﻿Nervous 
  System 
  of 
  Ccratodus 
  Forsteri. 
  171 
  

  

  layer 
  in 
  tlie 
  dorsal 
  walls 
  ; 
  the 
  other, 
  made 
  up 
  of 
  the 
  ordinary 
  

   neuroglia, 
  extends 
  to 
  the 
  outer 
  surface 
  ; 
  the 
  line 
  of 
  demarca- 
  

   tion 
  between 
  these 
  two 
  layers 
  is 
  irregular 
  ; 
  sometimes 
  isolated 
  

   portions 
  of 
  neuroglia 
  penetrate 
  into 
  the 
  stratum 
  of 
  cells, 
  and 
  

   on 
  the 
  other 
  hand 
  groups 
  of 
  cells 
  project 
  into 
  the 
  neuroglia 
  ; 
  

   the 
  cells 
  are 
  arranged 
  more 
  loosely 
  in 
  some 
  parts, 
  in 
  other 
  places 
  

   they 
  appear 
  to 
  be 
  more 
  closely 
  packed 
  together. 
  The 
  stratum 
  

   of 
  neuroglia 
  varies 
  in 
  width 
  from 
  one 
  third 
  to 
  nearly 
  one 
  half 
  

   of 
  the 
  thickness 
  of 
  the 
  walls, 
  the 
  remainder 
  of 
  the 
  space 
  

   being 
  occupied 
  by 
  the 
  cells. 
  

  

  Lobi 
  olfactorii. 
  

  

  Tn 
  the 
  structure 
  of 
  the 
  olfactory 
  lobes 
  (PI. 
  XITT. 
  fig. 
  21) 
  

   one 
  may 
  distinguish 
  four 
  layers 
  interposed 
  between 
  the 
  endo- 
  

   thelium, 
  which 
  lines 
  the 
  very 
  large 
  ventricles 
  and 
  which 
  is 
  con- 
  

   tinuous 
  with 
  that 
  of 
  the 
  cerebrum, 
  and 
  the 
  external 
  surface. 
  

  

  Internally 
  is 
  seen 
  a 
  layer 
  of 
  cells 
  resembling 
  in 
  every 
  respect 
  

   those 
  of 
  the 
  inner 
  layer 
  of 
  the 
  cerebrum, 
  witli 
  which 
  they 
  are 
  

   continuous 
  ; 
  their 
  measurements 
  also 
  are 
  very 
  much 
  about 
  

   the 
  same; 
  they 
  vary 
  in 
  diameter 
  from 
  10 
  /a 
  to 
  0145 
  millim. 
  

  

  External 
  to 
  this 
  layer 
  of 
  cells 
  is 
  a 
  space 
  occupied 
  by 
  a 
  

   finely 
  granular 
  neuroglia, 
  in 
  which 
  a 
  fibrillar 
  network 
  is 
  found 
  

   having 
  a 
  general 
  longitudinal 
  direction. 
  Longitudinal 
  fibres 
  

   of 
  a 
  larger 
  size 
  occur 
  scattered 
  through 
  this 
  layer 
  ; 
  these 
  are 
  

   in 
  some 
  places 
  collected 
  into 
  a 
  bundle 
  immediately 
  internal 
  

   to 
  the 
  outer 
  layer 
  ; 
  these 
  fibres 
  have 
  fusiform 
  cells 
  developed 
  

   along 
  their 
  course, 
  which 
  cells 
  are 
  of 
  a 
  larger 
  size 
  than 
  those 
  

   of 
  the 
  inner 
  layer; 
  their 
  size 
  varies 
  from 
  0*0259 
  millim. 
  long 
  

   and 
  0'0125 
  broad 
  to 
  12 
  /j, 
  long 
  and 
  0*0098 
  millim. 
  broad 
  ; 
  they 
  

   follow 
  the 
  curves 
  of 
  the 
  ventricle, 
  so 
  that 
  they 
  sometimes 
  

   become 
  visible 
  in 
  a 
  transverse 
  section, 
  as 
  in 
  the 
  figure. 
  

  

  As 
  Fritsch 
  has 
  justly 
  remarked, 
  the 
  glomeruli 
  are 
  as 
  cha- 
  

   racteristic 
  of 
  the 
  olfactory 
  lobes 
  as 
  the 
  Purkinje 
  cells 
  are 
  of 
  the 
  

   cerebellum 
  ; 
  they 
  occupy 
  the 
  external 
  part 
  of 
  the 
  lobe 
  in 
  

   Ceratodus, 
  as 
  in 
  Plagiostomata 
  and 
  Teleostei, 
  and 
  they 
  present 
  

   the 
  same 
  sort 
  of 
  structure. 
  The 
  fibrils 
  which 
  proceed 
  from 
  

   them 
  constitute 
  an 
  external 
  layer, 
  which 
  ultimately 
  contributes 
  

   to 
  form 
  the 
  olfactory 
  nerves. 
  

  

  At 
  the 
  anterior 
  end 
  of 
  the 
  ventricle 
  there 
  is 
  a 
  corresponding 
  

   arrangement 
  of 
  the 
  layers 
  from 
  behind 
  forward, 
  so 
  that 
  they 
  

   surround 
  the 
  ventricle 
  in 
  concentric 
  strata. 
  Anteriorly 
  the 
  

   fibrils 
  of 
  the 
  second 
  layer 
  form 
  a 
  distinct 
  bundle, 
  the 
  outer 
  

   fibrils 
  of 
  which 
  go 
  to 
  a 
  group 
  of 
  glomeruli 
  situated 
  in 
  the 
  

   centre 
  of 
  the 
  anterior 
  wall 
  of 
  the 
  ventricle, 
  while 
  the 
  inner 
  

   fibrils 
  pass 
  over 
  to 
  the 
  other 
  side. 
  

  

  