408 VETERINARY PHYSIOLOGY 



heard, two changes are taking place in the heart, either of 

 which would produce a sound. 



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



2nd. The auriculo-ventricular valves are being stretched. 



] st. That the first factor plays a part in the production 

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

 of an animal and listening to the organ with a stethoscope 

 while it is still beating — but without any blood passing 

 through it to stretch the valves. 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 subjected 

 on the one side to the high ventricular pressure, and on the 

 other to the low auricular pressure. If the valves are 

 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. It has been main- 

 tained that a trained ear can pick out in the first sound the 

 note corresponding to the valvular vibrations. 



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 — due partly 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. 



A third sound has been described by Gibson. It is heard 

 during the diastole of the ventricles, and it has been ascribed 

 to the rebound of the semilunar valves. 



