60 Physiology. 



the aorta and its branches to all the other parts of the 

 body. 



The elastic reaction of the arteries thus helps to make 

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

 heart. 



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, demands much 

 more blood than does such an inactive organ as a bone. 

 Further, the working tissues, such as the brain and mus- 

 cles, need a great deal more blood while they are at work 

 than when they are resting. An organ needing a large 

 supply of blood all the time 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 leading 



to the parts. In the walls of these 



Nucleus . , . i rn 



arteries are plain muscle fibers. 

 They are arranged circularly in the 



isolated Fibers walls of the arteries (see Fig. 36). 

 These fibers have, like all muscle 



Fibers joined fibers, the power of shortening. 

 When they shorten they reduce 

 the size of the artery, and, there- 

 fore, for the time, less blood can 

 flow through it. When the muscle 



Fig. 34. Plain Muscle Fibers. _. ., 



fibers relax, the artery widens, and 

 allows more blood to pass through it. 



Illustration of the Action of Muscles in Arterial Walls. To illus- 

 trate the action of the muscles in the walls of an artery, let the water 



