﻿Nervous 
  System 
  o/'Ceratodus 
  Forsteri. 
  159 
  

  

  dorsal 
  side 
  the 
  cranial 
  cavity 
  is 
  much 
  larger, 
  so 
  that 
  there 
  is 
  

   a 
  considerable 
  space 
  between 
  the 
  walls 
  of 
  the 
  cranium 
  and 
  

   the 
  surface 
  of 
  the 
  cerebrospinal 
  centres 
  ; 
  this 
  is 
  filled 
  by 
  a 
  

   coarse 
  and 
  loose 
  network 
  of 
  connective 
  tissue, 
  the 
  trabecule 
  

   ot 
  which 
  cany 
  vessels 
  and 
  capillaries 
  ; 
  this 
  arrangement 
  re- 
  

   sembles 
  that 
  of 
  Protopterus, 
  and, 
  as 
  Wiedersheim 
  * 
  remarks 
  

   may 
  possibly 
  serve 
  to 
  secrete 
  the 
  subarachnoid 
  fluid. 
  

  

  A 
  thick 
  membrane 
  of 
  great 
  toughness 
  encloses 
  the 
  brain 
  

   and 
  spinal 
  cord 
  ; 
  but 
  towards 
  the 
  posterior 
  end 
  of 
  the 
  latter 
  it 
  

   becomes 
  thinner 
  and 
  more 
  membranous. 
  

  

  _ 
  Two 
  glandular 
  bodies 
  are 
  developed 
  in 
  connexion 
  with 
  this 
  

   pia 
  mater, 
  as 
  it 
  may 
  be 
  called, 
  it 
  evidently 
  corresponding 
  to 
  

   that 
  structure, 
  at 
  least 
  in 
  position. 
  The 
  posterior 
  body, 
  Sf 
  a 
  

   spongy 
  character, 
  forms 
  a 
  cover 
  to 
  the 
  sinus 
  rhomboidalis, 
  or 
  

   fourth 
  ventricle, 
  resembling 
  that 
  of 
  Petromyzon 
  as 
  described 
  

   by 
  Ahlborn 
  f. 
  It 
  closes 
  the 
  whole 
  ventricle 
  with 
  the 
  excep- 
  

   tion 
  of 
  a 
  small 
  opening 
  at 
  the 
  posterior 
  end, 
  in 
  this 
  respect 
  

   resembling 
  the 
  corresponding 
  structure 
  in 
  Plagiostomata 
  • 
  its 
  

   structure 
  is 
  not 
  so 
  much 
  vascular 
  as 
  of 
  a 
  glandular 
  nature 
  

   being 
  composed 
  of 
  a 
  congeries 
  of 
  tubules 
  which 
  appear 
  to 
  

   connect 
  an 
  upper 
  and 
  lower 
  plate 
  ; 
  externally 
  the 
  whole 
  is 
  

   connected 
  with 
  and 
  passes 
  into 
  the 
  tough 
  pia 
  mater 
  enveloping 
  

   the 
  medulla 
  oblongata, 
  which 
  is 
  continuous 
  with 
  the 
  mem! 
  

   brane 
  that 
  encloses 
  the 
  remainder 
  of 
  the 
  brain. 
  

   < 
  The 
  anterior 
  body, 
  which 
  Beauregard 
  J 
  mistook 
  for 
  nervous 
  

   tissue, 
  resembles 
  the 
  cover 
  of 
  the 
  fourth 
  ventricle 
  in 
  structure 
  

   inasmuch 
  as 
  its 
  tissue 
  is 
  made 
  up 
  of 
  tubules, 
  but 
  differs 
  in 
  the 
  

   fact 
  that 
  these 
  tubules 
  are 
  more 
  contorted 
  ; 
  it 
  enters 
  the 
  third 
  

   ventricle, 
  forming 
  a 
  spongy 
  mass, 
  termed 
  by 
  Prof. 
  Huxley 
  6 
  

   the 
  tela 
  vasculosa, 
  but 
  in 
  which 
  I 
  failed 
  to 
  discover 
  any 
  

   capillaries, 
  its 
  structure 
  being 
  entirely 
  glandular. 
  

  

  This 
  spongy 
  mass 
  (PI. 
  IX. 
  fig. 
  5) 
  extends 
  to 
  the 
  anterior 
  

   end 
  of 
  the 
  cerebrum, 
  separating 
  the 
  dorsal 
  wall 
  of 
  one 
  side 
  

   from 
  that 
  of 
  the 
  other, 
  so 
  that 
  the 
  lateral 
  ventricles 
  are 
  not 
  

   closed 
  in 
  above 
  by 
  nervous 
  tissue. 
  At 
  the 
  anterior 
  end 
  of 
  the 
  

   cerebrum 
  the 
  lower 
  surface 
  of 
  this 
  structure 
  is 
  connected 
  by 
  a 
  

   lamina 
  of 
  connective 
  tissue 
  which 
  passes 
  between 
  the 
  inferior 
  

   internal 
  fold 
  of 
  the 
  cerebrum 
  to 
  join 
  the 
  general 
  covering 
  f 
  

   pia 
  mater 
  at 
  the 
  inferior 
  surface 
  of 
  the 
  brain. 
  This 
  fold 
  is 
  

  

  Z^t^tt^™- 
  ^'^ 
  188 
  °' 
  *■ 
  73 
  - 
  A1S0 
  J 
  -i.sche 
  

   t 
  Zeitechr. 
  f. 
  ^iss. 
  Zool. 
  Bd. 
  xxxix. 
  (1883). 
  

   X 
  L. 
  c. 
  p. 
  232. 
  

   § 
  L. 
  c. 
  p. 
  29. 
  

  

  12* 
  

  

  