CIRCULATION 



27 



brous, muscular and elastic tissue. The latter is partic- 

 ularly important. If fluid is put into an iron pipe until 

 it is full, 110 force will get any more of the liquid in ; but 

 if the same experiment is tried with a rubber tube, not 

 only can an additional amount be forced into the 

 stretched tube, but, as soon as the power is withdrawn, 

 the additional fluid will be squeezed out, even against 

 gravity, by the elastic contraction of the tube. When 

 the heart forces blood into the aorta, that tube expands. 

 As soon as the heart ceases to contract, the elastic coat 



.f'^'^&ij&S&S^^'' v "^^%^i ? -" 



:-M A ftoi v \ 



Fig. 5. Cross section of small artery and vein: A, artery; V, vein. 

 (Hill's Histology.) 



of the artery contracts. If the semilunar valves hold, no 

 blood can return to the heart and the contraction of the 

 elastic artery must drive the blood onward into the 

 smaller arteries. The elastic recoil of the arteries them- 

 selves, is, therefore, the second great force of the circula- 

 tion and one which acts continuously. The heart acts 

 only during systole, the recoil of the arteries continues 

 during diastole. As the arteries diminish in size they 

 offer greater resistance, partly by friction, to the pas- 

 sage of the blood ; and as their distance from the heart 

 increases and their strength diminishes, the heart's im- 



