﻿Nervous 
  System 
  of 
  Ceratodus 
  Forateri. 
  167 
  

  

  the 
  ventral 
  roots 
  of 
  the 
  spinal 
  nerves, 
  except 
  that 
  they 
  have 
  

   two 
  contiguous 
  roots 
  instead 
  of 
  one, 
  which 
  shows 
  that 
  each 
  

   one 
  ought 
  to 
  be 
  counted 
  as 
  two. 
  They 
  have 
  no 
  dorsal 
  roots, 
  

   and 
  take 
  a 
  long 
  course 
  in 
  the 
  vertebral 
  canal 
  before 
  passing 
  

   out 
  through 
  their 
  respective 
  foramina, 
  which 
  are 
  high 
  up 
  in 
  

   the 
  vertebral 
  arches 
  on 
  the 
  same 
  level 
  as 
  the 
  foramina 
  through 
  

   which 
  the 
  dorsal 
  roots 
  of 
  the 
  succeeding 
  spinal 
  nerves 
  emerge. 
  

  

  It 
  is 
  possible 
  that 
  they 
  may 
  correspond 
  to 
  the 
  two 
  ventral 
  

   roots 
  of 
  the 
  vagus, 
  as 
  described 
  by 
  Wiederslieim 
  *, 
  but 
  here 
  

   they 
  certainly 
  do 
  not 
  join 
  the 
  vagus. 
  They 
  cannot 
  be 
  looked 
  

   upon 
  as 
  the 
  hypoglossal, 
  as 
  the 
  course 
  usually 
  followed 
  by 
  

   that 
  nerve 
  in 
  fishes 
  is 
  here 
  pursued 
  by 
  the 
  second 
  and 
  third 
  

   spinal 
  nerves 
  ; 
  their 
  course 
  outside 
  the 
  skull 
  was 
  not 
  made 
  

   out, 
  but 
  it 
  is 
  improbable 
  that 
  they 
  would 
  join 
  those 
  nerves, 
  

   as 
  the 
  intervening 
  trunk, 
  which 
  is 
  the 
  first 
  spinal 
  nerve, 
  does 
  

   not 
  do 
  so. 
  

  

  The 
  ganglion 
  (PI. 
  VIII. 
  fig. 
  1, 
  sp.gn.) 
  of 
  the 
  spinal 
  nerves 
  

   forms 
  an 
  angle 
  in 
  the 
  cartilage 
  of 
  the 
  vertebral 
  arch, 
  and 
  

   extending 
  down 
  near 
  the 
  outer 
  edge 
  to 
  the 
  level 
  of 
  the 
  floor 
  

   of 
  the 
  vertebral 
  canal, 
  it 
  is 
  there 
  joined 
  by 
  the 
  ventral 
  root, 
  

   which 
  has 
  itself 
  passed 
  through 
  a 
  distinct 
  foramen. 
  

  

  The 
  hypoglossal 
  appears 
  to 
  be 
  represented 
  by 
  the 
  second 
  

   and 
  third 
  spinal 
  nerves, 
  which 
  pass 
  into 
  and 
  supply 
  the 
  

   anterior 
  fin. 
  

  

  Central 
  Cavities. 
  

  

  The 
  central 
  canal 
  (PI. 
  XI. 
  fig. 
  15, 
  c. 
  ca.) 
  in 
  the 
  spinal 
  

   cord 
  is 
  rather 
  large 
  comparatively 
  speaking 
  ; 
  its 
  long 
  diameter 
  

   is 
  placed 
  from 
  side 
  to 
  side 
  and 
  it 
  is 
  compressed 
  from 
  above 
  

   downward. 
  In 
  its 
  progress 
  forward 
  the 
  canal 
  gradually 
  

   widens, 
  the 
  lateral 
  ends 
  become 
  pointed 
  by 
  degrees, 
  and 
  on 
  

   approaching 
  the 
  posterior 
  end 
  of 
  the 
  fourth 
  ventricle 
  the 
  sharp 
  

   angle 
  formed 
  by 
  these 
  pointed 
  extremities 
  slightly 
  turns 
  

   downward. 
  At 
  this 
  point 
  the 
  canal 
  is 
  a 
  large 
  and 
  extensive 
  

   channel 
  occupying 
  about 
  one 
  third 
  of 
  the 
  extent 
  of 
  the 
  sec- 
  

   tion 
  ; 
  the 
  floor 
  is 
  convex 
  and 
  the 
  roof 
  is 
  vaulted. 
  A 
  short 
  

   distance 
  in 
  front 
  of 
  this 
  region 
  a 
  depression 
  or 
  notch 
  appears 
  

   in 
  the 
  roof, 
  which 
  is 
  attached 
  to 
  the 
  dorsal 
  edge 
  of 
  the 
  section 
  

   by 
  a 
  plug 
  of 
  connective 
  tissue 
  separating 
  one 
  side 
  from 
  the 
  

   other; 
  this 
  depression 
  increases 
  as 
  the 
  canal 
  deepens, 
  until 
  

   finally 
  the 
  fourth 
  ventricle 
  or 
  sinus 
  rhomboidalis 
  opens 
  out 
  ; 
  

   the 
  walls 
  recede, 
  the 
  floor 
  becomes 
  wider, 
  and 
  the 
  section 
  

   enlarges 
  in 
  every 
  direction 
  — 
  an 
  enlargement 
  due 
  partly 
  to 
  the 
  

  

  * 
  L. 
  c. 
  p. 
  76, 
  fig. 
  19. 
  

  

  