130 THE CIRCULATION. 



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 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 lulib 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. 



Scarcely any subject has been investigated with greater 

 care than that relating to the cause of the sounds of the 

 heart ; yet, although nearly all observers agree about the 



