HEART 407 



maintained. The pressure of the heart disappears as the 

 ventricles relax. 



9. Sounds of the Heart. — On listening over the region 

 of the heart, a pair of sounds may be heard with each cardiac 

 cycle, followed by a somewhat prolonged silence. These are 

 known respectively as the First and Second Sounds of the 

 Heart (fig. 173) {Practical Physiology). 



By placing a finger on the cardiac impulse, while listen- 

 ing to these sounds, it is easy to determine that the first sound 

 occurs synchronously with the cardiac impulse — I.e. synchron- 

 ously with the ventricular contraction. 



It develops suddenly, and dies away more slowly. In 

 character it is dull and rumbling, and may be imitated by 

 pronouncing the syllable liib. In pitch it is lower than the 

 second sound. 



The second sound is heard at the moment of ventricular 

 diastole. Its exact time in the cardiac cycle has been deter- 

 mined by recording it on a cardiac tracing by means of a 

 microphone. It develops suddenly and dies away suddenly. 

 It is a clearer, sharper, and higher-pitched sound than the 

 first. It may be imitated by pronouncing the syllable dupp. 



According to the part of the chest upon which the ear is 

 placed, these sounds vary in intensity. Over the apical 

 region the first sound is louder and more accentuated ; over 

 the base the second sound is more distinctly heard. 



A. The Cause of the Second. Sound is simple. At the 

 moment of ventricular diastole, when this sound develops, 

 the only occurrence which is capable of producing a sound is 

 the sudden stretching of the semilunar valves by the high 

 arterial pressure above them and the low intra-ventricular 

 pressure below them. The high arterial pressure comes on 

 them suddenly like the blow of a drum-stick on a drum-head, 

 and, by setting the valves in vibration, produces the sound. 



Aortic and Pnhnonary Areas. — The second sound has 

 thus a dual origin — from the aortic valve and from the 

 pulmonary valve ; and it is possible, by listening in suitable 

 positions, to distinguish the character of each of these. 



B. The Cause of the First Sound is twofold. When it is 



