﻿art. 
  27 
  ASYMMETRY 
  IX 
  SKULLS 
  OF 
  MAMMALS 
  HOWELL 
  13 
  

  

  The 
  above 
  theoretical 
  deflection 
  of 
  the 
  symphysis 
  by 
  20 
  mm. 
  is 
  

   purely 
  mechanical 
  in 
  character. 
  All 
  the 
  remaining 
  forces 
  of 
  asym- 
  

   metry 
  that 
  have 
  operated 
  to 
  alter 
  the 
  skull 
  are 
  either 
  myological 
  

   or 
  developmental 
  (growth 
  of 
  bone). 
  

  

  An 
  examination 
  of 
  the 
  superior 
  aspect 
  of 
  the 
  skull 
  indicates 
  that 
  

   in 
  comparison 
  with 
  normal 
  crania 
  the 
  one 
  under 
  discussion 
  seems 
  to 
  

   be 
  unusually 
  small, 
  as 
  well 
  as 
  can 
  be 
  judged 
  from 
  general 
  criteria 
  

   of 
  age. 
  Not 
  only 
  is 
  it 
  short, 
  but 
  the 
  sagittal 
  crest 
  is 
  more 
  poorly 
  

   developed. 
  The 
  cephalic 
  musculature 
  was 
  therefore 
  undoubtedly 
  

   below 
  normal, 
  although 
  one 
  may 
  presume 
  that 
  it 
  was 
  entirely 
  ade- 
  

   quate 
  for 
  the 
  feeding 
  needs 
  of 
  the 
  animal. 
  

  

  The 
  sagittal 
  crest 
  is 
  displaced 
  about 
  10 
  mm. 
  toward 
  the 
  left, 
  but 
  

   with 
  this 
  exception 
  there 
  is 
  no 
  decided 
  indication 
  that 
  the 
  posterior 
  

   portion 
  of 
  the 
  right 
  temporal 
  muscle 
  was 
  more 
  powerful 
  than 
  that 
  

   upon 
  the 
  left. 
  There 
  is 
  striking 
  evidence 
  to 
  this 
  effect, 
  however, 
  

   in 
  the 
  anterior 
  part 
  of 
  the 
  right 
  temporal 
  fossa. 
  As 
  clearly 
  shown 
  

   in 
  the 
  illustration, 
  this 
  portion 
  of 
  the 
  right 
  temporal 
  was 
  several 
  

   times 
  the 
  larger, 
  not 
  only 
  displacing 
  the 
  sagittal 
  crest 
  but 
  extending 
  

   well 
  forward 
  upon 
  the 
  frontal 
  and 
  modifying 
  the 
  shape 
  and 
  size 
  of 
  

   the 
  right 
  supraorbital 
  process. 
  The 
  sinistral 
  displacement 
  of 
  the 
  

   rostrum 
  is 
  largely 
  attributable 
  to 
  the 
  disproportionately 
  great 
  

   development 
  of 
  this 
  part 
  of 
  the 
  muscle, 
  coupled 
  with 
  the 
  fact 
  that 
  the 
  

   right 
  zygomatic 
  arch 
  and 
  masseter 
  were 
  free 
  to 
  accomplish 
  anterior 
  

   growth. 
  On 
  the 
  other 
  hand 
  the 
  stunting 
  of 
  the 
  left 
  zygomatic 
  arch 
  

   resisted 
  normal 
  growth 
  of 
  the 
  left 
  half 
  of 
  the 
  skull, 
  and 
  necessarily 
  

   limited 
  the 
  size 
  of 
  the 
  mass 
  of 
  muscle 
  that 
  could 
  pass 
  within 
  the 
  

   zygomatic 
  vacuity 
  of 
  the 
  temporal 
  fossa. 
  These 
  would 
  seem 
  to 
  be 
  

   the 
  two 
  causes 
  that 
  tended 
  to 
  limit 
  the 
  growth 
  to 
  the 
  left 
  temporal 
  

   muscle, 
  while 
  the 
  reduced 
  length 
  of 
  the 
  zygomatic 
  arch 
  upon 
  that 
  

   side 
  prevented 
  normal 
  growth, 
  and 
  undoubtedly 
  strength, 
  of 
  its 
  

   attached 
  masseter. 
  Thus, 
  during 
  the 
  development 
  of 
  the 
  animal, 
  

   the 
  root 
  or 
  base 
  of 
  the 
  left 
  side 
  of 
  the 
  rostrum 
  was 
  held 
  back, 
  while 
  

   that 
  of 
  the 
  right 
  side 
  was 
  pushed 
  forward, 
  resulting 
  in 
  the 
  great 
  dis- 
  

   placement 
  toward 
  the 
  left 
  exhibited 
  by 
  that 
  part 
  of 
  the 
  skull. 
  

   Another 
  consequence 
  has 
  been 
  the 
  disproportionate, 
  dextral 
  bowing 
  of 
  

   the 
  mesethmoid, 
  and 
  undoubtedly 
  of 
  the 
  cartilaginous 
  septum, 
  which 
  

   resulted 
  in 
  a 
  crowding 
  of 
  the 
  right 
  ethmoid 
  and, 
  therefore, 
  expansion 
  

   of 
  the 
  left 
  nasal 
  passage, 
  from 
  which 
  the 
  ethmoid 
  is 
  now 
  missing. 
  

   This 
  whole 
  process 
  has 
  naturally 
  effected 
  a 
  disproportionate 
  devel- 
  

   opment 
  of 
  the 
  individual 
  bones 
  of 
  the 
  rostrum 
  which 
  is 
  of 
  interest; 
  

   but 
  there 
  is 
  no 
  necessity 
  here 
  for 
  dwelling 
  at 
  greater 
  length 
  upon 
  

   their 
  individualities. 
  

  

  Returning 
  to 
  an 
  examination 
  of 
  the 
  mandible, 
  one 
  notes 
  that 
  there 
  

   is 
  no 
  indication 
  of 
  asymmetry 
  in 
  its 
  muscular 
  insertions. 
  It 
  is 
  clear, 
  

   however, 
  that 
  the 
  displacement 
  of 
  20 
  mm., 
  caused 
  by 
  the 
  shifting 
  

  

  