236 



PHYSIOLOGY OF THE HEART 



[CH. XXI. 



just precedes the pulse at the wrist. The second or diastolic sound 

 is shorter and sharper, with a somewhat flapping character, and 

 follows the end of ventricular systole, and is audible just after the 

 radial pulse is felt. The sounds are often but somewhat inaptly 

 compared to the syllables, lull dup. 



Causes. The exact cause of the first sound of the heart is a 

 matter of discussion. Two factors probably enter into it, viz., first, 

 the miration of the auriculo-ventricular valves and the chordae tendinece. 

 This vibration is produced by the increased intraventricular pressure 

 set up when the ventricular systole commences, which puts the valves 

 on the stretch. It is not unlikely, too, that the vibration of the 

 ventricular walls themselves, and of the aorta and pulmonary artery, 



all of which parts are suddenly 

 put into a state of tension at 

 the moment of ventricular con- 

 traction, may have some part 

 in producing the first sound. 

 The second factor is a 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 contrac- 

 tions 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 as 

 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. 



