﻿Nervous 
  System 
  o/* 
  Ceratodus 
  Forsteri. 
  161 
  

  

  When 
  the 
  pia 
  mater 
  is 
  stripped 
  from 
  the 
  prosencephalon, 
  

   the 
  nervous 
  tissue 
  that 
  remains 
  is 
  not 
  of 
  any 
  great 
  amount 
  ; 
  

   it 
  forms 
  a 
  pair 
  of 
  thin 
  plates, 
  each 
  folded 
  so 
  as 
  to 
  form 
  the 
  

   outer 
  walls 
  and 
  the 
  lower 
  part 
  of 
  the 
  inner 
  walls 
  of 
  two 
  im- 
  

   perfect 
  lateral 
  ventricles, 
  which, 
  as 
  above 
  said, 
  are 
  completed 
  

   by 
  the 
  tela 
  choroidea. 
  1 
  look 
  upon 
  them 
  as 
  lateral 
  ventricles, 
  

   notwithstanding 
  that 
  their 
  walls 
  are 
  not 
  completed 
  by 
  nervous 
  

   tissue 
  ; 
  but 
  if 
  it 
  is 
  considered 
  essential 
  to 
  the 
  connotation 
  that 
  

   the 
  nervous 
  walls 
  should 
  be 
  complete, 
  then 
  of 
  course 
  the 
  term 
  

   ventriculus 
  communis 
  should 
  be 
  applied. 
  The 
  chamber 
  thus 
  

   formed 
  is 
  elongated 
  from 
  before 
  backward, 
  but 
  is 
  narrow 
  from 
  

   side 
  to 
  side 
  in 
  its 
  dorsal 
  part 
  ; 
  anteriorly 
  it 
  communicates 
  with 
  

   the 
  ventricle 
  of 
  the 
  olfactory 
  lobe 
  by 
  a 
  foramen 
  passing 
  

   through 
  the 
  neck 
  of 
  the 
  same. 
  Beauregard 
  * 
  supposed 
  that 
  

   the 
  anterior 
  part 
  of 
  the 
  prosencephalon 
  belonged 
  to 
  the 
  rhinen- 
  

   cephalon; 
  but 
  microscopic 
  examination 
  shows 
  that 
  it 
  has 
  the 
  

   same 
  structure 
  as 
  that 
  of 
  the 
  remainder 
  of 
  the 
  cerebrum. 
  

  

  Below, 
  this 
  chamber 
  communicates 
  with 
  an 
  expanded 
  bulla 
  

   by 
  a 
  large 
  oval 
  opening 
  extending 
  for 
  the 
  whole 
  length 
  of 
  

   the 
  cerebrum 
  ; 
  the 
  inner 
  wall 
  of 
  this 
  bulla 
  is 
  in 
  contact 
  with 
  

   that 
  of 
  the 
  other 
  side, 
  but 
  separated 
  by 
  a 
  layer 
  of 
  pia 
  mater, 
  

   and 
  is 
  only 
  attached 
  behind 
  by 
  a 
  very 
  small 
  transverse 
  com- 
  

   missure. 
  

  

  The 
  prosencephalon 
  in 
  Ceratodus 
  differs 
  from 
  that 
  of 
  Pro- 
  

   topterus 
  in 
  the 
  incompleteness 
  of 
  the 
  nervous 
  tissue 
  of 
  the 
  

   walls 
  f, 
  but 
  agrees 
  in 
  the 
  hemispheres 
  being 
  entirely 
  separ- 
  

   able 
  except, 
  for 
  the 
  above-mentioned 
  narrow 
  transverse 
  com- 
  

   missure, 
  which 
  in 
  Protopterus 
  runs 
  between 
  the 
  two 
  crura 
  

   cerebri, 
  but 
  in 
  Ceratodus 
  between 
  the 
  posterior 
  ends 
  of 
  the 
  

   cerebrum 
  itself. 
  The 
  latter 
  also 
  differs 
  in 
  the 
  greater 
  deve- 
  

   lopment 
  of 
  the 
  rhinencephalon, 
  which 
  in 
  Wiedersheim's 
  

   figures 
  is 
  not 
  shown, 
  but 
  which 
  Fulliquet 
  \ 
  by 
  transverse 
  

   sections 
  demonstrated 
  to 
  be 
  present 
  indeed, 
  but 
  small 
  and 
  

   scarcely 
  separable 
  from 
  the 
  cerebrum. 
  

  

  The 
  thalamencephalon 
  is 
  a 
  deep 
  trough, 
  open 
  above, 
  where 
  

   pia 
  mater 
  alone 
  closes 
  it. 
  It 
  is 
  wider 
  in 
  front, 
  where 
  it 
  com- 
  

   municates 
  with 
  the 
  lateral 
  ventricles 
  by 
  a 
  broad 
  opening, 
  which 
  

   is 
  divided 
  into 
  two 
  by 
  the 
  tela 
  choroidea, 
  narrower 
  behind, 
  

   where 
  it 
  passes 
  into 
  the 
  aqueduct 
  of 
  Sylvius 
  by 
  a 
  foramen 
  

   nearly 
  as 
  large 
  as 
  the 
  ventricle 
  of 
  the 
  optic 
  lobe 
  j 
  above 
  this 
  

   latter 
  opening 
  is 
  situated 
  the 
  posterior 
  commissure, 
  which 
  

   is 
  not 
  visible 
  by 
  the 
  naked 
  eye, 
  but 
  is 
  shown 
  in 
  tne 
  micros^o- 
  

  

  * 
  L. 
  c. 
  p. 
  231. 
  

   t 
  Wiedersheim, 
  /. 
  c. 
  p. 
  70. 
  

  

  \ 
  " 
  Recherches 
  sur 
  le 
  cerveau 
  du 
  Protoptei'vs 
  annectens" 
  Recueil 
  

   Zoologique 
  Suisse, 
  tome 
  iii. 
  (1886). 
  

  

  