SOUNDS OF THE HEART. 105 



closer and more secure is their apposition. The corpora 

 Arantii meet at the centre of the arterial orifice when the 

 valves are down, and they probably assist in the closure ; but 

 they are not essential to it, for, not unfrequently, they are 

 wanting in the valves of the pulmonary artery, which are 

 then extended in larger, thin, flapping margins. In valves of 

 this form, also, the inlaid cords are less distinct than in those 

 with corpora Arantii ; yet the closure by contact of their sur- 

 faces is not less secure. 



Sounds of the Heart. 



When the ear is placed over the region of the heart, two 

 sounds may be heard at every beat of the heart, which follow 

 in quick succession, and are succeeded by a pause or period of 

 silence. The first sound is dull and prolonged ; its commence- 

 ment coincides with the impulse of the heart, and just precedes 

 the pulse at the wrist. The second is a shorter and sharper 

 sound, with a somewhat flapping character, and follows close 

 after the arterial pulse. The period of time occupied respec- 

 tively 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 different 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 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 difference may be also ex- 

 pressed, as Dr. C. J. B. Williams has remarked, by saying the 

 words lubb dup. 



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

 sound, are the contraction of the ventricles, the first part of 

 the dilatation of the auricles, the closure of the auriculo-ven- 

 tricular valves, the opening of the semilunar valves, and the 

 propulsion of blood into the arteries. The sound is succeeded, 

 in about one-thirtieth of a second, by the pulsation of the facial 

 artery, 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 contrac- 

 tion, and corresponds with the closure of the semilunar valves, 

 the continued dilatation of the auricles, the commencing dila- 

 tation of the ventricles, and the opening of the auriculo-ven- 



