50 PHYSIOLOGY. 



The elastic reaction of the arteries thus helps to make 

 steady the flow of blood, which is intermittent as it leaves 

 the heart. The medium-sized arteries also have elastic 

 tissue in their walls, and regulate the blood flow in the 

 same way. 



Variation of the Amount of Blood Needed. Each 

 organ requires a supply of blood in proportion to its 

 activity. An actively working organ, like the brain, de- 

 mands much more blood than bone, practically inactive. 

 Further, working tissues, such as the brain and muscles, 

 need a great deal more blood while they are at work than 

 when they are resting. An organ needing a constant large 

 supply of blood might secure this by having a large artery. 

 But how can the supply be regulated so that an organ 

 may receive, now more, now less, according to its needs ? 



Plain Muscle Fibers in the Walls of the Arteries. 

 This is regulated by the medium-sized and small arteries 



Connective Tissue 



Endothelium 



Muscle Fiber 



Fig. 20. Plain Muscle Fiber. Isolated and in Wall of Artery. 



leading to the parts. In the walls of these arteries are 

 muscle fibers of a different kind from those of the skele- 



