﻿MIND 
  OP 
  AN 
  INSECT 
  — 
  SNODGRASS 
  391 
  

  

  Now, 
  it 
  is 
  clear 
  that 
  this 
  diversity 
  of 
  fundamental 
  structure 
  gives 
  

   a 
  possibility 
  of 
  great 
  diversification 
  of 
  action, 
  because 
  each 
  of 
  the 
  

   three 
  elements 
  in 
  the 
  structural 
  series 
  can 
  be 
  endlessly 
  modified 
  and 
  

   multiplied. 
  In 
  the 
  first 
  place, 
  different 
  kinds 
  of 
  sense 
  organs 
  may 
  

   be 
  developed, 
  each 
  sensitized 
  to 
  one 
  particular 
  group 
  of 
  environ- 
  

   mental 
  stimuli, 
  thus 
  giving 
  the 
  animal 
  the 
  power 
  of 
  responding 
  sep- 
  

   arately 
  to 
  light, 
  heat, 
  odor, 
  touch, 
  etc. 
  In 
  the 
  second 
  place, 
  the 
  

   conducting 
  apparatus 
  can 
  be 
  so 
  modified 
  by 
  the 
  insertion 
  of 
  a 
  switch- 
  

   board 
  mechanism 
  into 
  its 
  course 
  that, 
  instead 
  of 
  transmitting 
  direct 
  

   from 
  receptor 
  to 
  effector, 
  it 
  can 
  send 
  the 
  stimulus 
  from 
  one 
  receptor 
  

   to 
  this 
  or 
  that 
  effector, 
  or 
  to 
  many 
  effectors 
  at 
  the 
  same 
  time. 
  

   Finally, 
  as 
  already 
  noted, 
  the 
  results 
  in 
  the 
  effector 
  apparatus 
  can 
  

   be 
  immensely 
  varied 
  by 
  multiplication 
  of 
  muscles 
  and 
  by 
  diversifi- 
  

   cation 
  in 
  the 
  connected 
  machinery 
  of 
  skeletal 
  parts. 
  Of 
  the 
  three 
  

   divisions 
  of 
  the 
  receiving-transmitting-operative 
  system, 
  the 
  second 
  

   is 
  of 
  particular 
  importance, 
  because, 
  with 
  the 
  other 
  two 
  factors 
  alike, 
  

   two 
  animals 
  may 
  act 
  quite 
  differently 
  under 
  the 
  effect 
  of 
  the 
  same 
  

   stimulus 
  through 
  some 
  slight 
  difference 
  in 
  the 
  nerve 
  connections 
  of 
  

   the 
  switch, 
  llie 
  particular 
  pattern 
  of 
  the 
  connections 
  determines 
  

   the 
  nature 
  of 
  the 
  reflex. 
  The 
  acts 
  of 
  the 
  animal 
  are 
  thus 
  results 
  of 
  

   its 
  specific 
  anatomical 
  structure; 
  and 
  physical 
  heredity, 
  therefore, 
  

   will 
  account 
  for 
  the 
  fact 
  that 
  all 
  individuals 
  of 
  a 
  species 
  respond 
  in 
  

   a 
  characteristic 
  manner 
  to 
  the 
  same 
  stimulus. 
  

  

  The 
  nervous 
  mechanism 
  of 
  response 
  to 
  stimuli, 
  as 
  understood 
  from 
  

   a 
  study 
  of 
  the 
  minute 
  structure 
  of 
  nerve 
  centers, 
  may 
  be 
  simply 
  

   explained 
  by 
  the 
  diagrams 
  in 
  Figure 
  1. 
  In 
  a 
  primitive 
  reflex 
  ap- 
  

   paratus 
  (A), 
  a 
  primary 
  sense 
  cell 
  (SCl-i^) 
  lying 
  in 
  or 
  beneath 
  the 
  skin 
  

   makes 
  a 
  direct 
  connection 
  with 
  a 
  muscle 
  cell 
  (Mel) 
  by 
  means 
  of 
  a 
  

   prolongation, 
  or 
  fiber 
  cailed 
  the 
  axon 
  {Nv), 
  from 
  its 
  inner 
  surface. 
  

   A 
  sensory 
  cell, 
  together 
  with 
  its 
  axon 
  and 
  any 
  other 
  branches, 
  con- 
  

   stitutes 
  a 
  neuron. 
  In 
  animals 
  having 
  a 
  central 
  nervous 
  system 
  (B), 
  

   the 
  primary 
  sensory 
  cell 
  (jSCI^) 
  is 
  buried 
  within 
  the 
  body, 
  while 
  a 
  

   secondary 
  sensory 
  cell 
  {SCL) 
  sends 
  an 
  axon 
  inward 
  that 
  makes 
  con- 
  

   nections 
  with 
  basal 
  branches 
  from 
  the 
  axon 
  of 
  the 
  primary 
  cell. 
  A 
  

   double 
  system 
  is 
  thus 
  established, 
  in 
  which 
  the 
  first 
  cell 
  and 
  its 
  

   branches 
  constitute 
  the 
  motor 
  neuron, 
  and 
  the 
  second 
  cell 
  and 
  its 
  

   branches 
  the 
  sensory 
  neuron. 
  The 
  central 
  connection, 
  or 
  root 
  asso- 
  

   ciations, 
  is 
  called 
  a 
  synapse 
  {Syn) 
  , 
  and 
  a 
  nervous 
  system 
  having 
  this 
  

   structure 
  (B, 
  C, 
  D) 
  is 
  said 
  to 
  be 
  of 
  the 
  synaptic 
  type 
  to 
  distinguish 
  

   it 
  from 
  the 
  direct 
  or 
  nonsynaptic 
  type 
  (A). 
  The 
  actual 
  connections 
  

   in 
  the 
  synaptic 
  structure 
  are 
  usually 
  more 
  complicated 
  than 
  in 
  the 
  

   simple 
  condition 
  represented 
  at 
  B, 
  through 
  the 
  interposition 
  of 
  an 
  

   association 
  neuron 
  {AGl)^ 
  as 
  shown 
  at 
  C, 
  branches 
  of 
  which 
  make 
  a 
  

   two-way 
  connection 
  between 
  the 
  sensory 
  roots 
  and 
  the 
  motor 
  roots. 
  

  

  