136 STUDIES IN PHYSIOLOGY 



relax, the semilunars close, the mitral and tricuspid valves 

 open, and the blood passes into these lower cavities. For a 

 very short time, therefore, blood can flow freely from the 

 mouth of the veins, through the auricles, down into the 

 ventricles. But as the latter fill, the valves float up toward 

 a horizontal position. Then comes the contraction of the 

 auricles, which begins the whole series of events just de- 

 scribed. 



We may summarize as follows what we have learned in 

 regard to the action of the valves : (1) when the auricles 

 contract, the mitral and tricuspid valves are open and the 

 semilunar valves are closed ; (2) when the ventricles con- 

 tract, the semilunar valves are open and the mitral and 

 tricuspid valves are closed ; (3) during the pause before the 

 beginning of the next heart beat, the mitral and tricuspid 

 valves are open, and the semilunar valves are closed [as 



Sounds of the Heart. If one listens at the chest wall, one 

 can hear two distinct sounds during each heart beat. The 

 * first sound is longer and more muffled ; it may be compared to 

 the syllable lub, and is probably caused by the vibration of the 

 valve flaps and the chordae tendineae when the contraction of 

 the ventricles closes the mitral and tricuspid valves. The 

 second sound is short and sharp, like the syllable dup. It 

 follows the contraction of the ventricles and is due to the 

 quick closing of the semilunar valves by the pressure of the 

 blood in the arteries. 



The Blood Supply for the Heart. One of the hardest worked 

 organs of the whole body is the heart. During every minute 

 of our lifetime it contracts from sixty to a hundred and 

 twenty times, and the only time it gets for rest is during 

 the pause of a fraction of a second after each heart beat. 

 The heart muscle must, therefore, be oxidized to furnish 

 energy for all this work, and consequently new building 

 material must be continually furnished. An abundance of 

 blood passes through the cavities of the heart, but the walls 



