2 2 THE INVOL UNTAR Y NER VO US S YSTEM 



cells of these involuntary muscles, and the motor fibres from 

 these cells are the true homologues of the motor fibres to the 

 voluntary muscles. How then are we to look upon the fibres 

 from the spinal cord which connect these cells with the central 

 nervous system, the white rami communicantes ? If it be a true 

 way of looking at the main chain of the sympathetic system, that 

 it consists of groups of motor cells arranged segmentally, which 

 give origin to the motor fibres of a special group of muscles and 

 were originally in the central nervous system, then it follows 

 that the connexions between the central nervous system and such 

 cell groups, the white rami communicantes, are not of the nature 

 of motor nerves, but must be looked upon as efferent tracts con- 

 necting one part of the central nervous system with another ; 

 just as the optic nerve is not a sensory nerve but a nerve tract 

 connecting two parts of the central nervous system. That this 

 is the right way of looking at this system is made certain by 

 Langley's observations on the innervation of the unstriped 

 muscles which move the hairs of the skin ; for he has shown 

 that, whereas the stimulation of each grey ramus from each sym- 

 pathetic ganglion of the main chain causes a movement of hairs 

 over the same segmental area of skin as is supplied by the sensory 

 nerves of the corresponding posterior root ganglion, the stimula- 

 tion of the connecting fibres in the corresponding anterior root 

 that is to say the preganglionic fibres of the ramus communicans 

 causes a movement of hairs over a large number of such seg- 

 mental areas. Further, he has given reasons for the belief that 

 this effect is due to the fact that these preganglionic fibres do not 

 connect with the cells of only one ganglion, but each fibre sends 

 off collaterals and thus connects with many of the segmental 

 ganglia. In other words, these fibres of the rami communicantes, 

 like the fibres of the pyramidal tracts, connect with the motor 

 cells of many segments by means of collaterals, and are con- 

 nector, not motor, fibres. 



The distribution of the motor nerves of the sympathetic 

 system manifests therefore a segmental arrangement of the same 

 kind as that of the motor nerves of the voluntary muscles ; but 

 the anterior roots of the spinal nerves are not, as in the case of 

 the voluntary system, the manifestation of that segmental ar- 

 rangement ; for they contain, not the motor fibres of the sym- 

 pathetic musculature, but the connector fibres to the segmentally 



