MECHANICS OF THE HEART-BEAT 89 



perhaps the resonant tissue of the lungs. Further, as we shall see 

 later on (p. 734), the sound caused by a contracting muscle readily 

 calls forth sympathetic resonance in the ear, and the peculiar boom- 

 ing character of the first sound may be due to the superposition of 

 these various resonance tones upon the muscular note. But, in 

 addition, the vibration of the auriculo-ventricular valves un- 

 doubtedly contributes to the production of the sound, and 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. In the excised 

 empty heart the deeper tone of the first sound is alone heard, while 

 the higher note is elicited when in a dead heart the auriculo-ventric- 

 ular valves are suddenly put under tension (Haycraft). 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 recognized 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 con- 

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

 paratively rare cases where they can be distinctly recognized, in the 

 third to the fifth interspace, a little to the right of the sternum. 



The second sound is caused by the vibrations of the semilunar 

 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 prevented 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 pul- 

 monary factor in the second. The place where the second sound is 

 best heard (over the junction of the second right costal cartilage and 

 sternum) is that at which any change produced by disease of the 

 aortic valves is most easily recognized. The sound is conducted up 

 from the valves along the aorta, which comes nearest to the surface 

 at this point. Changes connected with disease of the pulmonary 

 valves are most readily detected over the second left intercostal 

 space near the edge of the sternum, for here the pulmonary artery 

 most nearly approaches the chest- wall. ^The first sound is ' systolic ' 

 that is, it occurs during the ventricular systole; the second is 

 ' diastolic,' beginning at the commencement of the diastole \ 



