THE NERVOUS SYSTEM 11 



heart-beat; the second retards it and may 

 even bring it to a complete standstill. Such 

 a control of a muscle is by no means usual 

 and suggests at once that the heart must 

 be an exceptional organ. But what is of still 

 more significance than this peculiar form o£ 

 control is the fact that the heart will keep on 

 beating for many hours after all its nervous 

 connections with the rest of the body have been 

 severed. This condition would be very difficult 

 to explain from the usual standpoint of the 

 relation of nerve to muscle, were it not known, 

 from the time of Remak, over half a century 

 ago, that the heart muscle is permeated with a 

 network of nerve cells and thus may be said 

 to carry its own nervous mechanism within it- 

 self. Hence it is clear that the sympathetic and 

 vacuus nerves are not related to the heart mus- 

 ele as ordinary motor nerves are to the muscles 

 that they control, but are to be regarded as an 

 auxiliary nervous apparatus superimposed on 

 the heart whose true nervous mechanism may 

 be within its own substance. 



It is not my purpose to enter here into the 

 vexed question of the nature of the heart-beat 

 in the adult vertebrate. As you are probably 

 aware, this question has divided physiologists 



