INNERVATION OF THE HEART. 277 



cut off from the auriculo-ventricular orifices, as no tension 

 can occur. 



From the foregoing statements it would appear probable that 

 both the tension of the valves and the muscle tone are concerned 

 in the production of the first sound. 



The production of the second sound is more easily explained. 

 Occurring just after the ventricle is emptied, it is synchronous 

 with the sudden tension of the semilunar valves at the aorta and 

 pulmonary 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 rapidly 

 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 

 extreme delicacy of the nervous mechanism by which it is regu- 

 lated. 



The vast majority of the active contractile tissues of the higher 

 animals are under the immediate direction of the central nervous 

 system. All the great muscular organs are connected 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. 

 Other muscles, as has been seen in the pharynx, oesophagus, etc., 

 though not under the control of the will, are yet governed 

 altogether by the cerebro-spinal axis ; while others, of which the 

 most striking example is the heart, have nerve elements in im- 

 mediate relation to the contractile tissue, capable of exciting 

 them to contraction. 



It will materially help us in comprehending the nervous mech- 

 anism of the heart's rhythm, if we bear in mind what now seems 

 to be proved beyond doubt, namely, that the muscle tissue of .the 

 heart has quite independently of any nervous influences an 



