﻿Nervous 
  System 
  q/"Ceratodus 
  Forsteri. 
  177 
  

  

  Spinal 
  Cord. 
  

  

  In 
  a 
  section 
  through 
  the 
  spinal 
  cord 
  taken 
  a 
  short 
  distance 
  

   behind 
  the 
  posterior 
  end 
  of 
  the 
  sinus 
  rhomboidalis 
  the 
  follow- 
  

   ing 
  structures 
  may 
  be 
  distinguished 
  : 
  — 
  

  

  Three 
  pairs 
  of 
  columns 
  in 
  the 
  white 
  substance, 
  viz. 
  the 
  

   ventral, 
  between 
  the 
  two 
  ventral 
  roots 
  of 
  the 
  spinal 
  nerves, 
  

   the 
  dorsal, 
  between 
  the 
  two 
  dorsal 
  roots, 
  and 
  the 
  lateral, 
  

   between 
  the 
  dorsal 
  and 
  ventral 
  roots. 
  

  

  The 
  nerve-fibres 
  of 
  the 
  ventral 
  columns 
  are 
  on 
  an 
  average 
  

   of 
  a 
  larger 
  size 
  than 
  those 
  of 
  the 
  other 
  two 
  columns 
  ; 
  but 
  

   they 
  do 
  not 
  predominate 
  to 
  such 
  a 
  degree 
  as 
  in 
  the 
  Teleostei. 
  

   On 
  the 
  other 
  hand, 
  many 
  large-sized 
  fibres 
  are 
  scattered 
  

   through 
  the 
  lateral 
  columns, 
  which 
  thus 
  show 
  a 
  general 
  

   average 
  of 
  larger-sized 
  fibres 
  than 
  in 
  Teleostei. 
  

  

  Contrary 
  to 
  what 
  is 
  the 
  case 
  in 
  Protopterus, 
  where 
  the 
  

   grey 
  substance, 
  according 
  to 
  Fulliquet 
  *, 
  repeats 
  exactly 
  the 
  

   exterior 
  form 
  of 
  the 
  spinal 
  cord, 
  it 
  here 
  has 
  a 
  highly 
  irregular 
  

   outline. 
  It 
  extends 
  on 
  each 
  side 
  of 
  the 
  central 
  canal 
  in 
  a 
  

   semilunar 
  form, 
  the 
  concavity 
  being 
  directed 
  downwards 
  • 
  

   the 
  two 
  ventral 
  horns 
  expand 
  into 
  club-shaped 
  extremities. 
  

   The 
  dorsal 
  is 
  more 
  irregular 
  in 
  shape, 
  consisting 
  of 
  lamella? 
  

   of 
  grey 
  substance, 
  springing 
  from 
  the 
  substantia 
  gelatinosa 
  

   centralis 
  on 
  each 
  side 
  of 
  the 
  canal 
  and 
  being 
  directed 
  towards 
  

   the 
  dorsal 
  surface. 
  

  

  The 
  grey 
  substance 
  gradually 
  diminishes 
  in 
  going 
  back 
  

   until 
  at 
  the 
  posterior 
  end 
  of 
  the 
  cord 
  it 
  nearly 
  disappears. 
  

  

  The 
  concavity 
  of 
  the 
  grey 
  substance 
  coincides 
  with 
  the 
  

   convexity 
  of 
  the 
  ventral 
  columns, 
  on 
  the 
  summit 
  of 
  which 
  

   the 
  two 
  multiaxial 
  fibres 
  are 
  imbedded 
  ; 
  these 
  occupy 
  a 
  

   position 
  corresponding 
  to 
  that 
  of 
  the 
  Mauthner's 
  fibres 
  in 
  

   Teleostei. 
  As 
  far 
  as 
  I 
  can 
  discover, 
  they 
  are 
  distinct 
  from 
  

   any 
  other 
  kind 
  of 
  nerve- 
  fibre 
  in 
  their 
  structure, 
  but 
  seem 
  to 
  

   approach 
  nearest 
  to 
  those 
  fibres, 
  of 
  which 
  they 
  appear 
  to 
  be 
  an 
  

   amplification. 
  They 
  occupy 
  a 
  considerable 
  amount 
  of 
  space 
  

   on 
  the 
  upper 
  and 
  outer 
  sides 
  of 
  the 
  ventral 
  columns, 
  being 
  

   conterminous 
  with 
  the 
  lower 
  concave 
  edge 
  of 
  the 
  grey 
  substance 
  

   on 
  their 
  dorsal 
  sides. 
  They 
  vary 
  in 
  shape 
  in 
  different 
  parts 
  

   of 
  their 
  course 
  from 
  round 
  to 
  almond-shaped. 
  

  

  The 
  peculiarity 
  of 
  their 
  structure 
  consists 
  in 
  the 
  fact 
  that 
  

   from 
  forty 
  to 
  fifty 
  axis-cylinders 
  are 
  contained 
  in 
  a 
  single 
  

   medullary 
  sheath, 
  which 
  is 
  common 
  to 
  all 
  of 
  them. 
  This 
  

   sheath, 
  notwithstanding 
  its 
  immense 
  extent 
  and 
  thickness, 
  

   has 
  the 
  appearance 
  and 
  structure 
  of 
  the 
  medullary 
  sheaths 
  of 
  

  

  * 
  L. 
  c. 
  p. 
  41. 
  

  

  