618 RESPIRATORY UNDULATIONS. [BOOK n. 



section of the phrenic nerves, since in such a case the whole working 

 of the respiratory pump is materially affected. 



383. In addition to the influence thus exerted by the thoracic 

 movements on the great veins leading to, and the great arteries 

 leading from the heart, we have to consider the behaviour of the 

 pulmonary vessels themselves under the varying thoracic pressure. 

 These, like the vena? cavae and aorta, tend to expand under the in- 

 fluence of the inspiratory expansion of the chest, and thus to become 

 fuller of blood, very much as they would if the whole lung were placed 

 under a large cupping-glass. The first effect of this increased filling 

 of the pulmonary vessels would be to retain for a while a certain 

 quantity of blood in the lungs and thus to lessen the amount falling 

 into the left auricle. But this would be temporary only; and the 

 widening of the pulmonary vessels would speedily produce an 

 exactly contrary effect, namely, an increased flow through the lungs 

 due to the diminished resistance offered by the widened passages. 

 Conversely, the first effect of expiration would be an increased flow 

 into the left auricle due to tin- additional quantity of blood driven 

 inwards by the partial collapse of the pulmonary vessels, followed 

 by a more significant diminished flow caused by the greater resist- 

 ance now offered by the narrower vascular channels. Thus the 

 effect of inspiration in this way would be first to diminish the 

 flow into the left auricle and so into the left ventricle, but 

 afterwards, for the rest of the inspiration until the beginning 

 of expiration, to increase the flow into the ventricle; while con- 

 versely the effect of expiration would be first, for a brief period, to 

 increase and afterwards, during the rest of the movement, to 

 diminish the flow of blood into the left ventricle. Further, while 

 this may be considered as the effect on the pulmonary vessels, 

 large and small taken altogether, the influence both of the 

 thoracic negative pressure during inspiration, and the return 

 in a positive direction during expiration, will bear more on the 

 thin-walled pulmonary veins than on the stouter pulmonary 

 artery ; that is to say, as inspiration becomes established, there 

 will be a diminution of pressure in the pulmonary veins greater 

 than that in the pulmonary artery, and this will be an additional 

 influence favouring the flow into the left ventricle ; during 

 expiration a similar difference of effect will be felt in the contrary 

 direction. During the increase of flow into the ventricle, the 

 quantity of blood ejected at each stroke will increase, and each 

 stroke will ( 1C2) be increased in vigour, in consequence of which 

 the arterial pressure will rise. Conversely, during the decrease of 

 flow into the ventricle, the arterial pressure will fall. Hence 

 the general effect of the movements of the chest on the pulmonary 

 vessels will be during the beginning of inspiration to continue 

 the lowering of arterial pressure which was taking place during 

 expiration but subsequently to raise the arterial pressure ; and 

 conversely at the beginning of expiration to continue the rise 



