270 THE CIRCULATION. 



charged into the arteries. Each one of these successive actions is 

 called a beat, or pulsation of the heart. 



Fig. 83. 



COURSE OF BLOOD THROUGH THE HEART. a, a. Vena cava, superior and inferior. 

 6. Right ventricle, c. Pulmonary artery, d. Pulmonary vein. e. Left ventricle. /. Aorta. 



Each pulsation of the heart is accompanied by certain important 

 phenomena, which require to be studied in detail. These are the 

 sounds, the movements, and the impulse. 



The sounds of the heart are two in number. They can readily be 

 heard by applying the ear over the cardiac region, when they are 

 found to be quite different from each other in position, in tone, and 

 in duration. They are distinguished as the first and second sounds 

 of the heart. The first sound is heard with the greatest intensity 

 over the anterior surface of the heart, and more particularly over 

 the fifth rib and the fifth intercostal space. It is long, dull, and 

 smothered in tone, and occupies one-half the entire duration of a 

 single beat. It corresponds in time with the impulse of the heart 

 in the precordial region, and the stroke of the large arteries in the 

 immediate vicinity of the chest. The second sound follows imme- 

 diately upon the first. It is heard most distinctly at the situation 

 of the aortic and pulmonary valves, viz., over the sternum at the 

 level of the third costal cartilage. It is short, sharp, and distinct 

 in tone, and occupies only about one-quarter of the whole time of 



