﻿[47] 
  DEVELOPMENT 
  OF 
  THE 
  CETACEA. 
  473 
  

  

  expansion 
  of 
  the 
  myelon 
  at 
  the 
  beginning 
  of 
  the 
  caudal 
  region, 
  whence 
  

   it 
  is 
  continued, 
  gradually 
  diminishing 
  to 
  a 
  point 
  along 
  the 
  neural 
  canal 
  

   in 
  the 
  upper 
  lobe 
  of 
  the 
  tail. 
  In 
  some 
  bony 
  fishes 
  (trout, 
  Blenny) 
  the 
  

   caudal 
  ganglion 
  is 
  not 
  quite 
  terminal, 
  and 
  is 
  less 
  marked 
  than 
  in 
  the 
  

   cod 
  and 
  bream, 
  in 
  which 
  it 
  is 
  of 
  a 
  hard 
  texture, 
  but 
  receives 
  the 
  last 
  

   pair 
  of 
  spinal 
  nerves.* 
  

  

  A 
  little 
  further 
  on 
  the 
  same 
  author 
  states 
  that 
  in 
  Mola 
  the 
  myelon 
  

   " 
  has 
  shrunk 
  into 
  a 
  short, 
  conical, 
  and, 
  according 
  to 
  Arsaki,t 
  gaugliated 
  

   appendage 
  to 
  theencephalon. 
  Alike 
  singular 
  modification, 
  but 
  without 
  

   the 
  ganglionic 
  structure, 
  obtains 
  in 
  Tetrodon 
  and 
  Diodon, 
  in 
  a 
  species 
  of 
  

   which 
  latter 
  genus 
  I 
  found 
  the 
  myelon 
  only 
  four 
  lines 
  long 
  in 
  a 
  fish 
  of 
  

   7 
  inches 
  in 
  length 
  and 
  measuring 
  3 
  inches 
  across 
  the 
  head. 
  The 
  neural 
  

   canal 
  in 
  these 
  plectoguathic 
  fishes 
  is 
  chiefly 
  occupied 
  by 
  a 
  long 
  ' 
  cauda 
  

   equina.' 
  But, 
  insignificant 
  as 
  the 
  myelon 
  here 
  seems, 
  it 
  is 
  something 
  

   more 
  than 
  merely 
  unresolved 
  nerve 
  fibers 
  5 
  transverse 
  white 
  strise 
  are 
  

   discernible 
  in 
  it, 
  with 
  gray 
  matter, 
  showing 
  it 
  to 
  be 
  a 
  center 
  of 
  nervous 
  

   force, 
  not 
  a 
  mere 
  conductor. 
  In 
  the 
  Lophius 
  a 
  long 
  cauda 
  equina 
  partly 
  

   conceals 
  a 
  short 
  myelon, 
  which 
  terminates 
  in 
  a 
  point 
  about 
  the 
  twelfth 
  

   vertebra. 
  In 
  other 
  fishes 
  the 
  myelon 
  is 
  very 
  nearly 
  or 
  quite 
  co-exten- 
  

   sive 
  with 
  the 
  neural 
  canal, 
  and 
  there 
  is 
  no 
  cauda 
  equina 
  or 
  buudle 
  of 
  

   nerve 
  roots 
  in 
  the 
  canal 
  ; 
  a 
  tendinous 
  thread 
  sometimes 
  ties 
  the 
  terminal 
  

   ganglion 
  to 
  the 
  end 
  of 
  the 
  canal." 
  (Owen, 
  Anat. 
  Vertebrates, 
  I, 
  272.) 
  

  

  In 
  Gastrostomus 
  Bairdii, 
  a 
  fish 
  with 
  an 
  attenuated 
  flagelliform 
  tail, 
  

   the 
  medulla 
  spinalis 
  at 
  its 
  extreme 
  posterior 
  end 
  becomes 
  very 
  greatly 
  

   depressed 
  so 
  as 
  to 
  assume 
  in 
  sections 
  the 
  form 
  of 
  a 
  flattened 
  band 
  in 
  

   which 
  it 
  is 
  almost 
  impossible 
  to 
  discern 
  the 
  existence 
  of 
  a 
  central 
  canal. 
  

   In 
  very 
  young 
  eels 
  the 
  hinder 
  end 
  of 
  the 
  medulla 
  appears 
  to 
  be 
  con- 
  

   nected 
  with 
  a 
  globular 
  enlargement 
  which 
  is 
  quite 
  terminal 
  and 
  possibly 
  

   external 
  to 
  the 
  neural 
  canal. 
  

  

  Amongst 
  the 
  reptiles, 
  says 
  Owen, 
  " 
  With 
  the 
  exception 
  of 
  the 
  anu- 
  

   rous 
  Batrachia, 
  the 
  myelon 
  (spinal 
  chord) 
  is 
  continued 
  into 
  the 
  tail, 
  

   gradually 
  decreasing 
  to 
  a 
  point, 
  and 
  is 
  not 
  resolved 
  into 
  a 
  ' 
  cauda 
  

   equina.' 
  Such, 
  indeed, 
  is 
  its 
  condition 
  in 
  the 
  tadpole 
  state 
  of 
  the 
  frogs 
  

   and 
  toads 
  ; 
  but, 
  with 
  the 
  acquisition 
  of 
  the 
  mature 
  form, 
  the 
  myelon 
  

   shrinks 
  in 
  length 
  and 
  terminates 
  midway 
  between 
  the 
  fore 
  and 
  hind 
  

   limbs, 
  being 
  resolved 
  in 
  the 
  frog 
  into 
  the 
  three 
  pairs 
  of 
  nerves 
  which 
  

   form 
  the 
  sciatic, 
  and 
  into 
  a 
  few 
  filaments 
  passing 
  on 
  to 
  the 
  sacrum." 
  

   (Anat, 
  Vertebrates, 
  I, 
  295-G.) 
  

  

  The 
  development 
  of 
  the 
  Anura 
  therefore 
  confirms 
  the 
  rule, 
  which 
  was 
  

   laid 
  down 
  above, 
  as 
  to 
  the 
  genesis 
  of 
  a 
  cauda 
  equina. 
  In 
  them 
  the 
  

   whole 
  of 
  the 
  caudal 
  musculature 
  aborts, 
  together 
  with 
  the 
  caudal 
  end 
  

   of 
  the 
  myelon, 
  while 
  the 
  hind 
  limbs 
  attain 
  an 
  extraordinary 
  develop- 
  

   ment 
  and 
  specialization 
  of 
  the 
  muscular 
  system, 
  calling 
  for 
  an 
  extraor- 
  

   dinary 
  motor 
  nerve 
  supply 
  such 
  as 
  is 
  rarely 
  encountered 
  amongst 
  fishes, 
  

  

  " 
  Owen, 
  Anat. 
  Vertebrates, 
  I, 
  271. 
  

  

  tDe 
  Pisciimi 
  Cerebro 
  et 
  Medulla 
  Spinali, 
  4to, 
  1813. 
  

  

  