration the increase of pressure outside the 

 aortic arch of course tends to increase also the blood-pressure within the 

 aorta, acting in fact just in the same way as if the coats of the aorta them- 

 selves contracted. Thus, as far as arterial blood-pressure is concerned, the 

 effects of the respiratory movements on the great veins and great arteries 

 respectively are antagonistic to each other ; the effect on the veins being to 

 increase arterial pressure during inspiration and to diminish it during expira- 

 tion, while the effect on the arteries is to diminish arterial pressure during 

 inspiration and to increase it during expiration. But we should naturally 

 expect the effect on the thin-walled veins to be greater than that on the 

 stout, thick-walled arteries, so much so that the direct effect on the arteries 

 may be neglected. That is to say, we should expect the blood-pressure to 

 rise during inspiration and to fall'during expiration. This, as we have seen, 



