CHAP, iv.] THE VASCULAR MECHANISM. 239 



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

 cycle, as far as the sounds are concerned, might be represented 

 by : lubb, dup, pause. 



The second sound, which is short and sharp, presents na diffi- 

 culties. 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 semilunar 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. When 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 

 producing 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 pulmonary 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 explanation. 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 come nearest to the 

 surface, and to which sounds generated in the ventricles would be 

 best conducted. 



It is more closely coincident with the closure and consequent 

 vibrations of the auriculo T 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 

 at least up to the closure of the semilunar valves, and on 

 the other hand the auriculo- ventricular valves cease to be tense 



