﻿ASYMMETRY 
  IN 
  THE 
  SKULLS 
  OF 
  MAMMALS 
  

  

  By 
  A. 
  Brazier 
  Howell 
  

  

  Of 
  the 
  Bureau 
  of 
  Biological 
  Survey, 
  United 
  States 
  Department 
  of 
  Agriculture 
  

  

  Reference 
  to 
  moderate 
  asymmetry 
  in 
  human 
  skulls 
  is 
  not 
  infre- 
  

   quent 
  throughout 
  medical 
  literature, 
  but 
  marked 
  asymmetry 
  in 
  the 
  

   crania 
  of 
  the 
  other 
  mammals 
  (save 
  the 
  toothed 
  whales, 
  in 
  which 
  this 
  

   state 
  is 
  the 
  normal 
  one) 
  must 
  be 
  considered 
  as 
  a 
  very 
  rare 
  condition. 
  

   Careful 
  examination 
  of 
  the 
  material 
  in 
  any 
  large 
  collection 
  would 
  

   doubtless 
  result 
  in 
  the 
  discovery 
  of 
  a 
  number 
  of 
  specimens 
  showing 
  

   some 
  disparity 
  between 
  the 
  development 
  of 
  the 
  two 
  sides 
  of 
  the 
  

   skull; 
  but 
  it 
  is 
  exceedingly 
  seldom 
  that 
  one 
  occurs 
  in 
  which 
  such 
  

   condition 
  is 
  readily 
  to 
  be 
  noted. 
  Application 
  to 
  those 
  in 
  charge 
  of 
  

   some 
  of 
  the 
  larger 
  mammal 
  collections 
  of 
  North 
  America 
  have 
  pro- 
  

   duced 
  but 
  four 
  specimens, 
  two 
  of 
  which 
  exhibit 
  more 
  emphatic 
  dis- 
  

   tortion 
  than 
  seems 
  ever 
  to 
  have 
  been 
  recorded. 
  

  

  Asymmetry 
  in 
  a 
  skull 
  may 
  be 
  brought 
  about 
  by 
  a 
  change 
  in 
  the 
  

   size 
  or 
  relationship, 
  through 
  accident 
  or 
  disease, 
  of 
  the 
  individual 
  

   bones 
  of 
  one 
  side 
  of 
  the 
  head, 
  an 
  alteration, 
  through 
  the 
  same 
  agency, 
  

   to 
  one 
  or 
  more 
  of 
  the 
  large 
  muscles 
  upon 
  a 
  single 
  side, 
  or 
  a 
  combi- 
  

   nation 
  of 
  these 
  two 
  factors. 
  A 
  further 
  analysis 
  of 
  these 
  reasons 
  

   will 
  suggest, 
  as 
  fundamental 
  causes, 
  a 
  more 
  or 
  less 
  permanently 
  pain- 
  

   ful 
  condition 
  of 
  some 
  part 
  of 
  the 
  head, 
  as 
  a 
  sore 
  tooth, 
  a 
  diseased, 
  

   mandibular 
  condyle, 
  or 
  other 
  such 
  state 
  causing 
  the 
  animal 
  to 
  chew 
  

   entirely 
  upon 
  the 
  teeth 
  of 
  one 
  side, 
  or 
  otherwise 
  to 
  use 
  the 
  muscles 
  

   in 
  an 
  uneven 
  manner 
  so 
  as 
  to 
  ease 
  the 
  pain 
  of 
  the 
  offending 
  part 
  as 
  

   much 
  as 
  possible. 
  This 
  at 
  first 
  is 
  voluntary, 
  although 
  it 
  may 
  later 
  

   become 
  entirely 
  involuntary, 
  and 
  it 
  implies 
  a 
  long-continued 
  period 
  

   of 
  painfulness 
  of 
  the 
  part 
  originally 
  affected. 
  Again 
  there 
  may 
  be 
  

   a 
  definite 
  alteration 
  in 
  the 
  shape 
  of 
  a 
  bone, 
  through 
  fracture 
  and 
  

   later 
  healing 
  in 
  a 
  twisted 
  position, 
  changing 
  the 
  interrelationship 
  of 
  

   other 
  parts 
  of 
  the 
  skull, 
  as 
  the 
  mandible. 
  Accidental 
  severance 
  of 
  

   certain 
  nerves 
  may 
  also 
  be 
  productive 
  of 
  similar 
  results. 
  In 
  all 
  such 
  

   cases 
  of 
  asymmetry 
  in 
  the 
  skull, 
  initial 
  injury 
  at 
  a 
  comparatively 
  

   early 
  age 
  is 
  a 
  necessity— 
  the 
  earlier 
  the 
  injury 
  the 
  more 
  pronounced 
  

   will 
  be 
  its 
  effects, 
  other 
  things 
  being 
  equal. 
  

  

  The 
  pathological 
  conditions 
  resulting 
  from 
  the 
  healing 
  of 
  a 
  severe 
  

   injury 
  to 
  the 
  bone 
  after 
  the 
  animal 
  has 
  attained 
  full 
  growth 
  does 
  not 
  

  

  NO. 
  2599.-PROCEEDINGS 
  U. 
  S. 
  NATIONAL 
  MUSEUM, 
  VOL. 
  67, 
  ART. 
  27. 
  

   63194— 
  26t 
  1 
  1 
  

  

  