166 ELEMENTARY PHYSIOLOGY 



vessels, and the pressure upon the blood in them, were 

 everywhere the same, the result of this excess of pressure 

 on tho surface would be to drive all the blood from the 

 arteries and veins of the rest of the body into the heart 

 and great vessels contained in the thorax. And thus 

 the diminution of the pressure upon the thoracic blood 

 cavities produced by inspiration, would, practically, suck 

 the blood from all parts of the body towards the thorax. 

 But the suction thus exerted, while it hastened the flow 

 of blood to the heart in the veins, would equally oppose 

 the flow from the heart to the arteries, and the two 

 effects might balance one another. 



As a matter of fact, however, we know — 



(1) That the blood in the great arteries is constantly 

 under a very considerable pressure, exerted by their 

 elastic walls ; while that of the veins is under little 

 pressure. 



(2) That the walls of the arteries are strong and 

 resisting, while those of the veins are weak and 

 flabby. 



(3) That the veins have valves opening towards the 

 heart ; and that, during the diastole, there is no resistance 

 of any moment to the free passage of blood into the heart ; 

 while, on the other hand, the cavity of the arteries is shut 

 off from that of the ventricle, during the diastole, by the 

 closure of the semilunar valves. 



Hence it follows that equal pressures applied to the 

 surface of the veins and to that of the arteries must 

 produce very different effects. In the veins the pressure is 

 something which did not exist before ; and partly from 

 the presence of valves, partly from the absence of resist- 

 aiice in the heart, partly from the presence of resistance 

 in the capillaries, it all tends to accelerate the flow of blood 

 toivards the heart. In the arteries, on the other hand, the 

 [)ressure is only a fractional addition to that which existed 

 before ; so that, during the .systole, it only makes a com- 

 paratively small addition to the resistance which has to 



