CIECULATION 247 



sternum in the second left interspace. The pulmonary element 

 of the second sound may best be heard here. 



The Cause of the First Sound is by no means so simple. 

 When it is heard, two changes are taking place in the heart, 

 either of which would produce a sound. 



1st. The muscular wall of the ventricles is contracting. 



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



1st. That the first factor plays an important part in the pro- 

 duction of the first sound is proved by rapidly cutting out the 

 heart of an animal, and while it is still beating but without 

 any blood passing through it to stretch the valves 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 auriculo-ventricular valves are being closed and 

 subjected on the one side to the high ventricular pressure and 

 on the other to the low auricular pressure. 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 corresponding 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 tlie 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 

 which 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 



