CHAP, ii.] HESPIRATION. 369 



effect of expiration to diminish, arterial tension. That is to say, 

 we should expect the blood-pressure to rise during inspiration 

 and to fall during expiration. This as we have seen is frequently the 

 case, and indeed when the breathing is deep and laboured the 

 influence in this direction on the blood-pressure curve of the 

 pumping action of the chest is unmistakeable. 



In addition to the influence thus exerted by the thoracic move- 

 ments 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 venae 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 resis- 

 tance 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 and thus to 

 diminish for a brief initial period the blood-pressure in the aorta, 

 but afterwards, for the rest of the inspiration until the beginning of 

 expiration, to increase the flow into the ventricle and thus to raise 

 the arterial pressure ; while conversely 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 ven- 

 tricle, and through that the amount of arterial pressure. 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. The general effect then 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 



F. 24 



