252 CIRCULATION. 



action of the elastic arteries must actually assist the circula- 

 tion. The resiliency of the vessels is continually pressing their 

 contents toward the periphery, as regurgitation is rendered 

 impossible by the action of the semilunar valves. The dila- 

 tation of the vessels with each systole, of course, admits an 

 increased quantity of blood; and it has been experimentally 

 demonstrated, that the same intermittent force exerted on 

 an inelastic tube, will discharge a less quantity of liquid 

 from openings of equal caliber. 



Superadded, then, to the force of the heart, we must 

 recognize, as a cause influencing the flow of blood in the 

 arteries, the resiliency of the vessels, especially those of large 

 size. 



Thus it will be seen that the arteries are constantly kept 

 distended with blood by the heart, and by virtue of their 

 elasticity and the progressive increase in the capacity of this 

 system as they branch, the powerful contractions of the cen- 

 tral organ only serve to keep up an equable current in the 

 capillaries. The small vessels, by virtue of their contractile 

 walls, regulate the distribution of the blood ; acting as the 

 guards or sentinels of the process of nutrition, and, in fact, 

 all the numerous functions in which the blood is concerned. 

 Obeying the commands transmitted through the sympathetic 

 nervous system, they allow the passage to every part of the 

 proper quantity of the nutritive fluid at the proper time. 



locomotion of the Arteries and Production of the Pulse. 

 At each contraction of the heart, the arteries are increased in 

 length, and many of them undergo a considerable locomo- 

 tion. This may be readily observed in vessels which are 

 tortuous in their course, and is frequently very marked in the 

 temporal artery in old persons. The elongation may also be 

 seen if we watch attentively the point where an artery bifur- 

 cates, as at the division of the common carotid. It is simply 

 the mechanical eifect of sudden distention ; which, while it 



