122 HANDBOOK OF PHYSIOLOGY. 



dull and prolonged; its commencement coincides with the movement or 

 impulse of the heart against the chest wall, and just precedes the pulse 

 at the wrist. The second is a shorter and sharper sound, with a some- 

 what napping character; and follows close after the arterial pulse. The 

 period of time occupied respectively by the two sounds taken together, 

 and by the pause, are almost exactly equal. The relative length of time 

 occupied by each sound, as compared with the other, is a little uncer- 

 tain. The difference may be best appreciated by considering the differ- 

 ent forces concerned in the production of the two sounds. In one case 

 there is a strong, comparatively slow, contraction of a large mass of 

 muscular fibres, 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 valves, as 

 there was to their opening. The sounds may be expressed by saying the 

 words lilbb diip (C. J. B. Williams). 



The events which correspond, in point of time, with the first sound 

 are (1) the contraction of the ventricles, (2) the first part of the dilata- 

 tion of the auricles, (3) the tension of the auriculo- ventricular valves, 

 (4) the opening of the semilunar valves, and (5) the propulsion of blood 

 into the arteries. The sound is succeeded, in about one-thirtieth of a 

 second, by the pulsation of the facial arteries, and in about one-sixth of 

 a second, by the pulsation of the arteries at the wrist. The second 

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

 ventricular contraction, and corresponds with (a) the tension of the semi- 

 lunar valves, (b) the continued dilatation of the auricles, (c) the com- 

 mencing dilatation of the ventricles, and (d) the opening of the auriculo- 

 ventricular valves. The pause immediately follows the second sound, 

 and corresponds in its first part with the completed distention of the 

 auricles, and in its second with their contraction, and the completed 

 distention of the ventricles; the auriculo-ventricular valves being, all 

 the time of the pause, open, and the arterial valves closed. 



Causes. The exact causes of the first sound of the heart are not 

 exactly known. Two factors probably enter into it, viz., the vibration 

 of the auriculo-ventricular valves and chordae tendinese, due to their 

 stretching, and also, but to a less extent, of the ventricular walls, and 

 coats of the aorta and pulmonary artery, all of which parts are suddenly 

 put into a state of tension at the moment of ventricular contraction; 

 and secondly the muscular sound produced by contraction of the mass 

 of muscular fibres which form the ventricle. The first factor is probably 

 the more important. 



The cause of the second sound is more simple than that of the first. 

 It is probably due entirely to the vibration consequent on the sudden 

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



