IMPORTANCE OF ARTERIAL ELASTICITY 49 



closure of the semi-lunar valves. Consequently it can go 

 only toward the periphery. 



Now it is evident that the flow in the beginning of the 

 aorta is intermittent ; but it is found that, in vessels as 

 large as the carotids the flow has resumed a remittent char- 

 acter. The smaller the vessel the nearer the flow becomes 

 continuous until this condition is established in the capil- 

 laries. 



It is the elastic coat of the arteries which allows them 



FIG. 25. Transverse section of part of the wall of the posterior 

 tibial artery. (Man.) (From Yeo after Shafer.} 



a, endothelium lining the vessel, appearing thicker than natural from the con- 

 traction of they outer coats; b, the elastic layer of the intima; c, middle coat 

 composed of muscle fibers and elastic tissue; d, outer coat consisting chiefly of 

 white fibrous tissue. 



to expand and contract, thus forcing the contents onward. 

 Furthermore it is this elasticity that causes the intermit- 

 tent and remittent flow to become continuous. So the func- 

 tion of the elastic coat is two-fold ; first, it forces the blood 

 current continuously toward the periphery, and second, it is 

 chiefly the cause of the change from an intermittent flow 

 to a constant flow, which is of so much importance in the 

 capillaries 



Rate of Flow. The velocity of the blood current is equal 

 to the volume flowing through a determined section in one 

 second divided by the cross section. The rate is determined 

 by the pressure, the friction in the vessels, and the cross sec- 

 tion of the vessels. 



