THE CIRCULATION 233 



listening to the organ with a stethoscope. With each beat 

 the lub sound is distinctly heard. 



Apparently the wave of contraction, passing along the 

 muscular fibres of the heart, sets up vibrations, and when 

 these are conducted to the ear the external meatus picks out 

 the vibration corresponding to its fundamental note, and thus 

 produces the characters of the sound. 



2nd. The stretching of the auriculo-ventricular valves also 

 plays a part. If the valves be destroyed or diseased the 

 characters of the first sound are materially altered, or the 

 sound may be entirely masked by a continuous musical 

 sound a murmur. Again, it has been maintained that a 

 trained ear can pick out in the first sound the note corre- 

 sponding to the valvular vibration. 



The idea that the impulse of the heart against the chest 

 wall plays a part in the production of this sound is based upon 

 the fallacious idea that the heart " hits " the chest wall. All 

 that it does is to press more firmly against it. 



Mitral and Tricuspid Areas. On account of the part 

 played by the valves in the production of the first sound it 

 may be considered to be double in nature partly due to 

 the mitral valve, partly to the tricuspid. The mitral valve 

 element may best be heard not over the area of the mitral 

 valve which lies very deep in the thorax but over the 

 apex of the heart, as at this situation the left ventricle, in 

 wnich the valve lies, comes nearest to the thoracic wall and 

 conducts the sound thither. The tricuspid element may be 

 best heard over the area of the valve, and in listening to it 

 it is usual to go to the right extremity of the area in order 

 as far as possible to eliminate the mitral sound. The best 

 situation to select is at the junction of the fifth right costal 

 cartilage with the sternum. 



Cardiac Murmurs. When these valves are diseased and 

 fail to act properly, certain continuous sounds called cardiac 

 murmurs are heard. 



These owe their origin to the fact that, while a current of 

 fluid passing along a tube of fairly uniform calibre is not 

 thrown into vibrations and therefore produces no sound, 

 when any marked alterations in the lumen of the tube 

 occurs either a sudden narrowing or a sudden expansion 



