IMPORTANCE OF ARTERIAL ELASTICITY 49 



like amount of fluid be allowed to escape at the other end the 

 tube will resume its original caliber. Thus the pulse volume 

 enters with much force the aorta or pulmonary artery; the artery 

 is very elastic and expands under this influence, but immedi- 

 ately recoils with a great pressure on the contents. The pressure 

 tends to force the blood along the vessel in both directions, 

 but its return into the ventricle is effectually prevented by the 

 close of the semi-lunar valves. Consequently it can go only 

 toward the periphery. 



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 contrac- 

 tion of the 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. 



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 character. The 

 smaller the vessel the nearer the flow becomes continuous until 

 this condition is established in the capillaries. 



It is the elastic coat of the arteries that allows them to 

 expand and then contract on the contents forcing them 

 onward. Furthermore it is this elasticity that causes the inter- 

 mittent and remittent flow to become continuous. So the 

 function 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. 



4 



