THE HEART 125 



of the sympathetic always shows that the heart con- 

 tinues to beat rapidly for a long time after the stimu- 

 lation has ceased. Even^emojiionaj. increase_in jhe 

 heart's rate, then, is apparently due to^j^ temporary 

 diminution of vagus tone^lmft the same is true of the 

 acceleration observed during swallowing and as the 

 result of inspiration. At the same time such temporary 

 suspension of vagus control could not exert such an 

 immediate and marked influence upon the rate of the 

 heart were it not for the constant action of the sympa- 

 thetic, which merely waits, as it were, for the oppor- 

 tunity afforded by the relaxed control of the vagus in 

 order to assert its influence. Conversely, when the 

 heart has been slowed by increased vagus action, the 

 sympathetic limits the slowing, and enables the heart 

 to return more quickly to its normal rate. This action 

 is probably of importance in counteracting influences 

 which tend to cause reflex slowing, such, for example, 

 as injury to the abdominal viscera. It therefore 

 minimizes shock. 



The accelerator centres and nerves would appear to 

 be very resistant to influences such as a low blood- 

 pressure, extreme asphyxia, and certain drugs, which 

 quickly depress other nerve centres. This is fortunate, 

 for they are thus, in virtue of their tonic activity, in a 

 state to aid the heart in an emergency when it most 

 needs help. 



The degree of control exercised by the nervous system 

 over the heart probably varies at different ages. This 

 certainly seems to be so in the case of the inhibitory 

 fibres, at any rate. These appear to be almost inactive 



