66 ELEMENTARY PHYSIOLOGY less. 



8. The Working of the Arteries.— We must now 

 consider what happens in the arteries wlien the contents 

 of the ventricles are suddenly forced into these tubes 

 (which, it must be recollected, are already full). 



If the vessels were tul)es of a rigid material, like gas- 

 pipes, the forcible discliarge of the contents of the left 

 ventricle into the beginning of the aorta would send a 

 shock, travelling with great rapidity, right along the 

 whole system of tuljes, through the arteries into the 

 capillaries, through the capillaries into the veins, and 

 through these into the right auricle ; and just as much 

 blood would be driven from the end of the veins into the 

 right auricle as had escaped from the left ventricle into 

 the beginning of the aorta ; and that, at almost the 

 same instant of time. And tlie same would take place 

 in the pulmonary vessels between the right ventricle and 

 left auricle. 



However, the vessels are not rigid, but, on the contrary, 

 very yielding tubes ; and the great arteries, as we have 

 seen, have especially elastic walls. On the other hand, 

 the friction in the small arteries and capillaries which 

 opposes a resistance to the flow of blood, and is hence 

 spoken of as the peripheral resistance, is so great that 

 the blood cannot pass through tliem into the veins 

 as quickly as it escapes from the ventricle into the 

 aorta. Hence the contents of the ventricle, driven by 

 the force of the systole past the semilunar valves, are 

 at first lodged in the first part of the aorta, the walls 

 of which are stretched and distended by the extra 

 quantity of blood thus driven into it. But as soon as 

 the ventricle has emptied itself and no more blood 

 is driven out of it to stretch the aorta, the elastic 

 walls of this vessel come into play ; they strive to 

 go back again and make the tube as narrow as it was 

 before ; thus they return back to the blood the pressure 

 which they received from the ventricle. The effect of this 

 elastic recoil of the arterial walls is on the one hand to 



