THE BLOOD AND ITS CIRCULATION 



371 



of muscle called valves. The auricles receive blood from the veins. 

 The ventricles pump the blood into the arteries. From each ven- 

 tricle, large arteries leave the heart ; that of the left side is called the 

 aorta. Through the aorta, blood passes to all parts of the body. 

 From the right ventricle the pulmonary artery carries blood to the 

 lungs. The openings to these arteries are guarded by three half- 

 moon-shaped flaps, which open so as to allow blood to pass away 

 from the ventricle, but 

 not to go back into it 

 when the muscles relax. 

 The Heart in Action. 

 The heart is con- 

 structed on the same 

 plan as a force pump, 

 the valves preventing 

 the reflux of blood into 

 the auricle after it is 

 forced out of the ven- 

 tricle. Blood enters 

 the auricles from the 

 veins because the mus- 

 cles of that part of 

 the heart relax; this 

 allows the space within 

 the auricles to fill. Al- 



Tbe heart is a force pump ; prove it from these diagrams. 



most immediately the muscles of the ventricles relax, thus allowing 

 blood to pass into the chambers within the ventricles. Then, 

 after a short pause, during which time the muscles of the heart are 

 resting, a wave of muscular contraction begins in the auricles and 

 ends in the ventricles, with a sudden strong contraction which 

 forces the blood out into the arteries. Blood is kept on its course 

 by the valves, which act in the same manner as do the valves in a 

 pump, thus forcing the blood to pass into the arteries upon the con- 

 traction of ventricle walls. 



The Course of the Blood in the Body. Although the two sides 

 of the heart are separate and distinct from each other, yet every 

 drop of blood that passes through the left heart likewise passes 

 through the right heart. There are two distinct systems of cir- 



