﻿182 
  Mr. 
  A. 
  Sanders 
  on 
  the 
  

  

  passing 
  through 
  this 
  body 
  and 
  having 
  arrived 
  at 
  its 
  dorsal 
  

   end 
  its 
  fibres, 
  or 
  some 
  of 
  them, 
  pass 
  into 
  the 
  optic 
  lobe 
  to 
  

   join 
  the 
  fibres 
  from 
  the 
  lateral 
  columns 
  of 
  the 
  cord 
  ; 
  together 
  

   they 
  enter 
  the 
  second 
  layer 
  and 
  contribute 
  to 
  form 
  the 
  trans- 
  

   verse 
  commissure 
  in 
  the 
  mid 
  line 
  on 
  the 
  dorsal 
  edge 
  of 
  this 
  

   lobe. 
  The 
  whole 
  seems 
  to 
  correspond 
  to 
  the 
  commissura 
  

   ansulata 
  of 
  Teleostei. 
  

  

  At 
  and 
  slightly 
  posterior 
  to 
  the 
  exit 
  of 
  the 
  ocalomotores 
  

   this 
  commissure 
  has 
  nearly 
  disappeared, 
  only 
  again 
  to 
  increase 
  

   in 
  size 
  until 
  it 
  terminates 
  at 
  the 
  posterior 
  end 
  of 
  the 
  cul-de-sac 
  

   which 
  projects 
  a 
  short 
  distance 
  behind 
  the 
  infundibulum 
  ; 
  in 
  

   its 
  course 
  at 
  this 
  part 
  it 
  connects 
  the 
  two 
  sides 
  of 
  the 
  aqueduct 
  

   of 
  Sylvius, 
  at 
  the 
  part 
  which 
  probably 
  homologizes 
  with 
  the 
  

   thalamus 
  opticus. 
  This 
  commissure 
  is 
  on 
  the 
  ventral 
  side 
  of 
  

   the 
  brain 
  ; 
  on 
  the 
  dorsal 
  side 
  also 
  there 
  is 
  another 
  commissure, 
  

   which, 
  commencing 
  at 
  the 
  posterior 
  border 
  of 
  the 
  third 
  ven- 
  

   tricle, 
  is 
  continuous 
  backward 
  with 
  the 
  commissure 
  in 
  the 
  roof 
  

   of 
  the 
  optic 
  lobe 
  ; 
  it 
  is 
  the 
  posterior 
  commissure 
  which 
  is 
  

   found 
  both 
  in 
  Teleostei 
  and 
  Plagiostomata, 
  and 
  corresponds 
  

   both 
  in 
  position 
  and 
  general 
  arrangement 
  with 
  the 
  posterior 
  

   commissure 
  in 
  the 
  third 
  ventricle 
  of 
  the 
  Mammalia. 
  

  

  On 
  each 
  side 
  of 
  the 
  fissure 
  on 
  the 
  ventral 
  side 
  of 
  the 
  aque- 
  

   duct 
  of 
  Sylvius, 
  close 
  behind 
  its 
  entrance 
  into 
  the 
  third 
  ven- 
  

   tricle, 
  there 
  is 
  a 
  ganglion 
  containing 
  moderate-sized 
  cells 
  

   from 
  which 
  a 
  few 
  fibres 
  pass 
  back 
  and 
  ultimately 
  join 
  the 
  

   anterior 
  prolongations 
  of 
  the 
  ventral 
  columns 
  of 
  the 
  cord. 
  

  

  Deep 
  Origins 
  of 
  the 
  Cranial 
  Nerves. 
  

  

  Optic 
  Nerves. 
  — 
  There 
  is 
  no 
  chiasma 
  nervorum 
  opticorum 
  

   visible 
  externally, 
  but 
  internally 
  there 
  is 
  at 
  the 
  base 
  of 
  the 
  

   thalamencephalon 
  a 
  complicated 
  decussation 
  of 
  fibres. 
  Some 
  

   go 
  from 
  one 
  nerve 
  to 
  the 
  other 
  side, 
  some 
  of 
  the 
  dorsal 
  fibres 
  

   go 
  from 
  one 
  nerve 
  to 
  the 
  other, 
  but 
  none 
  could 
  be 
  seen 
  to 
  go 
  

   from 
  the 
  nerve 
  to 
  the 
  optic 
  tract 
  of 
  the 
  same 
  side. 
  

  

  The 
  optic 
  tract 
  passes 
  upward 
  and 
  forward 
  along 
  the 
  external 
  

   edge 
  of 
  the 
  thalamencephalon 
  until 
  it 
  reaches 
  the 
  optic 
  lobe, 
  

   where 
  its 
  fibres 
  spread 
  out 
  longitudinally, 
  forming 
  the 
  outer 
  

   layer 
  of 
  fibres 
  in 
  that 
  structure. 
  

  

  Oculomotores 
  (PL 
  XII. 
  fig. 
  8, 
  n.S). 
  — 
  As 
  Ahlborn 
  has 
  

   pointed 
  out 
  in 
  Petromyzon, 
  this 
  nerve 
  arises 
  from 
  the 
  under 
  

   surface 
  of 
  the 
  medulla 
  oblongata 
  ; 
  although 
  the 
  position 
  of 
  

   its 
  ganglion 
  is 
  beneath 
  the 
  floor 
  of 
  the 
  aqueduct 
  of 
  Sylvius, 
  

   yet 
  this 
  is 
  part 
  of 
  the 
  anterior 
  prolongation 
  of 
  the 
  medulla 
  

   oblongata. 
  In 
  Ceratodus 
  it 
  corresponds 
  in 
  position 
  and 
  mode 
  

   of 
  origin 
  with 
  the 
  same 
  nerve 
  in 
  Teleostei 
  and 
  Plagiostomata. 
  

  

  