﻿184 
  Mr. 
  A. 
  Sanders 
  on 
  the 
  

  

  the 
  ventral 
  column, 
  running 
  close 
  beneath 
  the 
  endothelial 
  

   lining 
  of 
  the 
  floor 
  ; 
  arrived 
  at 
  the 
  summit 
  of 
  the 
  ventral 
  

   columns, 
  it 
  appears 
  to 
  turn 
  back, 
  and 
  is 
  traceable 
  for 
  some 
  

   distance 
  as 
  a 
  bundle 
  of 
  fibres 
  smaller 
  than 
  the 
  remainder, 
  

   which 
  I 
  have 
  reason 
  to 
  believe 
  become 
  part 
  of 
  the 
  multiaxial 
  

   fibre 
  ; 
  in 
  its 
  course 
  along 
  the 
  floor 
  this 
  nerve 
  traverses 
  a 
  col- 
  

   lection 
  of 
  cells 
  from 
  which 
  some 
  of 
  its 
  fibres 
  may 
  be 
  derived. 
  

   Vagus 
  (PI. 
  XII. 
  fig. 
  14, 
  n. 
  11). 
  — 
  The 
  first 
  root 
  arises 
  from 
  

   the 
  outer 
  angle 
  of 
  the 
  medulla 
  oblongata 
  at 
  some 
  distance 
  

   behind 
  the 
  exit 
  of 
  the 
  acusticus, 
  but 
  at 
  a 
  slightly 
  higher 
  level 
  ; 
  

   it 
  passes 
  into 
  the 
  floor 
  of 
  the 
  ventricle, 
  where 
  it 
  enters 
  the 
  

   tuberosity 
  which 
  contains 
  the 
  tract 
  of 
  the 
  facial 
  ; 
  the 
  tube- 
  

   rosity 
  enlarges 
  for 
  its 
  reception 
  ; 
  it 
  then 
  passes 
  back 
  close 
  to 
  

   the 
  outer 
  side 
  of 
  that 
  tract. 
  After 
  the 
  disappearance 
  of 
  the 
  

   facial 
  the 
  tuberosity 
  becomes 
  larger 
  and 
  is 
  composed 
  of 
  finely 
  

   granular 
  neuroglia 
  ; 
  the 
  dorsal 
  part 
  is 
  occupied 
  by 
  cells 
  in 
  

   which 
  the 
  bundle 
  in 
  question 
  disappears. 
  

  

  This 
  tuberosity 
  extends 
  nearly 
  to 
  the 
  posterior 
  end 
  of 
  the 
  

   fourth 
  ventricle 
  and 
  receives 
  in 
  succession 
  the 
  remaining 
  roots 
  

   of 
  the 
  vagus, 
  which 
  are 
  all 
  derived 
  from 
  the 
  same 
  origin 
  ; 
  it 
  

   is 
  continuous, 
  and 
  not 
  discrete, 
  as 
  in 
  Scyllium 
  and 
  Acantluas. 
  

   There 
  are 
  five 
  roots 
  discernible, 
  the 
  three 
  posterior 
  of 
  which 
  

   are 
  double, 
  although 
  the 
  external 
  nerve 
  resulting 
  is 
  single 
  ; 
  

   this 
  would 
  make 
  the 
  trunk 
  of 
  the 
  vagus 
  equivalent 
  to 
  eight 
  

   nerves. 
  All 
  these 
  roots 
  are 
  dorsal, 
  and 
  in 
  this 
  species 
  there 
  

   are 
  no 
  ventral 
  roots. 
  Behind 
  the 
  last 
  root 
  of 
  the 
  vagus 
  two 
  

   nerves 
  are 
  given 
  off 
  which 
  have 
  only 
  ventral 
  roots 
  ; 
  they 
  arise 
  

   in 
  precisely 
  the 
  same 
  way 
  as 
  the 
  ventral 
  roots 
  of 
  the 
  spinal 
  

   nerves, 
  the 
  only 
  difference 
  being 
  that 
  they 
  have 
  each 
  two 
  

   roots 
  in 
  the 
  medulla 
  oblongata. 
  In 
  Ceratodus 
  at 
  least 
  they 
  

   do 
  not 
  belong 
  to 
  the 
  vagus. 
  

  

  Spinal 
  Nerves. 
  — 
  The 
  dorsal 
  roots 
  enter 
  the 
  spinal 
  cord 
  at 
  

   a 
  point 
  close 
  to 
  the 
  mid 
  line, 
  but 
  they 
  diverge 
  as 
  they 
  pursue 
  

   their 
  course 
  through 
  the 
  parenchyma, 
  so 
  that 
  in 
  the 
  interior 
  

   of 
  the 
  cord 
  they 
  are 
  further 
  from 
  the 
  central 
  line 
  than 
  they 
  

   are 
  at 
  their 
  point 
  of 
  emergence. 
  The 
  individual 
  fibres 
  are 
  of 
  

   a 
  large 
  size, 
  equalling 
  those 
  of 
  the 
  ventral 
  root. 
  The 
  entering 
  

   fibres 
  turn 
  forward 
  in 
  the 
  substance 
  of 
  the 
  cord, 
  immediately 
  

   outside 
  the 
  grey 
  substance 
  of 
  the 
  dorsal 
  horn. 
  

  

  The 
  ventral 
  roots, 
  after 
  entering 
  the 
  cord, 
  follow 
  a 
  forward 
  

   and 
  upward 
  course 
  towards 
  the 
  inner 
  edge 
  of 
  the 
  multiaxial 
  

   fibre, 
  usually 
  passing 
  between 
  that 
  fibre 
  and 
  the 
  central 
  canal 
  

   over 
  its 
  dorsal 
  edge 
  ; 
  they 
  are 
  lost 
  in 
  the 
  grey 
  matter 
  at 
  its 
  

   outer 
  angle. 
  Sometimes 
  a 
  fibre 
  belonging 
  to 
  this 
  root 
  may 
  

   be 
  traced, 
  after 
  having 
  followed 
  the 
  path 
  above 
  mentioned 
  

   in 
  a 
  longitudinal 
  direction, 
  into 
  one 
  of 
  the 
  fibres 
  of 
  the 
  

  

  