﻿art. 
  27 
  SlSI 
  MMETRY 
  IN 
  SKULLS 
  OF 
  MAMMALS 
  HOWELL 
  t 
  

  

  An 
  alternative 
  hypothesis, 
  in 
  no 
  wise 
  dependent 
  upon 
  the 
  original 
  

   injury 
  to 
  the 
  skull, 
  was 
  suggested 
  by 
  Capt. 
  R. 
  W. 
  Leigh 
  (MS), 
  of 
  

   the 
  Army 
  Medical 
  Museum, 
  to 
  account 
  for 
  the 
  alveolar 
  condition. 
  

   This 
  is 
  to 
  the 
  effect 
  that 
  the 
  molariform 
  teeth 
  grew 
  into 
  position 
  

   with 
  abnormally 
  large 
  spaces 
  between 
  them, 
  into 
  which 
  became 
  more 
  

   or 
  less 
  permanently 
  wedged 
  particles 
  of 
  fibrous 
  foods, 
  this 
  finally 
  forc- 
  

   ing 
  the 
  recession 
  of 
  the 
  alveolar 
  borders. 
  I 
  have 
  occasionally 
  noted 
  

   just 
  this 
  state 
  of 
  affairs 
  between 
  two 
  or 
  three 
  teeth 
  of 
  ungulates 
  and 
  

   rodents, 
  and 
  I 
  deem 
  it 
  very 
  likely 
  that 
  the 
  same 
  situation 
  operated 
  

   to 
  aggravate 
  the 
  abnormal 
  condition 
  in 
  the 
  gorilla; 
  but 
  that 
  this 
  

   was 
  the 
  original 
  and 
  sole 
  cause 
  for 
  the 
  recession 
  of 
  the 
  bony 
  borders 
  

   I 
  strongly 
  doubt. 
  

  

  An 
  examination 
  of 
  the 
  skull 
  impresses 
  one 
  with 
  the 
  probability 
  

   that 
  certain 
  muscles 
  suffered 
  considerable 
  violence, 
  either 
  directly 
  or 
  

   in 
  consequence 
  of 
  severe 
  infection 
  after 
  injury. 
  The 
  evidence 
  indi- 
  

   cates 
  that 
  the 
  muscles 
  thus 
  involved 
  were 
  chiefly 
  the 
  digastric, 
  tra- 
  

   chelo-mastoid, 
  sterno 
  and 
  cleido-mastoids, 
  splenius, 
  rectus 
  capitis 
  

   lateralis, 
  and 
  the 
  levator 
  and 
  tensor 
  palatis. 
  It 
  is 
  not 
  improbable 
  

   that 
  the 
  obliquus 
  superior 
  was 
  also 
  affected. 
  There 
  is 
  no 
  satisfactory 
  

   evidence 
  that 
  any 
  other 
  muscle 
  suffered 
  directly 
  except 
  in 
  so 
  far 
  as 
  

   the 
  pathological 
  condition 
  of 
  the 
  articulation 
  of 
  the 
  right 
  side 
  of 
  the 
  

   jaw 
  means 
  previous 
  infection, 
  to 
  some 
  extent, 
  of 
  the 
  muscles 
  sur- 
  

   rounding 
  it. 
  

  

  The 
  direct 
  results 
  of 
  injury, 
  reflected 
  in 
  the 
  stresses 
  exerted 
  by 
  

   other 
  muscles 
  of 
  the 
  head, 
  were 
  of 
  a 
  more 
  profound 
  and 
  lasting 
  char- 
  

   acter. 
  Injury 
  to 
  and 
  subsequent 
  healing 
  of 
  a 
  muscle 
  causes, 
  under 
  

   certain 
  conditions, 
  shrinkage 
  of 
  its 
  length, 
  and 
  other 
  strains 
  may 
  

   develop. 
  It 
  should 
  here 
  be 
  understood 
  that 
  the 
  term 
  "pull" 
  of 
  a 
  

   lesioned 
  muscle, 
  "shrinkage," 
  etc., 
  is 
  merely 
  relative 
  and 
  not 
  actual. 
  

   Shrinkage 
  is 
  very 
  seldom 
  sufficient 
  for 
  a 
  muscle 
  so 
  affected 
  to 
  exert 
  

   true 
  tension 
  upon 
  a 
  part; 
  but 
  a 
  muscle 
  of 
  this 
  character 
  not 
  only 
  

   resists 
  full 
  relaxation, 
  but 
  by 
  the 
  same 
  token 
  resists 
  normal 
  growth. 
  

   Hence, 
  for 
  all 
  practical 
  purposes, 
  it 
  does 
  not 
  matter, 
  when 
  speaking 
  

   of 
  a 
  case 
  where 
  asymmetry 
  of 
  a 
  bone 
  is 
  due 
  to 
  difference 
  in 
  the 
  

   development 
  of 
  either 
  of 
  a 
  pair 
  of 
  muscles, 
  whether 
  one 
  conceive 
  

   that 
  the 
  abnormal 
  side 
  of 
  a 
  bone 
  has 
  been 
  pulled 
  around 
  by 
  con- 
  

   traction 
  (i. 
  e., 
  resistance 
  to 
  normal 
  growth), 
  or 
  that 
  the 
  growth 
  of 
  

   the 
  normal 
  side 
  has 
  pushed 
  the 
  other 
  out 
  of 
  line. 
  

  

  With 
  respect 
  to 
  the 
  long 
  axis 
  of 
  the 
  skull, 
  the 
  foramen 
  magnum 
  

   is 
  displaced 
  toward 
  the 
  right 
  in 
  such 
  a 
  manner 
  as 
  to 
  suggest 
  that 
  

   the 
  articulation 
  of 
  the 
  condyle 
  with 
  the 
  atlas 
  was 
  forced 
  in 
  that 
  

   direction, 
  logically, 
  by 
  the 
  pull 
  of 
  the 
  lesioned 
  obliquus 
  superior 
  and 
  

   rectus 
  capitus 
  lateralis, 
  and 
  perhaps 
  other, 
  muscles 
  upon 
  that 
  side. 
  

   The 
  basioceipital 
  naturally 
  is 
  obliged 
  to 
  follow 
  this 
  tendency, 
  and 
  it 
  

  

  