﻿Nervous 
  System 
  ofCeratodus 
  Forsteri. 
  169 
  

  

  some 
  distance, 
  and 
  ends 
  in 
  a 
  cul-de-sac 
  immediately 
  in 
  front 
  

   of 
  the 
  point 
  where 
  the 
  infundibulum 
  detaches 
  itself 
  from 
  the 
  

   under 
  surface 
  of 
  the 
  brain 
  to 
  pass 
  down 
  to 
  the 
  hypophysis. 
  

  

  From 
  the 
  point 
  where 
  the 
  ventricle 
  of 
  the 
  optic 
  lobe 
  joins 
  

   the 
  infundibulum 
  the 
  latter 
  proceeds 
  obliquely 
  both 
  upwards 
  

   and 
  downwards, 
  upwards 
  through 
  a 
  membranous 
  tube 
  to 
  the 
  

   epiphysis 
  and 
  downwards 
  to 
  the 
  hypophysis 
  ; 
  at 
  the 
  point 
  of 
  

   junction 
  with 
  this 
  body 
  the 
  pia 
  mater 
  splits 
  into 
  two 
  layers, 
  

   one 
  going 
  to 
  envelop 
  the 
  hypophysis, 
  while 
  the 
  other 
  passes 
  

   over 
  to 
  the 
  opposite 
  side 
  and 
  shuts 
  in 
  the 
  lower 
  end 
  of 
  the 
  

   infundibulum, 
  so 
  that 
  there 
  is 
  no 
  actual 
  communication 
  

   between 
  this 
  passage 
  and 
  the 
  cavity 
  of 
  the 
  hypophysis, 
  as 
  is 
  

   the 
  case 
  in 
  Scy 
  Ilium 
  ; 
  neither 
  is 
  this 
  passage 
  completed 
  by 
  

   nervous 
  tissue, 
  but 
  the 
  lower 
  end 
  is 
  closed 
  merely 
  by 
  this 
  

   membrane. 
  

  

  Anteriorly 
  the 
  third 
  ventricle 
  communicates 
  by 
  means 
  of 
  a 
  

   ventriculus 
  communis 
  with 
  the 
  two 
  lateral 
  ventricles 
  in 
  the 
  

   cerebrum, 
  the 
  walls 
  of 
  which 
  are 
  completed 
  by 
  the 
  tela 
  cho- 
  

   roidea 
  ; 
  each 
  of 
  these 
  again 
  communicates 
  anteriorly 
  with 
  a 
  

   ventricle 
  in 
  the 
  olfactory 
  lobe. 
  

  

  Microscopic 
  Anatomy 
  of 
  the 
  Brain. 
  

  

  Cerebrum. 
  

  

  On 
  inspecting 
  a 
  section 
  through 
  the 
  dorsal 
  wall 
  of 
  the 
  

   cerebrum 
  (PI. 
  XIII. 
  fig. 
  22) 
  it 
  is 
  possible 
  to 
  make 
  out 
  four 
  

   layers 
  counting 
  from 
  the 
  surface 
  turned 
  toward 
  the 
  ventricle, 
  

   which 
  is 
  lined 
  by 
  an 
  endothelium 
  continuous 
  with 
  that 
  of 
  the 
  

   remainder 
  of 
  the 
  cavities 
  of 
  the 
  brain. 
  The 
  cells 
  of 
  this 
  endo- 
  

   thelium 
  send 
  processes 
  into 
  the 
  parenchyma 
  of 
  the 
  cerebrum, 
  

   and 
  carry 
  on 
  their 
  internal 
  surface 
  a 
  flat 
  disk, 
  which, 
  with 
  

   those 
  of 
  the 
  contiguous 
  cells, 
  form 
  a 
  membrane 
  which 
  lines 
  

   the 
  ventricle 
  and 
  in 
  some 
  places 
  shows 
  the 
  remains 
  of 
  cilia. 
  

  

  The 
  most 
  internal 
  of 
  the 
  four 
  layers 
  of 
  the 
  parenchyma 
  of 
  

   the 
  cerebrum 
  consists 
  of 
  cells 
  which 
  occupy 
  about 
  one 
  quarter 
  

   (more 
  or 
  less) 
  of 
  the 
  thickness 
  of 
  the 
  cerebral 
  wall. 
  

  

  External 
  to 
  these 
  cells 
  the 
  second 
  layer, 
  composed 
  of 
  a 
  

   finely 
  granular 
  neuroglia, 
  occupies 
  rather 
  more 
  than 
  one 
  

   fourth 
  of 
  the 
  total 
  thickness. 
  

  

  The 
  third 
  layer 
  consists 
  of 
  cells 
  larger 
  but 
  much 
  less 
  nume- 
  

   rous 
  than 
  those 
  of 
  the 
  first 
  layer. 
  

  

  The 
  fourth 
  or 
  external 
  layer 
  is 
  occupied 
  by 
  a 
  zone 
  of 
  granu- 
  

   lar 
  neuroglia, 
  which 
  extends 
  to 
  nearly 
  half 
  of 
  the 
  total 
  thick- 
  

   ness, 
  filling 
  the 
  space 
  between 
  the 
  third 
  layer 
  and 
  the 
  outer 
  

   edge. 
  

  

  