194 HANDBOOK OF PHYSIOLOGY. 



The Sounds of the Heart. 



When the ear is placed over the region of the heart, two sounds may 

 be heard at every beat of the heart, which follow in quick succession, 

 and are succeeded by a pause or period of silence. The first sound is 

 dull and prolonged; its commencement coincides with the impulse of 

 the heart against the chest wall, and just precedes the pulse at the wrist. 

 The second is shorter and sharper, with a somewhat napping character, 

 and follows close after the arterial pulse. The periods of time occupied 

 respectively by the two sounds taken together and by the pause between 

 the second and the first, are unequal. According to Foster, the interval 

 of time between the beginning of the first sound and the second sound 

 is .3 second, while between the second and tbe succeeding first it is 

 nearly .5 (see fig. 165). Tbe relative length of time occupied by each 

 sound, as compared with the other, may be best appreciated by consider- 

 ing the different forces concerned in the production of the two sounds. 

 In one case there is a strong, comparatively slow, contraction of a large 

 mass of muscular fibres, urging forward a certain quantity of fluid 

 against considerable resistance; while in the other it is a strong but 

 shorter and sharper recoil of the elastic coat of the large arteries shorter 

 because there is no resistance to the flapping back of the semilunar valves, 

 as there was to their opening. The sounds may be expressed by the 

 words lubb dup. 



The events which correspond, in point of time, with the first sound, 

 are (1) the contraction of the ventricles, (2) the first part of the dilata- 

 tion of the auricles, (3) the tension of the auriculo-ventricular valves, 

 (4) the opening of the semilunar valves, and (5) the propulsion of blood 

 into the arteries. The sound is succeeded, in about one-thirtieth of a 

 second, by the pulsation of the facial arteries, and in about one-sixth of 

 a second, by the pulsation of the arteries at the wrist. The second sound, 

 in point of time, immediately follows the cessation of the ventricular 

 contraction, and corresponds with (a) the tension of the semilunar 

 valves, (b) the continued dilatation of the auricles, (c) the commencing 

 dilatation of the ventricles, and (d) the opening of the auriculo-ventric- 

 ular valves. The pause immediately follows the second sound, and 

 corresponds in its first part with the completed distention of the auri- 

 cles, and in its second with their contraction, and the completed disten- 

 tion of the ventricles ; the auriculo-ventricular valves being all the time 

 of the pause open, and the arterial valves closed. 



Causes. The exact cause of the first sound of the heart is not 

 known. Two factors probably enter into it, viz., firstly the vibration 

 of the auriculo-ventricular valves and of the chordae tendinese. This 

 vibration is produced by the increased intraventricular pressure set up 



