﻿Nervous 
  System 
  of 
  Ceratodus 
  Forsteri. 
  179 
  

  

  framework 
  of 
  the 
  cord 
  ; 
  some 
  of 
  them 
  can 
  be 
  traced 
  occasion- 
  

   ally 
  to 
  the 
  outer 
  edge 
  of 
  the 
  section, 
  where 
  they 
  are 
  found 
  to 
  

   pass 
  into 
  the 
  longitudinal 
  columns, 
  where 
  they 
  become 
  the 
  

   finer 
  longitudinal 
  fibres 
  which 
  are 
  to 
  be 
  seen 
  in 
  that 
  position 
  ; 
  

   others 
  can 
  be 
  traced 
  to 
  the 
  centre 
  of 
  the 
  motor 
  tract, 
  where 
  

   they 
  become 
  the 
  fine 
  fibres 
  which 
  occupy 
  the 
  spaces 
  between 
  

   the 
  usual 
  larger-sized 
  fibres 
  of 
  the 
  ventral 
  and 
  lateral 
  columns 
  ; 
  

   in 
  most 
  cases 
  these 
  processes 
  of 
  the 
  cells 
  turn 
  backward, 
  in 
  a 
  

   few 
  they 
  turn 
  forward. 
  In 
  one 
  case 
  I 
  found 
  a 
  process 
  from 
  

   a 
  cell 
  turning 
  backward 
  in 
  the 
  ventral 
  column 
  and 
  another 
  

   process 
  from 
  the 
  same 
  cell 
  turning 
  forward 
  in 
  the 
  dorsal 
  part 
  

   of 
  the 
  lateial 
  column 
  ; 
  occasionally 
  a 
  process 
  may 
  be 
  traced 
  

   into 
  the 
  ventral 
  column 
  of 
  the 
  opposite 
  side 
  of 
  the 
  cord 
  ; 
  some- 
  

   times 
  a 
  branch 
  is 
  traceable 
  into 
  the 
  substantia 
  gelatinosa 
  

   centralis, 
  passing 
  dorsad 
  of 
  the 
  multiaxial 
  fibre. 
  

  

  Cells 
  (PL 
  XII. 
  fig. 
  18) 
  with 
  two 
  nuclei 
  are 
  occasionally 
  to 
  

   be 
  found, 
  each 
  nucleus 
  being 
  provided 
  with 
  a 
  nucleolus 
  ; 
  per- 
  

   haps 
  these 
  cells 
  may 
  be 
  more 
  properly 
  regarded 
  as 
  two 
  distinct 
  

   cells 
  connected 
  together 
  by 
  a 
  short 
  thick 
  process. 
  

  

  Cells 
  of 
  the 
  other 
  species 
  are 
  found 
  in 
  the 
  substantia 
  gela- 
  

   tinosa 
  centralis, 
  where 
  they 
  form 
  a 
  ganglion 
  corresponding 
  in 
  

   position 
  to 
  the 
  central 
  ganglion 
  in 
  Teleostei 
  ; 
  they 
  are 
  smaller 
  

   in 
  size 
  and 
  present 
  a 
  smoother 
  and 
  more 
  regular 
  contour 
  than 
  

   those 
  in 
  the 
  ventral 
  horn 
  ; 
  they 
  are 
  flask-shaped 
  or 
  oval, 
  and 
  

   usually 
  give 
  off 
  one 
  or 
  two 
  processes 
  ; 
  their 
  long 
  axis 
  is 
  

   generally 
  turned 
  towards 
  the 
  dorsal 
  edge. 
  Although 
  no 
  posi- 
  

   tive 
  connexion 
  could 
  be 
  traced 
  between 
  them 
  and 
  the 
  fibres 
  of 
  

   the 
  dorsal 
  root, 
  yet 
  the 
  probability 
  is 
  that 
  they 
  unite 
  and 
  that 
  

   this 
  ganglion 
  belongs 
  to 
  that 
  root. 
  Cells 
  of 
  this 
  species 
  are 
  

   also 
  found 
  grouped 
  in 
  the 
  fibras 
  rectre 
  ; 
  one 
  such 
  ganglion 
  is 
  

   found 
  in 
  this 
  position 
  immediately 
  behind 
  the 
  posterior 
  end 
  of 
  

   the 
  sinus 
  rhomboidalis. 
  Cells 
  resembling 
  these, 
  but 
  slightly 
  

   smaller, 
  are 
  occasionally 
  found 
  singly 
  scattered 
  through 
  the 
  

   field 
  of 
  the 
  ventral 
  columns 
  ; 
  they 
  differ, 
  however, 
  in 
  giving 
  

   off 
  more 
  than 
  one 
  process. 
  

  

  The 
  multiaxial 
  fibres 
  (PI. 
  XI. 
  fig. 
  19) 
  commence 
  at 
  the 
  

   posterior 
  end 
  of 
  the 
  spinal 
  cord 
  and 
  are 
  first 
  met 
  with 
  oppo- 
  

   site 
  the 
  hinder 
  end 
  of 
  the 
  abdomen 
  ; 
  here 
  they 
  consist 
  of 
  a 
  

   very 
  few 
  axis-cylinders 
  enclosed 
  in 
  a 
  comparatively 
  small 
  

   medullary 
  sheath 
  ; 
  the 
  axis-cylinders 
  gradually 
  increase 
  in 
  

   number 
  as 
  the 
  fibres 
  proceed 
  forward, 
  but 
  the 
  increase 
  is 
  not 
  

   uniform 
  ; 
  at 
  about 
  the 
  middle 
  of 
  their 
  course 
  they 
  become 
  

   much 
  smaller, 
  and 
  after 
  a 
  few 
  sections 
  again 
  enlarge 
  to 
  the 
  

   original 
  diameter. 
  But 
  this 
  diminution 
  does 
  not 
  occur 
  at 
  the 
  

   corresponding 
  point 
  on 
  the 
  opposite 
  sides 
  ; 
  but 
  the 
  fibre 
  of 
  one 
  

   side 
  first 
  suffers 
  a 
  diminution, 
  then 
  that 
  of 
  the 
  other 
  side. 
  

  

  