I2 THE CIRCULATION. 



The relative length of time occupied by each sound, as 

 compared with the other, is a little uncertain. 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 expressed, 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-ventricular 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 contraction, and corresponds 

 with the closure of the semilunar valves, the continued 

 dilatation of the auricles, the commencing dilatation of the 

 ventricles, and the opening of the auriculo-ventricular 

 valves. The pause immediately follows the second sound, 

 and corresponds in its first part with the completed disten- 

 sion of the auricles, and in its second with their contraction, 

 and the distension of the ventricles, the auriculo-ventricular 

 valves being all the time open, and the arterial valves 

 closed. 



The chief cause of the first sound of the heart appears 

 to be the vibration of the auriculo-ventricular valves, and 

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

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



