76 



ACTION OF LARGE ARTERIES. 



be heard, during the contraction of the ventricles. This is 

 supposed to be due to the vibrations of the walls of the ven- 

 tricles, and of the large valves. 



Action of the Large Arteries. The large arteries have 

 in their walls a yellow elastic tissue. When the blood is 

 forced into them they are stretched. As soon as the ventricle 

 ceases to contract, and sends no more blood into the arteries, 

 they " stretch back." We should not say contract, for it is 

 simply an elastic reaction. As the artery reacts it presses on 

 the blood, and hence the blood tries to escape in every possi- 

 ble way. It cannot go back, for it fills the pockets of the 

 semilunar valves, and closes them with a click. A rapid 

 wave is sent forward that gives the pulse, and a slower but 

 still rapid stream flows along the 

 arteries, through the pulmonary 

 artery to the lungs, and through 

 the aorta and its branches to all 

 the other parts of the body. 



The elastic reaction of the ar- 

 teries thus makes steady the flow 

 of blood which is intermittent as it 

 leaves the heart. 



Action of the Medium-Sized 

 Arteries. Each organ requires 

 a supply of blood in proportion to 

 its activity. An actively working 



organ, like the brain, demands 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? 



Nucleus 



''''"" Isolated Fibers 



Fibers Joined 



Fig. 24. Plain (Unstriated) Mus- 

 cular Fibers from the Bladder. 



