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  ANNUAL 
  EEPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  The 
  association 
  system 
  gives 
  limitless 
  possibilities 
  for 
  varying 
  the 
  

   response 
  to 
  a 
  stimulus, 
  for 
  the 
  single 
  sensory 
  impulse 
  can 
  be 
  distrib- 
  

   uted 
  to 
  many 
  motor 
  neurons 
  (D). 
  

  

  The 
  conception 
  outlined 
  above 
  of 
  the 
  anatomical 
  basis 
  of 
  animal 
  

   behavior 
  is 
  the 
  foundation 
  of 
  most 
  modern 
  ideas 
  as 
  to 
  the 
  nature 
  of 
  

  

  MNv 
  

  

  ACl 
  

  

  Fig., 
  1. 
  — 
  Diagrams 
  showing 
  the 
  evolution 
  of 
  the 
  reflex 
  mechanism 
  of 
  the 
  nervous 
  

  

  systemi 
  

  

  A, 
  tbeoi'etically 
  primitive 
  condition 
  in 
  which 
  a 
  sense 
  cell 
  (SCh) 
  of 
  the 
  ectoderm 
  

   (Eot) 
  sends 
  an 
  axon 
  process 
  or 
  nerve 
  (Nv), 
  to 
  a 
  muscle 
  fiber 
  (Mel). 
  The 
  stimula- 
  

   tion 
  is 
  here 
  direct 
  from 
  the 
  receptor 
  {SCI) 
  to 
  the 
  efEector 
  (Mel), 
  and 
  the 
  action 
  of 
  

   the 
  latter 
  is 
  correspondingly 
  limited. 
  

  

  B, 
  the 
  primary 
  sense 
  cell 
  (SCh) 
  removed 
  from 
  the 
  ectoderm 
  to 
  the 
  interior 
  of 
  the 
  

   body, 
  as 
  indicated 
  by 
  the 
  arrow, 
  where 
  it 
  becomes 
  ii 
  motor 
  cell 
  (MCI) 
  of 
  the 
  cen- 
  

   tral 
  nervous 
  system, 
  and 
  its 
  axon 
  going 
  to 
  the 
  muscle 
  (Mel) 
  becomes 
  a 
  motor 
  nerve 
  

   VJfJVv). 
  The 
  stimulus 
  is 
  here 
  received 
  indirectly 
  by 
  the 
  motor 
  nerve 
  from 
  another 
  

   sense 
  cell 
  (SCI.) 
  of 
  the 
  ectoderm, 
  which 
  transmits 
  its 
  stimulus 
  through 
  its 
  axon 
  

   (SNv) 
  to 
  the 
  motor 
  nerve 
  by 
  way 
  of 
  a 
  system 
  of 
  interlacing 
  root 
  branches 
  from 
  the 
  

   two 
  nerves, 
  forming 
  a 
  synapss 
  (Syn). 
  A 
  simple 
  synaptic 
  sjstem 
  of 
  this 
  kind 
  is 
  

   characteristic 
  of 
  the 
  worms. 
  

  

  C, 
  the 
  synaptic 
  nervous 
  system 
  of 
  insects 
  and 
  of 
  all 
  higher 
  animals 
  is 
  compli- 
  

   cated 
  by 
  the 
  interpolation 
  of 
  an 
  association 
  neuron 
  between 
  the 
  sensory 
  neuron 
  

   and 
  the 
  motor 
  neuron 
  ; 
  the 
  association, 
  or 
  internuncial, 
  neuron 
  consists 
  of 
  a 
  cell 
  

   (ACl) 
  with 
  branches 
  making 
  a 
  two-way 
  connection 
  with 
  the 
  roots 
  of 
  the 
  motor 
  

   nerve 
  (MNv) 
  and 
  those 
  of 
  the 
  sensory 
  nerve 
  (SNv). 
  

  

  D, 
  showing 
  possibilities 
  of 
  development 
  and 
  resulting 
  complexity 
  of 
  action 
  in 
  a 
  

   central 
  nervous 
  system 
  with 
  association 
  neurons, 
  where, 
  by 
  the 
  multiplication 
  of 
  

   nerve 
  associations 
  in 
  the 
  synapse, 
  a 
  sensory 
  impulse 
  can 
  be 
  distributed 
  to 
  various 
  

   effectors. 
  

  

  the 
  acts 
  of 
  living 
  creatures 
  called 
  reflexes 
  and 
  instincts. 
  An 
  instinct 
  

   is 
  but 
  a 
  series 
  of 
  correlated 
  reflexes, 
  and 
  in 
  the 
  insects 
  as 
  a 
  class 
  

   instinct 
  has 
  been 
  carried 
  to 
  its 
  highest 
  point 
  of 
  perfection. 
  Reflex 
  

   acts 
  performed 
  alike 
  by 
  all 
  members 
  of 
  a 
  species 
  in 
  response 
  to 
  a 
  

   given 
  external 
  stimulus 
  are 
  often 
  called 
  in 
  general 
  tropistns; 
  but 
  the 
  

   term 
  "tropism" 
  was 
  introduced 
  by 
  Loeb 
  into 
  the 
  study 
  of 
  animal 
  

  

  