SUMMARY 153 



the same peripheral ganglia : this fact suggests that they may- 

 have a close relation to each other, that they started as a 

 single cell in the central nervous system, and that the axis cylin- 

 der split to form two nerves, one of which was motor to the one 

 muscle, and the other inhibitory to the antagonistic muscle. It 

 is a striking characteristic of the cells of the central nervous 

 system of the leech that in some of them the axis cylinder does 

 split into two fibres, which pass out of the central nervous system 

 in separate nerves supplying different groups of muscles. One 

 of the nerve cells which is known to possess this peculiarity is 

 one of the adrenaline containing cells, which I have suggested 

 gave origin to the sympathetic nervous system. Further research 

 is badly needed on the meaning of this peculiarity, which is such 

 a marked feature in the central nervous system of the segmented 

 annelid group. 



With respect to the dermal system of muscles the motor cells 

 all belong to the sympathetic system and are situated in the 

 lateral chain of ganglia ; the inhibitory nerve cells of these mus- 

 cles are known in the case of the retractor penis muscles and of 

 the muscles round the anus, and all belong to the enteral system ; 

 at present, however, we know nothing of any inhibitory nerves to 

 the main mass of dermal muscles, which form the pilo-motor 

 group. 



The segmental duct system of musculature shows no sign of 

 such reciprocal innervation ; both motor and inhibitory nerve 

 cells have travelled out as far as the musculature itself, and both 

 belong entirely to the sympathetic system and are not connected 

 in any way with the enteral system. 



The vascular group of muscles, which is supplied with motor 

 fibres from the sympathetic system alone, shows many signs of 

 possessing a reciprocal innervation of the same character as in 

 other cases ; thus the dilatation of blood vessels, which causes erec- 

 tion, is brought about by the nervus erigens or pelvic nerve, 

 a connector nerve belonging to the sacral outflow and therefore 

 to the enteral system. In many other cases, where dilatation of 

 vessels is brought about by stimulation of connector nerves of 

 the bulbar outflow, it is always doubtful whether we are dealing 

 with the inhibitory nerves to the muscles of the blood vessel, or 

 whether the dilatation of the vessel may not be due to the influence 

 of chemical substances formed by the activity of the organs con- 



