198 PHYSIOLOGY CHAP. 



with the tension into which they are thrown after closure, when 

 the ventricular diastole commences. 



In conclusion, we may admit the following points as 

 established : 



1. The systolic sound is essentially a muscular bruit, with 

 which higher tones are associated that depend on the vibrations of 

 the auriculo-ventricular valves, the semilunars, and the mass of 

 the blood. 



2. The post-systolic sound is the result of higher tones depending 

 on the vibration either of the semilunars and the bulbi arteriosi 

 when thrown into tension, or of the mass of the blood. 



The importance of the heart-sounds, from a physiological stand- 

 point, consists in recognising them as the external signs of the 

 duration of the phases of the cardiac cycle, since there is good 

 reason for assuming that the commencement of the first sound 

 coincides with the commencement of systole, the commencement 

 of the second sound with the commencement of diastole ; and that 

 the interval between the first and second sounds represents the 

 duration of systole, the interval between the second and first, the 

 duration of perisystole plus presystole. We shall see how Edgren 

 has applied these criteria. 



It is not the physiologist's task to make any profound examina- 

 tion of the pathological changes in the heart's sounds, or to go into 

 their great significance from the diagnostic and clinical standpoint. 

 But in so far as these changes are a proof, and a further illustration, 

 of physiological theory, the most general aspects of them may be 

 summed up in a few words. 



Cardiac sounds under pathological conditions may be reinforced 

 or weakened, according as the heart's action is stronger or weaker 

 than in the normal state. The first may be a sign of hypertrophy, 

 the second of degeneration, of the myocardium. 



In auricular hypertrophy there may be a presystolic murmur, 

 immediately preceding the systolic, giving the effect of a duplica- 

 tion or abnormal lengthening of the first sound. On the other 

 hand there may be a real doubling of the second sound, when the 

 tension of the aortic seiniltmar valves (which normally have to 

 bear a greater pressure) precedes that of the semilunars of the 

 pulmonary (which normally sustain less pressure) in marked 

 degree. 



In stenosis of the orifices and in valvular insufficiency the 

 sounds are replaced by " blowing " murmurs, which are produced 

 by the vibrations of the blood as it passes through the narrowed 

 orifices or imperfectly closed valves. 



In these can be distinguished : 



(a) Post-systolic murmurs, the diagnostic sign of insufficiency 

 of aortic or pulmonary semilunars, according as they are more 

 audible in the region of the origin of the aorta (right sternal 



