THE HEART. 163 



short too short to be measurable but between the second and the suc- 

 ceeding first sound there is a distinct pause. The sounds have been 

 likened to the pronunciation of the syllables lubb, dup, so that the cardiac 

 cycle, as far as the sounds are concerned, might be represented by : liibb, 

 dup, pause. 



The second sound, which is short and sharp, presents no difficulties. It is 

 coincident in point of time with the closure of the semilunar valves, and is 

 heard to the best advantage over the second right costal cartilage close to its 

 junction with the sternum i. e., at the point where the aortic arch comes 

 nearest to the surface, and to which sounds generated at the aortic orifice 

 would be best conducted. Its characters are such as would belong to a 

 sound generated by membranes like the semilunar valves being suddenly 

 made tense and so thrown into vibrations. It is obscured and altered or 

 replaced by a " murmur " when the semilunar valves are affected by disease, 

 and may be artificially obliterated, a murmur taking its place, by passing a 

 wire down the arteries and hooking up the aortic valves. There can be no 

 doubt, in fact, that the second sound is due to the semilunar valves being 

 thrown into vibrations at their sudden closure. The sound heard at the 

 second right costal cartilage is chiefly that generated by the aortic valves, 

 and murmurs or other alterations in the sound caused by changes in the 

 aortic valves are heard most clearly at this spot. But even here the sound 

 is not exclusively of aortic origin, for in certain cases in which the semi- 

 lunar valves on the two sides of the heart are not wholly synchronous 

 in action the sound heard here is double (" reduplicated second sound "), 

 one being due to the aorta and one to the pulmonary artery. While 

 the sound is listened to on the left side of the sternum at the same 

 level, the pulmonary artery is supposed to have the chief share in pro- 

 ducing what is heard, and changes in the sound heard more clearly here 

 than on the right side are taken as indications of mischief in the pulmo- 

 nary valves. 



The first sound, longer, duller, and of a more " booming " character than 

 the second, heard with greatest distinctness at the spot where the cardiac 

 impulse is felt, presents many difficulties in the way of a complete explana- 

 tion. It is heard distinctly when the chest-walls are removed. The cardiac 

 impulse, therefore, can have little or nothing to do with it. In point of time 

 it is coincident with the systole of the ventricles, and may be heard to the 

 greatest advantage at the spot of the cardiac impulse that is to say, at the 

 place where the ventricles corne nearest to the surface, and to which sounds 

 generated in the ventricle would be best conducted. 



It is more closely coincident with the closure and consequent vibrations of 

 the auriculo-ventricular valves than with the entire systole ; for, on the one 

 hand, it dies away before the second sound begins, whereas, as we shall see, the 

 actual systole lasts up to, if not beyond, the closure of the semilunar valves ; 

 and, on the other hand, the auriculo-ventricular valve ceases to be tense and to 

 vibrate as soon as the contents of the ventricle are driven out. This suggests 

 that the sound is caused by the sudden tension of the auriculo-ventricular 

 valves, and this view is supported by the facts that the sound is obscured, 

 altered, or replaced by murmurs when the tricuspid or mitral valves are dis- 

 eased, and that the sound is also altered, or, according to some observers, 

 wholly done away with, when blood is prevented from entering the ven- 

 tricles by ligature of the venae cavse. On the other hand, the sound has not 

 the sharp character which one would expect in a sound generated by the 

 vibration of membranes such as the valves in question, but in its booming 

 qualities rather suggests a muscular sound. Further, according to some 

 observers, the sound, though somewhat modified, may still be heard when 



