244 PHYSIOLOGY. 



THE CAUSE OF THE FIRST SOUND is more difficult to determine 

 than is that of the second. The nature of this sound is more com- 

 plex, several factors entering into its evolvement. 



Since it is established that the first sound corresponds in point 

 of time with ventricular systole, it is reasonable to connect it with one 

 or several phenomena which take place in the heart at that moment. 

 They are : The precordial shock, contraction of the ventricles, occlusion 

 of the auriculo-ventricular valves, and opening of the sigmoid valves. 



While the above phenomena are synchronous with the first sound, 

 yet the majority of them are believed to have no action in producing 

 the first sound. Thus, the sound is audible in a heart before which 

 the chest-wall has been removed, so that precordial shock is not the 

 source of the sound. 



Fig. 74. The Action of the Tricuspid Valve. ( CHAUVEAU. ) 



Pv, Tracing of the variations of pressure in the right ventricle. 1, Means 

 first sound. 8, Tracing by the signal magnet, showing the action of the valve, 

 which by its movements closes and opens an electric current to the signal 

 magnet. The first sound (closure of the auriculo-ventricular valves) is simul- 

 taneous with the beginning of the ventricular systole and it produces during the 

 first sound a rapid ascent of the curve of ventricular pressure. 



That the opening of the sigmoid (semilunar) valves is not of 

 consequence has long been refuted by experiment. 



In the case of the second sound we just learned that the pro- 

 duction of it was due to the closure of the sigmoid valves. In like 

 manner the closure of the auriculo-ventricular valves is in part the 

 cause of the first sound. Wintrich, by means of proper resonators, was 

 able to analyze the first sound and so distinguish the clear, snappy 

 valvular component of this so-called solid sound. The very fact that 

 the sound is low and booming in nature demonstrates the fact that 

 there must be some other component entering into its causation. 



The tension and vibration of the chords tendineaB are factors 

 in producing sound, but the nature of it is similar in every respect 

 to that produced by valvular vibration. 



Even though the auriculo-ventricular valves and their chordae 

 tendinea3 be destroyed in an excised heart, yet will there be pro- 



