﻿art. 
  27 
  \SY.\I 
  MHTKY 
  IN 
  SKULLS 
  OF 
  MAMMALS 
  HOWELL 
  15 
  

  

  condition 
  of 
  the 
  cervical 
  region; 
  but 
  all 
  such 
  hypotheses 
  are 
  pure 
  

   speculation. 
  

  

  CONCLUSIONS 
  

  

  From 
  the 
  study 
  of 
  these 
  four 
  specimens 
  it 
  has 
  been 
  concluded 
  

   that 
  the 
  primary 
  cause 
  inducing 
  asymmetry 
  in 
  the 
  skulls 
  of 
  mam- 
  

   mals 
  other 
  than 
  toothed 
  cetaceans 
  is 
  probably, 
  in 
  most 
  instances, 
  by 
  

   accident 
  or 
  disease, 
  to 
  the 
  bones 
  or 
  muscles 
  of 
  a 
  single 
  side 
  of 
  the 
  

   head 
  at 
  a 
  comparatively 
  early 
  age, 
  and 
  that 
  this 
  must 
  be 
  of 
  such 
  a 
  

   character 
  as 
  to 
  result 
  in 
  a 
  stunted 
  or 
  infantile 
  condition 
  of 
  a 
  crucial 
  

   part 
  of 
  the 
  bony 
  framework, 
  and 
  a 
  reduction 
  in 
  the 
  rate 
  of 
  growth, 
  

   or 
  strength 
  through 
  lesions, 
  of 
  the 
  muscles 
  of 
  a 
  single 
  side. 
  Asym- 
  

   metry 
  usually 
  is 
  directly 
  dependent 
  upon 
  unevenness 
  in 
  the 
  strains 
  

   developed 
  upon 
  the 
  two 
  sides 
  of 
  the 
  head 
  while 
  an 
  animal 
  is 
  eating. 
  

  

  Certain 
  injury 
  to 
  the 
  bones 
  of 
  the 
  head 
  causes 
  a 
  premature 
  oblit- 
  

   eration 
  of 
  the 
  sutures, 
  as 
  already 
  indicated. 
  Published 
  data 
  respect- 
  

   ing 
  human 
  crania 
  have 
  shown 
  that 
  such 
  early 
  obliteration 
  of 
  the 
  

   sutures 
  may 
  also 
  occur 
  from 
  obscure 
  causes 
  without 
  violence 
  having 
  

   been 
  suffered 
  by 
  the 
  individual. 
  It 
  is 
  doubtless 
  fortuitous 
  that 
  no 
  

   skulls 
  of 
  this 
  character 
  have 
  been 
  available 
  in 
  the 
  present 
  study. 
  Con- 
  

   versely, 
  it 
  is 
  known 
  that 
  retarded 
  obliteration 
  of 
  certain 
  sutures 
  be- 
  

   yond 
  the 
  usual 
  time 
  for 
  their 
  disappearance 
  results 
  in 
  the 
  hypertrophy 
  

   of 
  the 
  corresponding 
  part 
  of 
  the 
  skull. 
  It 
  is 
  only 
  a 
  question 
  of 
  time 
  

   before 
  material 
  illustrating 
  the 
  latter 
  point 
  in 
  the 
  mammalia 
  other 
  

   than 
  man 
  is 
  brought 
  to 
  light. 
  

  

  One 
  of 
  the 
  most 
  conspicuous 
  results 
  of 
  this 
  investigation, 
  and 
  one 
  

   that 
  deserves 
  to 
  be 
  stressed, 
  is 
  the 
  conclusion 
  that 
  normal 
  develop- 
  

   ment 
  of 
  the 
  bones 
  of 
  the 
  skull 
  is 
  directly 
  dependent 
  upon 
  the 
  growth 
  

   of 
  the 
  attached 
  muscles. 
  In 
  other 
  words, 
  if 
  for 
  any 
  reason 
  the 
  mus- 
  

   cles 
  of 
  an 
  animal 
  remain 
  infantile 
  and 
  fail 
  properly 
  to 
  grow, 
  the 
  bones 
  

   to 
  which 
  they 
  are 
  secured 
  will 
  remain 
  proportionately 
  undersized. 
  

   This 
  assertion 
  can 
  not 
  be 
  proven 
  in 
  the 
  case 
  of 
  the 
  masseter 
  muscles 
  

   until 
  the 
  myology 
  of 
  asymmetrical 
  skulls 
  can 
  be 
  more 
  fully 
  investi- 
  

   gated. 
  It 
  is 
  also 
  apparent 
  that 
  the 
  smaller 
  the 
  origin 
  or 
  fossa 
  of 
  a 
  

   muscle 
  the 
  smaller 
  must 
  the 
  muscle 
  itself 
  be. 
  9 
  The 
  significance 
  of 
  

   these 
  facts, 
  when 
  considered 
  with 
  reference 
  to 
  specific 
  (and 
  higher) 
  

   variation 
  of 
  the 
  skull, 
  is 
  profound. 
  

  

  It 
  is 
  apparent 
  that 
  asymmetrical 
  development 
  of 
  a 
  skull 
  inclines 
  

   to 
  progress 
  both 
  forward 
  and 
  backward 
  from 
  a 
  center 
  that 
  is 
  rather 
  

   uniform. 
  In 
  other 
  words, 
  it 
  always 
  appears 
  as 
  though 
  a 
  part 
  of 
  the 
  

   skull 
  were 
  held 
  stationary 
  while 
  the 
  portions 
  craniad 
  and 
  caudad 
  

  

  »0f 
  interest 
  in 
  this 
  connection 
  is 
  a 
  paper 
  by 
  J. 
  A. 
  Howell 
  (An 
  experimental 
  study 
  of 
  the 
  effect 
  of 
  stress 
  

   and 
  strain 
  on 
  bone 
  development, 
  Anat. 
  Rec, 
  vol. 
  13, 
  1917, 
  pp. 
  233-252) 
  comparing 
  the 
  leg 
  bones 
  of 
  a 
  dog, 
  

   the 
  muscles 
  upon 
  one 
  side 
  of 
  which 
  had 
  been 
  transected 
  when 
  the 
  animal 
  was 
  very 
  young. 
  The 
  diameter 
  

   of 
  the 
  bone 
  was 
  very 
  greatly 
  diminished 
  thereby, 
  but 
  its 
  length 
  was 
  but 
  little 
  below 
  normal. 
  

  

  