CIRCULATION OF THE BLOOD. 



49 



The Beat of the Heart. The apex of the heart is 

 always in contact with the chest wall. Consequently, it 

 never strikes it. At each beat it pushes hard against the 

 chest wall. This push may be felt and seen, and is called 

 the heart beat. 



The Sounds of the Heart. There are two sounds of 

 the heart : 



1. A short, sharp sound made by the closing of the semi- 

 lunar valves. 



2. Just preceding this sound a longer, duller sound may 

 be heard during the contraction of the ventricles. This is 

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

 ventricles and of the large valves. 



As soon as 



Nucleus 



Isolated Fibers 



Fibers Joined 



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, 

 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 possible 

 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 for- 

 ward 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. 



Fig. 1 9. Plain (Unstriated) Muscu- 

 lar Fibers from the Bladder. 



