INNERVATION OF THE HEART. - 271 



moved from the animal, in which the valves could not become 

 tense (Ludwig). 



The sound has been analyzed with suitable resonators, and two 

 distinct tones made out : one, high and short, corresponding to the 

 closure of the valves ; the other, long and low, corresponding in 

 duration with the muscle contraction. From this it would ap- 

 pear that both the tension of the valves and the muscle tone have 

 something to do with the production of the sound. 



The good reasons given for thinking that the heart muscle can- 

 not produce a tone, suggest that the sudden state of tension of 

 the ventricular wall when tightened over the blood may give rise 

 to vibrations, and be an important item in causing the first sound. 

 This would explain the faintness of the sound, both when the 

 valves were injured and the muscle weak. It would also explain 

 the prsssystolic sound, which requires a certain auricular tension 

 for its production. 



The production of the second sound is more easily explained. 

 Occurring just after the ventricle is emptied, it is synchronous 

 with the closure of the semilunar valves at the aorta and pulmo- 

 nary orifices. The blood in the aorta forcibly closes the valves 

 as soon as the ventricular pressure begins to wane. This sudden 

 motion causes a vibration of the valves, which is immediately 

 checked by the continuous pressure of the column of blood. 



INNERVATION OF THE HEART. 



The most interesting phenomenon in the heart's action, and 

 that most difficult to explain, is the wonderful regularity of its 

 rhythmical contractions under normal circumstances, and the ex- 

 treme delicacy of the nervous mechanism by which it is regulated. 



The vast majority of the active contractile tissues of the higher 

 animals are under the immediate and exclusive direction of the 

 central nervous system. All the great muscular organs are con- 

 nected with the cerebro-spinal axis by means of nerves, along 

 which impulses pass stimulating the contractile tissue to action. 

 Thus the skeletal muscles are brought under the control of the 

 will, and the nerves coming from the brain carry stimuli to cer- 

 tain sets of muscles when we wish to perform any simple action, 



