THE SOUNDS OF THE HEART 



179 



pressed by the words lubb dub. The beginning of the first sound corre- 

 sponds in time with the three coincident events, namely, the beginning of 

 the contraction of the ventricles, the closure of the tricuspid and mitral 

 valves, and the first part of the dilatation of the auricles. The sound con- 

 tinues through a somewhat longer interval than the second sound. The 

 second sound, in point of time, immediately follows the cessation of the 



FIG. 153. Simultaneous Tracings of the Cardiac Impact, or Cardiogram (lower), and 

 the Heart Tones (upper), of Man. The cross strokes at the beginning of the cardiac sound 

 tracing and on the cardiogram mark the synchronous events. (Hiirthle.) 



ventricular contraction, and corresponds with the commencing dilatation of 

 the ventricles and the opening of the semilunar and mitral valves, figure 154. 

 The exact cause of the first sound of the heart is not absolutely known. 

 Two factors probably enter into it. First, the vibration of the semilunar and 

 mitral valves and of the chordae tendineae. Second, the vibration of the 



FIG. 154. Simultaneous Tracings of the Heart Tone and Pulse of the Carotid in the 

 Dog. A i and A2, First and second sounds; P, pulse; S, time in tenths and fiftieths of a 

 second. (Einthoven and Geluk.) 



muscular mass of the ventricles themselves. The same mechanical condi- 

 tions produce equal tension on the ventricular muscle itself and, according 

 to the second view, this is sufficient to account for the first sound. Looking 

 upon the contraction of the heart as a simple contraction and not as a series 

 of contractions, or tetanus, it is at first sight difficult to see why there should 

 be any muscular sound when the heart contracts. 



