SUMMARY 151 



differences in the innervation of these two groups of muscles. 

 For if we make use of the term nervous system to designate 

 these peripheral nerve cells and their nerve fibres, we see that 

 these two groups of muscles are supplied by motor fibres from 

 two distinct nervous systems. The first I designate the "sym- 

 pathetic nervous systerh " retaining the old name and the other 

 the " enteral nervous system ". The adrenaline group of muscles 

 is supplied by motor fibres exclusively from the nerve cells of 

 the sympathetic nervous system, and the endodermal group of 

 muscles from those of the enteral nervous system. The nerve 

 cells of these two systems cannot all be considered to be motor, 

 for in the involuntary nervous system I accept the existence of 

 inhibitory nerve cells and nerve fibres as well as motor nerve 

 cells and fibres ; the nerve cells therefore of both the sympathetic 

 and enteral nervous systems include inhibitory and motor nerve 

 cells, which supply respectively inhibitory and motor nerve fibres 

 to unstriped muscle. 



Further the motor nerve cells of these two nervous systems 

 are connected with the central nervous system, and their con- 

 nector nerves are confined to certain regions of the spinal cord 

 forming three marked outflows, which I have called the bulbar, 

 thoracico-lumbar and sacral outflows. The distinctiveness of the 

 sympathetic and enteral systems is again made manifest, for the 

 connector fibres to the nerve cells of the sympathetic system are 

 confined to the thoracico-lumbar outflow, and those to the nerve 

 cells of the enteral system to the bulbo-sacral outflow. 



Embryological investigations show that the nerve cells of 

 both these systems were originally in the central nervous system 

 and have travelled out from it. In that travelling out the sym- 

 pathetic cells have been accompanied by cells containing adren- 

 aline. These latter form the chromaffine system of cells, and the 

 close relationship found always to exist between chromaffine cells 

 and sympathetic cells, both from an anatomical as well as from 

 a physiological standpoint, strongly suggests their common origin 

 from a definite group of adrenaline containing nerve cells in the 

 central nervous system of the ancestor of the vertebrate. Such 

 nerve cells exist in the central nervous system of those segmented 

 annelids which possess a muscular vascular system. They are 

 well seen in the leeches and represent in my opinion the origin 

 of both the sympathetic and chromaffine cells of the vertebrate. 



