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SOUNDS OF THE HEART. 323 



that it is not entirely due to this cause, is also sufficiently evident from 

 two circumstances ; its prolonged character, which could scarcely be 

 given by a momentary impulse; and its continuance, though with 

 diminished intensity, when the parietes of the chest are wanting, and 

 even after the complete removal of the heart from the body. Moreover, 

 the duration of the first sound is much increased by any morbid state 

 of the orifices of the ventricles, which obstructs the exit of the blood. 

 Much discussion has taken place as to the cause of that part of it, which 

 is not due to the impulse ; some having attributed it to the muscular 

 contraction of the walls of the ventricles, others to the flow of blood 

 over the irregular surfaces of their interior, and others to the rush of 

 the fluid through the narrow orifices leading to the aorta and pulmonary 

 artery. There can be little doubt, that the first and last of these causes 

 are both concerned in producing the sound. For as a sound may be 

 distinctly heard by means of the stethoscope, when the heart is con- 

 tracting vigorously out of the body, and when no blood is propelled by it, 

 nothing else than muscular contraction can be then regarded as its source; 

 and there is other evidence, that sound may be produced by this cause, 

 since the vigorous contraction of any other large muscle gives rise to a 

 continued tingling, which may be heard through the stethoscope. But 



hen the heart is contracting in its natural position, and is propelling 

 e blood with its ordinary vigour, the sound is heard in its greatest in- 



ensity at the base of the heart, i. e., at the origin of the great arteries ; 

 and since any obstruction to the exit of the blood through them increases 

 the intensity as well as the length of the sound, it can scarcely be 

 doubted that it is partly due to the rush of the blood through the con- 

 tracted entrances of these vessels. A very similar sound, known as the 

 "bruit de soufflet" or bellows-sound, may be heard through the stetho- 

 scope, over any large artery, when it is compressed, so as to permit the 

 passage of blood less readily than usual. Thus the ordinary first sound 

 may be regarded as composite in its nature ; being made up of the 

 sound produced by the impulse of the heart against the parietes of the 

 chest, of the muscular sound occasioned by the forcible contraction of 

 the thick walls of the ventricles, and of the sound generated by the 

 friction of the particles of blood against each other, and against the 

 boundaries of the narrowing orifices which lead into the vessels. 



574. The cause of the second sound is simpler, and more easily un- 

 derstood. It is due to the sudden filling-out of ihe semilunar valves 

 with blood, at the moment when the ventricular systole has ceased, and 

 when the commencing diastole produces a tendency to the regurgitation 

 of blood from the aorta and pulmonary artery. The sudden passage of 

 the valves, from a state of complete relaxation to one of complete ten- 

 sion, occasions a sort of click ; which is the second sound of the heart. 

 That this is the real cause, has now been fully demonstrated. If one 

 of the valves be hooked back against the side of the artery, by the in- 

 troduction of a curved needle, so that a reflux of blood is permitted, the 

 sound is entirely suppressed. And if the eomplete closure of the valves 

 be prevented by disease, so that their tension is diminished, and a cer- 

 tain amount of regurgitation takes place, the second sound is no longer 

 heard in its proper intensity ; whilst, on the other hand, a sound analo- 



