THE SOUNDS OF THE HEART 



159 



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 ex- 

 pressed by the words lubb dUp. The beginning of the -first sound cor- 

 responds in time with the beginning of the contraction of the ventricles, the 

 closure of the auriculo-ventricular 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. 154. Simultaneous Tracings of the Heart Tone and Pulse of the Carotid in the Dog. 

 A i and Az, First and second sounds; P, pulse; S, time in tenths and fiftieths of a second. (Ein- 

 thoven and Geluk.) 



follows the cessation of the ventricular contraction, and corresponds with 

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

 ventricular valves, figure 154. 



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

 probably enter into it. First, the vibration of the auriculo-ventricular valves 

 and of the chordse tendineae. Second, the vibration of the muscular mass 

 of the ventricles themselves. The same mechanical conditions 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 con- 

 tractions, or tetanus, it is at first sight difficult to see why there should be 

 any muscular sound when the heart contracts. 



The cause of the second sound is more simple and definite than that of 

 the first. It is entirely due to the vibration consequent on the sudden closure 

 of the semilunar valves when they are pressed down across the orifices of 

 the aorta and pulmonary artery. The influence of these valves in producing 

 the sound was first demonstrated by Hope who experimented with the hearts 

 of calves. In these experiments two delicate curved needles were inserted, 

 one into the aorta, and another into the pulmonary artery below the line of 



