﻿Nervous 
  System 
  c/Ceratodus 
  Forsteri. 
  175 
  

  

  Four 
  layers 
  are 
  to 
  be 
  distinguished 
  from 
  within 
  outward 
  

   viz. 
  the 
  fibrous, 
  the 
  granular, 
  intermediate, 
  and 
  molecular. 
  

  

  The 
  fibrous 
  layer 
  consists 
  of 
  nerve-fibres 
  situated 
  close 
  

   beneath 
  the 
  endothelium 
  ; 
  they 
  run 
  obliquely 
  backward 
  and 
  

   eventually 
  form 
  the 
  crura 
  cerebelli 
  ad 
  meclullam 
  ; 
  some 
  fibres 
  

   also 
  of 
  this 
  layer 
  pass 
  through 
  the 
  granular 
  layer 
  ; 
  they 
  come 
  

   up 
  from 
  the 
  internal 
  tuberosity 
  on 
  each 
  side, 
  then 
  turn 
  down 
  

   along 
  the 
  main 
  lamina 
  of 
  the 
  cerebellum. 
  

  

  The 
  granular 
  layer 
  occupies 
  the 
  space 
  between 
  the 
  fibrous 
  

   layer 
  and 
  the 
  intermediate 
  ; 
  it 
  consists 
  of 
  spherical 
  cells 
  which 
  

   are 
  imbedded 
  in 
  a 
  loose 
  network 
  of 
  fibrillar, 
  to 
  which 
  they 
  

   give 
  off 
  processes. 
  

  

  The 
  intermediate 
  layer, 
  which 
  comprises 
  the 
  Purkinje 
  cells, 
  

   is 
  conterminous 
  with 
  the 
  molecular 
  layer, 
  to 
  which 
  it 
  might 
  

   appear 
  to 
  belong 
  ; 
  but 
  as 
  these 
  cells 
  send 
  processes 
  not 
  only 
  

   into 
  the 
  molecular 
  but 
  also 
  into 
  the 
  granular 
  layer, 
  they 
  

   might 
  be 
  said 
  to 
  belong 
  to 
  both 
  ; 
  therefore 
  it 
  is 
  perhaps 
  

   better 
  to 
  make 
  them 
  into 
  a 
  separate 
  layer, 
  as 
  they 
  are 
  struc- 
  

   tures 
  characteristic 
  of 
  the 
  cerebellum. 
  

  

  These 
  Purkinje 
  cells, 
  as 
  seen 
  in 
  a 
  horizontal 
  section, 
  have 
  

   a 
  very 
  irregular 
  shape 
  and 
  give 
  off 
  numerous 
  processes 
  ; 
  some 
  

   have 
  five, 
  four 
  of 
  which 
  go 
  more 
  or 
  less 
  obliquely 
  into 
  the 
  

   molecular 
  layer, 
  sometimes 
  dividing 
  dichotomously 
  ; 
  the 
  fifth 
  

   in 
  this 
  specimen 
  goes 
  directly 
  into 
  the 
  granular 
  layer. 
  In 
  

   other 
  cases 
  these 
  cells 
  present 
  a 
  quadrangular 
  form 
  and 
  

   show 
  four 
  processes, 
  the 
  long 
  axis 
  being 
  placed 
  radially 
  or 
  at 
  

   right 
  angles 
  to 
  the 
  surface 
  ; 
  in 
  these 
  forms 
  two 
  processes 
  pass 
  

   into 
  the 
  molecular 
  layer 
  and 
  two 
  into 
  the 
  granular 
  ; 
  in 
  one 
  

   case 
  one 
  of 
  the 
  latter 
  processes 
  could 
  be 
  traced 
  into 
  the 
  

   bundles 
  of 
  the 
  fibrous 
  layer. 
  Other 
  forms 
  also 
  are 
  found, 
  

   some 
  with 
  three 
  processes 
  and 
  others 
  only 
  with 
  two, 
  the 
  latter 
  

   being 
  fusiform 
  in 
  shape. 
  The 
  form 
  of 
  these 
  cells 
  in 
  Gera- 
  

   todus 
  does 
  not 
  appear 
  to 
  be 
  fixed, 
  since 
  they 
  vary 
  so 
  much 
  

   more 
  than 
  is 
  the 
  case 
  in 
  Plagiostomata 
  and 
  Teleostei. 
  

  

  In 
  longitudinal 
  and 
  vertical 
  sections 
  these 
  cells 
  are 
  more 
  

   often 
  fusiform 
  or 
  take 
  on 
  the 
  shape 
  of 
  an 
  isosceles 
  triangle, 
  

   which 
  shows 
  that 
  they 
  are 
  more 
  compressed 
  in 
  this 
  direction; 
  

   in 
  the 
  latter 
  case 
  the 
  apex 
  sends 
  a 
  process 
  into 
  the 
  granular 
  

   and 
  the 
  base 
  sends 
  two 
  processes 
  into 
  the 
  molecular 
  layer. 
  

  

  At 
  the 
  anterior 
  end 
  of 
  the 
  cerebellum 
  the 
  molecular 
  layer 
  

   forms 
  a 
  mere 
  cap 
  on 
  the 
  summit 
  of 
  this 
  segment 
  of 
  the 
  brain 
  ; 
  

   but 
  on 
  proceeding 
  further 
  back 
  it 
  is 
  seen 
  to 
  extend 
  laterally 
  

   over 
  the 
  crura, 
  carrying 
  the 
  other 
  layers 
  with 
  it, 
  but 
  not 
  ex- 
  

   tending 
  so 
  far 
  over 
  the 
  restiform 
  bodies 
  as 
  in 
  some 
  Plagio- 
  

   stomata 
  ; 
  but 
  where 
  the 
  latter 
  become 
  clear 
  of 
  the 
  cerebellum 
  

  

  13* 
  

  

  