THE HEART AND THE BLOOD VESSELS 141 



The blood that flows out of the right ventricle goes through 

 pulmonary artery to the lungs, whence it goes through 

 >ur vessels called the pulmonary veins to the left auricle of 

 heart; Fig. 76. The left side of the heart, which the 

 >lood now enters, is almost exactly like the right side; there 

 is a similar flap-like valve hanging down between the auricle 

 and ventricle. This mitral valve has two flaps instead of 

 three, but its action is precisely the same as that of the tricus- 

 pid. A large artery also leads out of the left ventricle, its 

 opening guarded by three semilunar valves, exactly like 

 those, at the origin of the pulmonary artery. This artery 

 into which blood is pumped by the left ventricle is called the 

 aorta, and through branches of it, as we shall see, blood is 

 distributed over the entire body. 



Rate of Heart Action 1 . The beat of the heart is really very 

 rapid, a whole beat occupying less than a single second. 

 About seventy times a minute, day and night, the heart goes 

 through the entire act of opening and closing its several valves 

 and forcing along the blood. The two sides beat at exactly 

 the same time, so that with each beat a small cupful of blood 

 is forced into the pulmonary artery from the right ventricle and 

 a similar amount from the left ventricle into the aorta. The 

 actual beat of the heart, i. e. the contraction of the muscles 

 to force the blood along, takes only about 0.3 of a second. 

 After the beat, the heart rests during the time that the auri- 

 cles are filling from the veins. This time, which is the only 

 rest the heart has from its continual work, is about 0.5 of a 

 second. The beating period is called the systole, and the 

 resting period, the diastole. 



Heart Sounds. If one places his ear over a person's heart 

 when it is beating normally, or if an instrument constructed 

 for the purpose (the stethoscope) is applied to the area above 

 the heart, each beat seems to be accompanied by two sounds, 

 one longer than the other, When a heart beat begins and the 



