THE PHENOMENA NOTED DUKING EACH CARDIAC CYCLE 291 



(c) The auriculoventricular valve flaps may be hooked back without markedly 

 impairing the quality of the sound. 



(d) The character of the sound is decrescent. 



(e) The contracting auricles also emit a sound which, however, remains below 

 the threshold of audibility, owing to the small mass of tissue involved. 



It is generally conceded, however, that the first sound also contains a 

 sUght valvular element, because if the play of the valve flaps is re- 

 stricted or prevented, it displays a somewhat different character. We 

 know that the ventricular systole insures first of all the closure of the 

 auriculoventricular valves (Fig. 147 a), and shortly thereafter, the open- 

 ing of the semilunar valves (6). As the outward movement of the latter 

 is accomplished practically without noise, it must be concluded that 

 the modification imparted to the muscular element of the first sound 

 must be dependent upon the initial contact and the subsequent after- 

 vibration of the closed mitral and tricuspid valves. 



Fig. 147. — Schema to Show the Relationship Between the Heart Sounds and 

 THE Curve of Intraventricular Pressure. 

 AB, systole; BC plateau; and CD, diastole; a, closure of auriculoventricular valve; 

 6, opening of semilunar valve; c, closure of semilunar valve; d, opening of auriculo- 

 ventricular valve; /, // and ///, heart sounds. 



The second sound occurs at the beginning of ventricular diastole 

 and follows immediately upon the closure of the semilunar valves. 

 It lasts 0.05-0.11 sec, while the interval between it and the first 

 sound amounts to 0.15-0.25 sec. It is most intense when the blood 

 pressure is high and when the arterial system is very elastic. 



In contradistinction to the first sound, the second sound possesses 

 no muscular element. It is purely valvular in its origin and is caused 

 by the tension and after-vibration of the closed semilunar valves. 

 This can be shown in the following way: 



(a) If the tension in the aorta and pulmonary artery is lessened by permitting 

 a quick escape of the arterial blood, the intensity of the second sound is greatly 

 diminished. 



(6) If the heart is rendered bloodless, it ceases to give a clear second sound. 



(c) If the semilunar valve-flaps are hooked back, the second sound gives way 

 to a murmur, due to the regurgitation of the blood into the ventricular cavity. 



(d) A sound very similar in character to the second sound may be produced in 



