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  ANNUAL, 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  are 
  so 
  nicely 
  balanced 
  against 
  the 
  impinging 
  forces 
  of 
  the 
  environ- 
  

   ment 
  that 
  any 
  change 
  in 
  the 
  latter 
  is 
  immediately 
  reflected 
  by 
  a 
  

   change 
  in 
  the 
  former. 
  The 
  response 
  in 
  the 
  protoplasm, 
  however, 
  

   involves 
  a 
  partial 
  decomposition 
  of 
  some 
  of 
  its 
  molecules, 
  resulting 
  

   in 
  the 
  production 
  of 
  simpler 
  substances 
  of 
  which 
  some 
  are 
  elimi- 
  

   nated, 
  and 
  in 
  the 
  liberation 
  of 
  energy. 
  The 
  second 
  property 
  of 
  

   protoplasm, 
  reversibility, 
  is 
  that 
  by 
  which 
  living 
  matter 
  reverts 
  to 
  

   its 
  original 
  structure, 
  and 
  by 
  which, 
  therefore, 
  it 
  remains 
  alive, 
  for 
  

   otherwise, 
  after 
  one 
  effort, 
  it 
  would 
  be 
  dead. 
  The 
  original 
  proto- 
  

   plasmic 
  structure 
  is 
  restored 
  by 
  the 
  taking 
  of 
  materials 
  from 
  the 
  

   environment 
  to 
  replace 
  those 
  lost 
  during 
  the 
  moment 
  of 
  response 
  to 
  

   the 
  environmental 
  change, 
  and 
  the 
  reconstructive 
  act 
  involves 
  an 
  

   absorption 
  of 
  energy. 
  These 
  simple 
  properties 
  of 
  protoplasm 
  con- 
  

   stitute 
  the 
  physiological 
  basis 
  of 
  all 
  animal 
  activities. 
  

  

  Life, 
  in 
  its 
  simplest 
  terms, 
  thus 
  appears 
  to 
  consist 
  of 
  the 
  produc- 
  

   tion 
  of 
  energy 
  by 
  the 
  upset 
  of 
  a 
  certain 
  equilibrium, 
  and 
  the 
  resto- 
  

   ration 
  of 
  the 
  equilibrium 
  to 
  give 
  the 
  power 
  of 
  more 
  energy 
  produc- 
  

   tion. 
  As 
  long 
  as 
  the 
  double 
  process 
  can 
  go 
  on, 
  life 
  continues. 
  All 
  

   energy 
  is 
  motion 
  of 
  some 
  sort, 
  either 
  of 
  masses, 
  or 
  of 
  the 
  constituent 
  

   molecules, 
  atoms, 
  or 
  electrons 
  of 
  matter, 
  or 
  of 
  the 
  impalpable 
  ether, 
  

   and 
  one 
  form 
  of 
  it 
  can 
  be 
  converted 
  into 
  any 
  other 
  form. 
  If 
  the 
  

   structure 
  of 
  the 
  protoplasmic 
  mass 
  is 
  such 
  as 
  to 
  permit 
  of 
  movement, 
  

   its 
  liberated 
  energy 
  may 
  be 
  converted 
  into 
  motion. 
  

  

  The 
  kind 
  of 
  movement 
  that 
  an 
  animal 
  can 
  make 
  will 
  depend 
  

   entirely 
  on 
  the 
  mechanical 
  construction 
  of 
  its 
  body. 
  In 
  most 
  cases 
  

   the 
  motor 
  principle 
  is 
  a 
  mere 
  change 
  in 
  shape 
  of 
  certain 
  tissues, 
  

   usually 
  the 
  contraction 
  of 
  a 
  muscle 
  cell 
  or 
  fiber. 
  The 
  variety 
  of 
  

   movements 
  of 
  which 
  animals 
  are 
  capable, 
  therefore, 
  results 
  not 
  from 
  

   differences 
  in 
  the 
  type 
  of 
  engine 
  or 
  the 
  kind 
  of 
  fuel 
  but 
  from 
  differ- 
  

   ences 
  in 
  the 
  connected 
  mechanism. 
  The 
  principle 
  of 
  motion 
  produc- 
  

   tion 
  is 
  the 
  same 
  in 
  an 
  amoeba, 
  a 
  worm, 
  an 
  insect, 
  a 
  bird, 
  or 
  a 
  man. 
  

  

  We 
  have 
  seen 
  that 
  live 
  protoplasm 
  does 
  not 
  change 
  except 
  in 
  

   response 
  to 
  the 
  stimulus 
  of 
  some 
  change 
  in 
  the 
  environment. 
  Muscle 
  

   tissue, 
  therefore, 
  does 
  not 
  contract 
  unless 
  it 
  is 
  given 
  a 
  stimulus. 
  But, 
  

   in 
  most 
  animals, 
  the 
  muscles 
  are 
  removed 
  from 
  direct 
  contact 
  with 
  

   the 
  exterior, 
  and 
  are 
  stimulated 
  indirectly 
  through 
  a 
  nerve 
  that 
  is 
  

   connected 
  with 
  a 
  specially 
  sensitive 
  end 
  organ 
  in 
  the 
  skin, 
  which 
  

   receives 
  the 
  stimulus 
  direct 
  from 
  the 
  environment 
  and 
  transmits 
  it 
  

   to 
  the 
  muscle 
  through 
  the 
  nerve. 
  In 
  the 
  many-celled 
  animals, 
  there- 
  

   fore, 
  movement 
  depends 
  upon 
  the 
  presence 
  of 
  a 
  receptor 
  apparatus, 
  

   or 
  sense 
  organ, 
  a 
  transmitting 
  apparatus, 
  or 
  nervous 
  system, 
  and 
  an 
  

   effector 
  apparatus, 
  or 
  muscle. 
  By 
  this 
  mechanism, 
  the 
  animal 
  

   responds 
  with 
  a 
  movement 
  to 
  an 
  external 
  stimulus. 
  

  

  