﻿Nervous 
  System 
  of 
  Ceratodus 
  Forsteri. 
  163 
  

  

  line 
  two 
  longitudinal 
  ridges, 
  separated 
  from 
  each 
  other 
  by 
  a 
  

   median 
  longitudinal 
  furrow, 
  are 
  faintly 
  visible 
  ; 
  at 
  the 
  ante- 
  

   rior 
  end 
  two 
  other 
  longitudinal 
  ridges 
  appear, 
  which 
  contain 
  

   the 
  ganglion 
  of 
  origin 
  of 
  the 
  anterior 
  root 
  of 
  the 
  trifacial. 
  

   These 
  two 
  pairs 
  of 
  ridges 
  pass 
  beneath 
  the 
  cerebellum, 
  and 
  

   the 
  central 
  ones 
  are 
  traceable 
  into 
  the 
  aqueduct 
  of 
  Sylvius. 
  

   Posteriorly 
  this 
  ventricle 
  passes 
  by 
  a 
  long 
  funnel-shaped 
  

   passage 
  into 
  the 
  central 
  canal 
  of 
  the 
  corda 
  spinalis. 
  

  

  Apparent 
  Origin 
  of 
  the 
  Cranial 
  Nerves. 
  

  

  Second 
  pair 
  (PI. 
  VIIL 
  fig. 
  1, 
  n. 
  2). 
  — 
  The 
  optic 
  nerves, 
  

   which 
  are 
  of 
  small 
  size, 
  pass 
  after 
  a 
  long 
  course 
  each 
  through 
  a 
  

   foramen 
  which 
  appears 
  to 
  correspond 
  to 
  the 
  optic 
  foramen. 
  

   Each 
  one 
  is 
  closely 
  attached 
  for 
  a 
  short 
  distance 
  to 
  the 
  first 
  branch 
  

   of 
  the 
  trifacial 
  nerve 
  ; 
  but 
  it 
  is 
  possible 
  to 
  separate 
  them 
  by 
  

   dissection 
  without 
  injury 
  to 
  either. 
  The 
  nerve, 
  then 
  passing 
  

   between 
  the 
  origins 
  of 
  the 
  eye-muscles, 
  enters 
  the 
  eyeball. 
  

  

  Third 
  p>air 
  (PI. 
  VIII. 
  fig. 
  1, 
  n. 
  3). 
  — 
  The 
  oculomotor 
  

   arises 
  in 
  the 
  usual 
  position 
  at 
  the 
  base 
  of 
  the 
  optic 
  lobe 
  in 
  

   what 
  Ahlborn 
  terms 
  the 
  epichordal 
  part 
  of 
  the 
  brain, 
  and 
  

   passing 
  out 
  of 
  the 
  brain 
  through 
  its 
  special 
  foramen, 
  it 
  

   immediately 
  enters 
  the 
  sheath 
  of 
  the 
  first 
  branch 
  of 
  the 
  tri- 
  

   facial, 
  from 
  which 
  it 
  cannot 
  be 
  dissected 
  without 
  injury; 
  

   after 
  a 
  short 
  distance 
  it 
  abandons 
  its 
  companion, 
  and, 
  taking 
  

   an 
  independent 
  course, 
  can 
  be 
  traced 
  to 
  the 
  muscles 
  of 
  the 
  

   eyeball. 
  In 
  Protopterus 
  Wiedersheim 
  was 
  uncertain 
  about 
  

   the 
  origin 
  of 
  the 
  corresponding 
  nerve, 
  thinking 
  it 
  might 
  be 
  a 
  

   branch 
  of 
  the 
  trifacial. 
  Fulliquet 
  *, 
  however, 
  found 
  its 
  

   origin 
  in 
  the 
  usual 
  position, 
  but 
  supposed 
  that 
  the 
  first 
  branch 
  

   of 
  the 
  trifacial 
  shared 
  its 
  distribution 
  ; 
  if 
  this 
  should 
  be 
  the 
  

   case 
  in 
  Protopterus 
  it 
  certainly 
  is 
  not 
  so 
  in 
  Ceratodus 
  , 
  although 
  

   of 
  course 
  in 
  the 
  latter 
  case 
  there 
  may 
  be 
  an 
  interchange 
  of 
  

   fibres. 
  Serres 
  f 
  absolutely 
  denies 
  the 
  existence 
  of 
  this 
  nerve 
  

   in 
  Protopterus 
  , 
  as 
  also 
  ot 
  the 
  fourth 
  and 
  sixth. 
  

  

  Fourth 
  pair. 
  — 
  The 
  trochleares 
  are 
  too 
  small 
  to 
  be 
  seen 
  in 
  

   a 
  macroscopic 
  dissection, 
  which 
  justifies 
  {Serres 
  in 
  denying 
  

   their 
  existence; 
  but 
  when 
  we 
  come 
  to 
  the 
  microscopic 
  de- 
  

   scription 
  it 
  will 
  be 
  seen 
  that 
  their 
  presence 
  is 
  demonstrable 
  in 
  

   the 
  transverse 
  section 
  of 
  the 
  part 
  between 
  the 
  optic 
  lobes 
  and 
  

   the 
  cerebellum, 
  where 
  their 
  decussation 
  and 
  emergence 
  from 
  

   the 
  brain 
  are 
  distinctly 
  to 
  be 
  seen 
  (PI. 
  XI. 
  tig. 
  9). 
  

  

  Fifth 
  pair. 
  — 
  Trifacial 
  arises 
  by 
  two 
  routs 
  immediately 
  

   behind 
  the 
  junction 
  between 
  the 
  cerebellum 
  and 
  the 
  restiform 
  

  

  * 
  L. 
  c. 
  p. 
  30. 
  t 
  Conipt. 
  Rend. 
  18G3, 
  p. 
  579. 
  

  

  