Circulation of the Blood. 59 



heart by regular exercise so it will not tire out in pumping 

 the blood to the muscles during an athletic contest, such as 

 a foot race or boat race. 



The Work of the Auricle. The auricle has three func- 

 tions: (i) to complete the filling of the ventricle; (2) to 

 complete the closing of the aur-vent valves ; (3) to act as a 

 reservoir for the blood entering the auricle while the ven- 

 tricle is contracting, that is, while the aur-vent valves are 

 closed. 



s The Work of the Ventricle. The contraction of each 

 ventricle forces the blood around to the ventricle of the 

 other side of the heart. 



The Sounds of the Heart. There are two sounds of the 

 heart: - 



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

 semilunar 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 aur-vent valves. 



Action of the Large Arteries. The arteries have elastic 

 tissue in their walls. When the blood is forced into them, 

 they are stretched. As soon as the ventricle ceases to con- 

 tract, 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 into the ventricles, for it 

 fills the pockets of the semilunar valves, and closes them 

 with a click. The blood therefore flows along the arteries, 

 through the pulmonary artery to the lungs, and through 



