846 
MR.  R.  OWEN  ON  THE  OSTEOLOGY  OF 
increased  power  of  mastication  required  for  the  due  action  of  the  large  permanent 
teeth.  The  muscular  impressions  in  the  occipital  region  of  the  cranium  are  less  strongly 
marked  in  the  Chimpanzee  than  in  the  Orang,  the  occipital  foramen  is  further  from 
the  posterior  plane  of  the  cranium,  and  its  position  is  less  obliqued  The  lambdoidal 
ridge,  the  spine  of  the  occiput,  and  the  crista  continued  from  the  latter  downwards 
towards  the  occipital /ommen,  although  slightly  developed  in  comparison  to  the  Orang, 
are  characters  of  the  adult  cranium  of  the  Chimpanzee  wihch  are  wanting  in  the  young 
animal.  There  is  a  greater  proportion  of  brain  behind  the  meatus  auditorius  externus  in 
the  Chimpanzee  than  in  the  Orang,  and  this  difference  is  greater  in  the  adult  than  in  the 
young  skull,  whence  it  results  that  in  the  former  the  supra-auditory  ridge  is  at  some 
distance  anterior  to  the  additamentum  suturee  lambdoidalis,  and  consequently  the  skull 
in  this  respect  more  nearly  approximates  the  human  structure. 
In  the  young  Chimpanzee,  the  articular  cavity  for  the  condyle  of  the  lower  jaw  is  an- 
terior to  the  bony  circle  of  the  meatus  auditorius,  and  on  a  higher  plane  ;  but,  as  the  zy- 
gomatic arch  increases  in  strength  with  the  increasing  power  of  the  maxillary  apparatus, 
without  any  corresponding  downward  increase  of  the  brain  and  cranium,  the  glenoid 
cavity  is  carried  so  near  to  the  lower  level  of  the  bony  meatus,  that  it  no  longer,  as  in 
the  young  animal  and  in  Man,  affords  to  the  condyle  of  the  jaw  a  support  against  back- 
ward dislocation.  To  remedy  the  effects  of  this  change,  a  process,  of  which  the  rudi- 
ment is  perceptible  in  the  young  Chimpanzee,  co-extends  in  downward  growth  with  the 
altered  position  of  the  articulation  of  the  jaw,  becomes  interposed  between  the  maxil- 
lary condyle  and  the  meatus,  and  compensates  for  the  loss  of  that  protection  which  is 
afforded  to  the  maxillary  articulation  by  the  downward  development  of  the  cranium 
posterior  to  the  glenoid  cavity  in  the  human  subject. 
The  lower  part  of  the  external  boundary  of  the  meatus  is  irregularly  jagged  in  the 
Chimpanzee,  for  the  better  attachment  of  the  cartilaginous  portion  of  the  auditory  pass- 
age. The  zygoma  is  proportionally  weaker  than  in  the  Orang ;  the  temporal  portion 
joins  the  malar  obliquely,  and  is  slightly  and  irregularly  wavy. 
The  most  characteristic  feature  of  the  Chimpanzee's  skull,  both  in  the  young  and  old 
state,  is  the  large  projecting  supra-orbital  ridges,  which,  being  continued  into  one 
another  across  the  glabella,  form  a  sort  of  barrier  between  the  cranium  and  face. 
Behind  the  junction  of  the  malar  with  the  frontal  bone  there  is  a  convex  ridge  leading 
obliquely  downwards  and  inwards,  and  strengthening  the  bony  septum  which  divides  the 
orbit  from  the  temporal /ossa. 
The  cranial  sutures,  which  are  obliterated  in  the  adults  of  the  Orang,  syndactylous 
Ape,  and  frequently  in  the  adult  crania  of  Baboons  and  other  Quadrumana,  are  for  the 
most  part  persistent  in  the  Chimpanzee,  and  the  coronal  and  sagittal  sutures  have  the 
true  denticulated  structure.  The  sagittal  suture  is  not  continued  along  the  frontal  bone. 
The  squamous  suture  is  partially  lost,  but  sufficient  remains  to  show  that  the  anterior 
'  Compare  Plates  LII.  and  LIV. 
