THE CIRCULATION 221 



above the head, and slowly lowering them again, has a remark- 

 able effect in quickening the circulation increasing the blood- 

 supply of the brain. Changes of posture, by relieving pressure 

 on subcutaneous veins, removes an impediment to the flow of 

 blood. 



The second of the factors to which we have referred as 

 adjuvant to the heart's action is the negative pressure of 

 inspiration. In explaining the effect of this force upon the 

 circulation, the relation of the lungs to the thorax must be 

 taken into account. The box in which the lungs are enclosed 

 is too big for them ; nevertheless, being extensible and elastic, 

 they always fill it. They follow its movements when in 

 inspiration the muscles between the ribs enlarge it, and when 

 in expiration it diminishes again. No air or fluid, save the 

 moisture which lubricates the surface of the pleura, reducing 

 friction, occupies the (potential) space between the lungs 

 and the chest. But the moment the chest is punctured the 

 lungs collapse. Air is sucked into the pleural cavity. The 

 lungs fill the chest only so long as there is neither air nor fluid 

 between it and them. Lung-tissue is extremely delicate. Each 

 air-cell is a cup of thin membrane holding together a basket- 

 work of capillary vessels. So long as the chest- wall is stationary 

 the negative pressure in the pleural cavity has no effect upon 

 these slender tubes. But when the chest expands, the capil- 

 laries are between two minus pressures, the pull of the chest- 

 wall and the resistance offered to the entrance of air into the 

 lungs by the passages through which it has to pass. The 

 calibre of the lung-capillaries is increased, just as it would be 

 increased were they hanging in an air-pump while the piston 

 was drawn out. More blood passes to the left heart through 

 the wider capillaries. Ejected into the aorta, it raises the 

 pressure in the arterial system. A record of the pressure in 

 an artery shows a rhythmic rise for each heart-beat. It shows 

 also a rise with inspiration and a fall with expiration. These 

 larger undulations correspond with the movements of the 

 chest, although they are necessarily somewhat late on respira- 

 tion, for the first effect of the dilatation of the capillaries is to 

 cause them to hold more blood and to deliver less. The first 

 effect of expiration, on the other hand, is to urge on the blood 

 which the dilated vessels contain. In any ca-se a single beat is 



