236 PHYSIOLOGY OF THE HEART [CH. XX. 



producing the first sound. Secondly, the muscular sound produced 

 by contraction of the mass of muscular fibres which forms the 

 ventricle. Looking upon the contraction of the heart as a single 

 contraction and not as a series of contractions or tetanus, it is at 

 first sight difficult to see why there should be any muscular sound 

 at all when the heart contracts, as a single muscular contraction 

 does not produce sound. It has been suggested, however, that it 

 arises from the repeated unequal tension produced when the wave 

 of muscular contraction passes along the very intricately arranged 

 fibres of the ventricular walls. Many regard the valvular element is 

 the more important of the two factors, because the sound is loudest 

 at first, when the vibration of the valves commences, and fades 

 away as the vibrations cease. If the sound was mainly muscular, 

 it would be loudest when the muscular contraction was most powerful, 

 which is approximately about the middle of the ventricular systole. 

 The facts of disease lend support to the theory that the first sound 

 is mainly valvular ; for when the valves are incompetent, the first 

 sound is largely replaced by a murmur due to regurgitation of blood 

 into the auricle. After the removal of the heart from the body, the 

 muscular contribution to the first sound is audible, but it is very 

 faint. It is stated to have a somewhat lower pitch than the valvular 

 sound. 



There is, on the other hand, much to be said against the view 

 that the cause of the first sound is entirely or even largely due to 

 vibration of the auriculo-ventricular valves. Any sound produced 

 by the valves must be very quickly damped by the high pressure 

 acting on their ventricular surfaces only. The sustained character 

 of the sound (throughout practically the whole of the ventricular 

 systole) is on the other hand exactly what is to be expected if it is 

 of muscular origin. The argument that the extent to which the 

 muscle sound contributes to the production of the first sound can 

 be judged from the sound heard in an isolated and empty heart is 

 quite fallacious, since under these conditions the muscle is contract- 

 ing against no resistance. 



The cause of the second sound is more simple than that of the 

 first. It is entirely due to the vibration consequent on the sudden 

 stretching of the semilunar valves when they are pressed down across 

 the orifices of the aorta and pulmonary artery. The influence of 

 these valves in producing the sound was first demonstrated by Hope, 

 who experimented with the hearts of calves. In these experiments 

 two delicate curved needles were inserted, one into the aorta, and 

 another into the pulmonary artery, below the line of attachment of 

 the semilunar valves, and, after being carried upwards about half an 

 inch, were brought out again through the coats of the respective 

 vessels, so that in each vessel one valve was included between the 



