HISTORY OF THE INVOLUNTARY NERVOUS SYSTEM 29 



on the same plan as the voluntary nervous system, with receptor, 

 connector, and excitor elements. 



The marked difference is that the excitor elements have left 

 the central nervous system and become peripheral, forming the 

 various ganglia found throughout the body ; while the receptor 

 elements have remained in the same position as those of the 

 voluntary system, namely in the posterior root ganglia, and con- 

 nect by means of sensory root fibres with the cells of the connector 

 elements, which have remained in the central nervous system ; 

 the connector fibres have passed out to reach their respective 

 peripherally situated excitor elements. This outflow of con- 

 nector fibres does not occur, at all events in the higher vertebrates, 

 in every segment, but is interrupted at the places where the 

 nerve plexuses for the upper and lower limbs come in ; so that 

 we can speak of three outflows of these fibres, a bulbar, a thoracico- 

 lumbar, and a sacral. Of these the thoracico-lumbar outflow of 

 connector nerves connect with all the excitor neurons of the 

 so-called sympathetic system, the sacral outflow connects with 

 another set of excitor neurons forming the pelvic gang! ionic 

 group, and the bulbar outflow connects with the excitor neu- 

 rons found in the course of the various mesomatic segmental 

 nerves. 



In addition to these three main groups of connector nerves 

 a fourth group exists confined to the region of the mid-brain, 

 which connects with the excitor neurons forming the ciliary 

 ganglion. 



These connector fibres of the involuntary system leave the 

 cord in the spinal region by the anterior roots, so that strictly 

 speaking the anterior root is made up not of motor fibres, but of 

 motor fibres to the voluntary system and connector fibres to the 

 involuntary system ; therefore as we do not think of speaking of 

 the fibres of the pyramidal tract as motor fibres of the voluntary 

 muscles, or as preganglionic fibres, so also we ought not to speak 

 of the corresponding fibres of the involuntary system as motor or 

 preganglionic but as connector fibres. 



When I speak of the motor neurons of the involuntary nervous 

 system as having travelled out from the central nervous system 

 in separate outflows, I do not in the least intend to imply that in 

 my opinion they have wandered out to their destination in the 

 shape of free cells, but simply that their ultimate position has 



