1 5 2 THE INVOL UNTAR Y NER VO US S YSTEM 



In the lowest vertebrate (Ammocoetes) they have left the 

 central nervous system mainly as chromaffine cells, and masses of 

 them are still found close against the cells of the posterior root 

 ganglia, with which the embryology of the higher vertebrates 

 shows that the sympathetic nerve cells leave the central nervous 

 system. As we pass from the lowest to the highest vertebrates 

 the sympathetic cells become more numerous and the chromaffine 

 cells diminish in number, until at last in the mammalia the sym- 

 pathetic system is fully developed and the chromaffine system 

 reduced to the cells found in the medulla of the suprarenal 

 capsules. 



The evidence of embryology and comparative anatomy thus 

 points strongly to the conclusion that the sympathetic nervous 

 system arose from nerve cells containing adrenaline, such as are 

 found in the central nervous system of all segmented annelids 

 which possess vascular muscles, and that, when these cells left 

 the central nervous system to become peripheral, they left not as 

 single cells but as two separate cells one of which contained all 

 the adrenaline and formed the chromaffine system, and the other 

 the nerve cells of the sympathetic system of the vertebrate. 



These nerve cells, which have travelled out from the central 

 nervous system, include inhibitory as well as motor cells in 

 both the sympathetic and enteral nervous systems : the usual rule 

 appears to be that the inhibitory cells of the one system have 

 travelled out with the motor cells of the other system and vice 

 versa. The most perfect illustration of this is found, when the 

 muscles of the two systems have become antagonistic in function, 

 as in the case of the sphincter musculature and the endodermal 

 musculature. The motor nerves of the sphincter group of 

 muscles belong to the sympathetic nervous system, and their 

 motor nerve cells have travelled out as far as the inferior mesen- 

 teric ganglia in the case of the sphincter system of the coprodaeum 

 and urodaeum, and as far as the superior mesenteric ganglia in 

 the case of the ileo-colic sphincter : the inhibitory nerves to the 

 corresponding antagonistic muscles arise, in the case of the endo- 

 dermal muscles of the bladder and large intestine, from nerve 

 cells in the inferior mesenteric ganglia, and in the case of the 

 small intestine from those in the superior mesenteric ganglia. 

 The motor cells of the sphincter muscles and the inhibitory cells 

 of the neighbouring endodermal muscles have therefore reached 



