1 84 



THE CIRCULATION OF THE BLOOD 



muscular fibers, urging forward a certain quantity of fluid against con- 

 siderable 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 cusps as there was to their 

 opening. The sounds may be expressed by the words lubb dub. The 

 beginning of the first sound corresponds 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 continues through a somewhat longer interval 

 than the second sound. The second sound, in point of time, immediately 



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.) 



follows the cessation of the ventricular contraction, and corresponds with 

 the commencing dilatation of the ventricles and the opening of the semi- 

 lunar 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. Ai and A 2, 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 



