HEART SOUNDS. 275 



One sound follows the other quickly, and then comes a short 

 pause; consequently, they are spoken of as the first and second 

 sounds. 



The first sound occurs at the same time as the ventricular sys- 

 tole. It is a low, soft, prolonged tone, and is most distinctly 

 heard over the fifth intercostal space. 



The second sound is produced when the two sets of semilunar 

 valves are closed, that is, at the moment when the blood ceases 

 to escape from the ventricles. It is a sharp, short sound, and is 

 best heard at the second costal cartilage on the right side. 



The cause of the first sound is not so evident. Possibly there 

 are several factors in its production. The principal events occur- 

 ring at this time may be enumerated thus : 



1. The heart's impulse. 



2. The contraction of the heart muscle. 



3. The rush of blood into the arteries. 



4. The sudden tension of the ventricular chambers and the 



auriculo-ventricular valves. 



It has already been seen that the heart's impulse is caused by 

 a sudden change in its shape and density, and not by a knock 

 against the chest. Moreover, the first sound is heard when the 

 chest wall is removed, so that the apex beating against the thorax 

 cannot even help to cause the sound. 



The sound is not unlike the tone which accompanies the con- 

 traction of the skeletal muscles. It corresponds in time and dura- 

 tion with the contraction of the cardiac muscle. In disease where 

 the heart muscle is weak, the sound becomes faint or inaudible, 

 although the valves are made tense by an intraventricular force 

 sufficient to overcome the pressure in the arteries ; for otherwise 

 the circulation would cease. An abnormal presystolic sound, 

 like in character to the systolic sound, is now recognized by phy- 

 sicians as being produced by the auricular systole, but this can- 

 not depend on the vibrations of valves. According to the most 

 recent and careful observers, the first sound may be heard when 

 the heart is empty. 



All this evidence tends to show that the sound is produced by 

 the contraction of the muscle tissue of the heart, or, in short, that 



