﻿130 
  

  

  Prof. 
  T. 
  H. 
  Huxley 
  on 
  the 
  Brain 
  [Dec. 
  17, 
  

  

  dura 
  and 
  the 
  optic 
  nerve, 
  in 
  Amphioxus, 
  are 
  represented 
  by 
  the 
  third, 
  

   fourth, 
  fifth, 
  and 
  sixth 
  pairs 
  of 
  cranial 
  nerves 
  of 
  the 
  higher 
  Vertebrata. 
  

   The 
  nerve 
  a, 
  in 
  fact, 
  has 
  the 
  characteristic 
  course 
  and 
  distribution 
  of 
  

   the 
  orbito-nasal 
  division 
  of 
  the 
  trigeminal 
  ; 
  while, 
  without 
  at 
  present 
  

   drawing 
  a 
  closer 
  parallel, 
  it 
  is 
  easy 
  to 
  see 
  that 
  the 
  nerves 
  b, 
  c, 
  d, 
  e,f, 
  and 
  

   g, 
  with 
  their 
  respective 
  myotomes, 
  supply 
  the 
  requisite 
  materials 
  for 
  

   metamorphosis 
  into 
  the 
  oculomotor, 
  pathetic, 
  trigeminal, 
  and 
  abducens 
  

   nerves, 
  with 
  the 
  muscles 
  of 
  the 
  eye 
  and 
  of 
  the 
  jaws, 
  in 
  the 
  more 
  differen- 
  

   tiated 
  vertebrate 
  types. 
  

  

  Thus, 
  that 
  part 
  of 
  the 
  cerebro-spinal 
  axis 
  of 
  Amphioxus 
  which 
  lies 
  

   in 
  front 
  of 
  the 
  seventh 
  myotome 
  answers 
  to 
  the 
  praeauditory 
  part 
  of 
  the 
  

   brain 
  in 
  the 
  higher 
  Vertebrata, 
  and 
  the 
  corresponding 
  part 
  of 
  the 
  head 
  to 
  

   the 
  trabecular 
  region 
  of 
  the 
  skull 
  in 
  them. 
  On 
  the 
  other 
  hand, 
  from 
  

   the 
  seventh 
  myotome 
  backwards, 
  a 
  certain 
  number 
  of 
  segments 
  answer 
  

   to 
  the 
  postauditory, 
  or 
  parachordal, 
  region 
  of 
  the 
  skull 
  of 
  the 
  higher 
  

   Vertebrata. 
  

  

  The 
  answer 
  to 
  the 
  question, 
  how 
  many 
  ? 
  involves 
  sundry 
  considera- 
  

   tions. 
  It 
  must 
  be 
  recollected 
  that 
  though 
  the 
  branchial 
  chamber 
  of 
  

   Amphioxus 
  is 
  the 
  homologue 
  of 
  the 
  branchial 
  chamber 
  of 
  other 
  Verte- 
  

   brata, 
  it 
  does 
  not 
  necessarily 
  follow 
  that 
  the 
  imperfect 
  branchial 
  skeleton 
  

   of 
  Amphioxus 
  corresponds 
  with 
  their 
  branchial 
  skeleton. 
  The 
  branchial 
  

   skeleton 
  of 
  the 
  higher 
  Vertebrata 
  consists 
  of 
  cartilaginous 
  rods, 
  which 
  

   seem 
  to 
  be 
  developed 
  in 
  the 
  somatopleure, 
  and 
  to 
  be 
  homologous 
  with 
  the 
  

   ribs, 
  while 
  the 
  branchial 
  skeleton 
  of 
  Amphioxus 
  consists 
  of 
  fibrous 
  bands 
  

   apparently 
  developed 
  in 
  the 
  splanchnopleure. 
  

  

  The 
  branchial 
  arches 
  of 
  the 
  higher 
  Vertebrata, 
  in 
  accordance 
  with 
  their 
  

   essentially 
  costal 
  nature, 
  receive 
  their 
  innervation 
  from 
  the 
  glosso- 
  

   pharyngeal 
  and 
  pneumogastric 
  nerves, 
  which 
  are 
  homologues 
  of 
  spinal 
  

   nerves 
  ; 
  and, 
  in 
  seeking 
  for 
  the 
  posterior 
  limits 
  of 
  that 
  region 
  in 
  Amphioxus 
  

   which 
  corresponds 
  with 
  the 
  skull 
  and 
  brain 
  in 
  other 
  Vertebrates, 
  we 
  must 
  

   only 
  take 
  into 
  account 
  as 
  many 
  pairs 
  of 
  those 
  nerves 
  which 
  arise 
  from 
  the 
  

   cerebro-spinal 
  axis 
  as 
  we 
  know 
  are, 
  in 
  the 
  Vertebrata 
  next 
  above 
  Amphi- 
  

   oxus, 
  devoted 
  to 
  the 
  branchial 
  arches. 
  In 
  none 
  of 
  these 
  are 
  there 
  more 
  

   than 
  seven 
  pairs 
  of 
  branchial 
  arches 
  ; 
  so 
  tha,t 
  not 
  more 
  than 
  eight 
  myotomes 
  

   (and 
  consequently 
  'protovertebrae) 
  of 
  Amphioxus, 
  in 
  addition 
  to 
  those 
  

   already 
  mentioned, 
  can 
  be 
  reckoned 
  as 
  the 
  equivalents 
  of 
  the 
  parachordal 
  

   region 
  of 
  the 
  skull 
  in 
  the 
  higher 
  Vertebrates. 
  Thus 
  it 
  would 
  appear 
  that 
  

   the 
  cranium 
  of 
  the 
  latter 
  is 
  represented 
  by 
  those 
  segments 
  of 
  the 
  body 
  

   of 
  Amphioxus 
  which 
  lie 
  in 
  front 
  of 
  the 
  fifteenth, 
  counting 
  from 
  before 
  

   backwards, 
  and 
  that 
  their 
  cranial 
  nerves 
  are 
  represented 
  by 
  the 
  corre- 
  

   sponding 
  anterior 
  pairs 
  of 
  nerves 
  in 
  Amphioxus. 
  

  

  In 
  all 
  Vertebrata 
  above 
  Amphioxus 
  the 
  nerves 
  which 
  answer 
  to 
  the 
  

   seven 
  posterior 
  pairs 
  in 
  Amphioxus 
  unite 
  into 
  one 
  or 
  two 
  trunks 
  on 
  

   each 
  side, 
  and 
  give 
  rise 
  to 
  the 
  nerves 
  called 
  pneumogastric 
  and 
  glosso- 
  

   pharyngeal 
  ; 
  and, 
  as 
  these 
  pass 
  out 
  of 
  the 
  skull 
  in 
  front 
  of 
  the 
  occipital 
  

  

  