﻿168 
  Mr. 
  A. 
  Sanders 
  on 
  the 
  

  

  increase 
  of 
  the 
  parenchyma, 
  but 
  principally 
  to 
  the 
  expansion 
  

   of 
  the 
  lumen. 
  

  

  The 
  posterior 
  part 
  of 
  the 
  floor 
  of 
  the 
  fourth 
  ventricle 
  pre- 
  

   sents 
  the 
  same 
  form 
  as 
  that 
  of 
  the 
  anterior 
  part 
  of 
  the 
  funnel- 
  

   shaped 
  termination 
  of 
  the 
  central 
  canal, 
  except 
  that 
  at 
  the 
  

   lateral 
  part 
  the 
  junction 
  of 
  the 
  floor 
  with 
  the 
  walls 
  is 
  more 
  

   rounded 
  and 
  less 
  sharp 
  than 
  further 
  back. 
  

  

  The 
  two 
  multiaxial 
  fibres, 
  a 
  description 
  of 
  which 
  will 
  be 
  

   given 
  presently, 
  form 
  a 
  couple 
  of 
  ridges 
  projecting 
  into 
  the 
  

   floor: 
  between 
  the 
  two 
  there 
  is 
  a 
  depression, 
  and 
  again 
  on 
  

   each 
  side 
  another 
  leading 
  to 
  the 
  lateral 
  parietes, 
  which 
  are 
  

   here 
  perpendicular. 
  Further 
  forward 
  the 
  two 
  prominences 
  

   become 
  higher 
  and 
  more 
  marked, 
  and 
  contain 
  not 
  only 
  the 
  

   multiaxial 
  fibres 
  but 
  also 
  some 
  of 
  the 
  fibres 
  of 
  the 
  ventral 
  

   columns 
  ; 
  they 
  correspond 
  to 
  the 
  longitudinal 
  ridges 
  which 
  

   are 
  seen 
  on 
  the 
  floor 
  of 
  the 
  sinus 
  rhomboidalis 
  in 
  the 
  Plagio- 
  

   stomata. 
  

  

  In 
  the 
  wall 
  of 
  the 
  ventricle 
  there 
  now 
  appear 
  two 
  tubero- 
  

   sities, 
  one 
  forming 
  a 
  club-shaped 
  dorsal 
  termination, 
  like 
  a 
  

   coping 
  to 
  a 
  wall, 
  which 
  belongs 
  to 
  the 
  trifacial 
  (PI. 
  X. 
  tig. 
  13, 
  

   v. 
  tri.), 
  the 
  other 
  situated 
  lower 
  down, 
  just 
  above 
  the 
  junction 
  

   with 
  the 
  floor 
  ; 
  towards 
  the 
  posterior 
  end 
  this 
  is 
  a 
  long, 
  low, 
  

   flat 
  swelling 
  (PI. 
  XII. 
  fig. 
  14, 
  ff.), 
  but 
  further 
  forward 
  it 
  

   gradually 
  sinks 
  down 
  until 
  it 
  becomes 
  a 
  small 
  rounded 
  

   eminence 
  ; 
  this 
  corresponds 
  to 
  the 
  bead-like 
  tubercles 
  on 
  the 
  

   flcor 
  of 
  this 
  ventricle 
  in 
  Seyllium, 
  although 
  it 
  is 
  here 
  a 
  ridge 
  

   and 
  not 
  discrete 
  formations. 
  

  

  The 
  fourth 
  ventricle 
  passes 
  beneath 
  the 
  cerebellum 
  without 
  

   any 
  diminution 
  in 
  size 
  (PI. 
  X. 
  fig. 
  11) 
  ; 
  on 
  the 
  contrary, 
  it 
  

   gradually 
  widens 
  from 
  behind 
  forward 
  ; 
  at 
  this 
  part 
  a 
  low 
  

   broad 
  tuberosity 
  belonging 
  to 
  the 
  trifacial 
  is 
  visible. 
  Towards 
  

   the 
  anterior 
  end 
  of 
  the 
  cerebellum 
  (PI. 
  XIII. 
  fig. 
  10) 
  this 
  

   swelling 
  disappears 
  and 
  the 
  depression 
  between 
  the 
  ventral 
  

   columns 
  loses 
  its 
  gentle 
  contour 
  and 
  becomes 
  a 
  triangular 
  

   trough, 
  with 
  the 
  sides 
  meeting 
  at 
  an 
  angle 
  at 
  the 
  bottom 
  ; 
  

   here 
  the 
  lumen 
  is 
  more 
  contracted, 
  and 
  its 
  space 
  is 
  diminished 
  

   by 
  the 
  projection 
  of 
  a 
  process 
  from 
  the 
  roof 
  of 
  the 
  cerebellum. 
  

  

  In 
  the 
  ventricle 
  (PI. 
  XI. 
  fig. 
  9) 
  of 
  the 
  mesencephalon 
  or 
  

   aqueduct 
  of 
  Sylvius 
  the 
  floor 
  gradually 
  deepens 
  into 
  a 
  narrow 
  

   fissure, 
  which 
  eventually 
  opens 
  into 
  the 
  infundibulum 
  ; 
  in 
  

   addition 
  to 
  the 
  fissure 
  in 
  the 
  floor 
  of 
  the 
  optic 
  lobe 
  there 
  is 
  

   one 
  in 
  the 
  roof 
  which 
  extends 
  for 
  the 
  whole 
  length 
  of 
  the 
  

   lobe. 
  

  

  In 
  the 
  region 
  of 
  the 
  anterior 
  end 
  of 
  the 
  optic 
  lobe 
  an 
  offshoot 
  

   of 
  the 
  fissure 
  on 
  the 
  floor 
  is 
  observed 
  to 
  be 
  directed 
  backward 
  ; 
  

   this 
  ean 
  be 
  traced 
  beneath 
  the 
  floor 
  of 
  the 
  optic 
  ventricle 
  for 
  

  

  