502 RESPIKATOEY UNDULATIONS. [Book ii. 



cannot be settled by eliminating the movements of the dia- 

 phragm through section of the phrenic nerves, since in such a case 

 the whole working of the respiratory pump is materially affected. 



§ 312. 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 be- 

 haviour of the pulmonary vessels themselves under the varying 

 thoracic pressure. These, like the vense cavse and aorta, tend 

 to expand under the influence 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 the additional 

 quantity of blood driven onwards by the partial collapse of the 

 pulmonary vessels, followed by a more significant diminished 

 flow caused by the greater resistance now offered by the nar- 

 rower 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 conversely the effect of expira- 

 tion would be first, for a brief period, to increase and after- 

 wards, 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 nega- 

 tive 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 dimi- 

 nution 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 quan- 

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

 stroke will (§ 140) 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 in- 



