78 A MANUAL OF PHYSIOLOGY 



tial part of the sound, which perhaps acquires its peculiar 

 booming character from the resonance tones of the ear, and 

 possibly of the chest-wall, set up by the muscular contrac- 

 tion. Some observers have been able to distinguish in 

 the first sound the valvular and the muscular elements, 

 the former being higher in pitch than the latter, but a 

 minor third below the second sound. Further, when the 

 mitral valve is prevented from closing by experimental 

 division of the chordae tendineae, or by pathological lesions, 

 the first sound of the heart is altered or replaced by a 

 * murmur.' This evidence is not only important as regards 

 the physiological question, but of great practical interest 

 from its bearing on the diagnosis of cardiac disease. It 

 may be added that the point of the chest-wall at which the 

 first sound is most easily recognised is also the point at 

 which a changed sound or murmur connected with disease 

 of the mitral valve is most distinctly heard, The sound is, 

 therefore, best conducted from the mitral valve along the 

 heart to the point at which it comes in contact with the wall 

 of the chest. Changes in the first sound connected with 

 disease of the tricuspid valve are heard best, in the com- 

 paratively rare cases where they can be distinctly recognised, 

 in the third to the fifth interspace, a little to the right of 

 the sternum. 



Sir Richard Quain has recently revived the theory that the first 

 sound is due, not to the vibrations of the auriculo-ventricular valves, 

 nor to the muscle-sound of the contracting ventricles, but to the impact 

 of the ventricular blood on the semilunar valves at the moment of 

 systole, and the resistance which it encounters as it passes through 

 the orifices of the aorta and pulmonary artery. But although some 

 of the facts which he cites seem to favour such a view, there are many 

 difficulties in the way of its acceptance. 



The second sound is caused by the vibrations of the semi- 

 lunar valves when suddenly closed, 'the recoiling blood 

 forcing them back, as one unfurls an umbrella, and with an 

 audible check as they tighten ' (Watson). The sharpness 

 of its note is lost, and nothing but a rushing noise or bruit 

 can be heard, when the valves are hooked back and pre- 

 vented from closing. It is altered, or replaced by a murmur 

 when the valves are diseased. As there is a mitral and a 

 tricuspid factor in the first sound, so there is an aortic and 

 a pulmonary factor in the second. The place where the 



