368 EFFECTS ON CIRCULATION. [BOOK n. 



at rest, at the end of an expiration, the pressure on the heart and 

 great vessels is slightly (by about 5 mm. mercury) below that of the 

 atmosphere. 



During an inspiration then the pressure around the heart and 

 great blood-vessels becomes considerably less than that of the atmo- 

 sphere on the vessels outside the thorax. During expiration this 

 pressure returns towards 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 exceeds that of 

 the atmosphere. But if during inspiration the pressure bearing on 

 the right auricle and the vense cavse becomes less than the pressure 

 which is bearing on the jugular, subclavian, and other veins out- 

 side the thorax, this must result in an increased flow from the 

 latter 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 tension 

 proportionately increased. During expiration the converse takes 

 place. The pressure on the intra-thoracic blood-vessels returns to 

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

 into the venae cavse and right auricle is no longer assisted, and in 

 consequence less blood passes through the heart into the aorta, 

 and arterial tension 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 different from that on the veins. During inspiration the 

 diminution of pressure in the thorax around the aortic arch tends 

 to draw the blood from the arteries outside the thorax back to the 

 arch of the aorta, or, in other words, tends to check the onward 

 flow of blood. At the same time, and this is the point to which 

 we wish to call attention, the aortic arch itself tends to expand ; in 

 consequence the pressure of blood withm it, i. e. the arterial tension, 

 tends to diminish. During expiration, the increase of pressure 

 outside the aortic arch of course tends to increase also the blood- 

 pressure within it, acting in fact just in the same way as if the 

 coats of. the aorta themselves contracted. Thus as far as arterial 

 blood-pressure is concerned the effects of the respiratory move- 

 ments on the great veins and great arteries respectively are 

 antagonistic to each other ; the effect on the veins being to increase 

 arterial tension during inspiration and to diminish it during 

 expiration, while the^ effect on the arteries is to diminish arterial 

 tension 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 that 

 the total effect of inspiration would be to increase, and the total 



