THE  CHIMPANZEE  AND  ORANG  UTAN. 
379 
tution,  where  the  transverse  diameter  is  13  lines  and  a  half.,  this  space  measures  only 
2  lines,  or  less  than  one  sixth  ;  while  in  the  Pongo,  where  the  same  diameter  is  17  lines 
and  a  half,  it  is  no  less  than  7  lines  and  a  half,  or  nearly  equal  to  one  half  the  breadth 
of  the  orbit.'' 
The  cause  of  the  proportional  differences  in  the  size  of  the  orbits,  and  extent  of  the 
inter-orbital  space,  is  explained  in  the  text,  p.  358.  And  the  Figures  in  this  plate  show 
that  they  operate  equally  in  the  Simia  Troglodytes  as  in  the  Simia  Satyrus,  and  that  the 
proportionally  greater  breadth  of  the  inter-orbital  space  which  distinguishes  the  young 
Chimpanzee  from  the  young  Orang  is  equally  characteristic  of  the  adults  of  the  two 
species. 
A  comparison  of  Fig.  5  with  Fig.  6,  and  of  Fig.  7  with  Fig.  8,  will  show  that  in  each 
species  a  gradual  change  takes  place  in  the  plane  of  the  occipital  foramen :  this  is  occa- 
sioned by  the  downward  and  lateral  growth  of  the  upper  jaw,  and  is  accompanied  with 
a  corresponding  enlargement  of  the  glenoid  cavities,  and  of  the  whole  base  of  the  skull, 
which,  as  the  cerebral  portion  remains  stationary  in  its  development,  throws  the  occi- 
pital/oramew  more  backwards.  This  change  is  of  course  proportionally  greater  in  the 
Simia  Satyrus  than  in  the  Simia  Troglodytes,  corresponding  to  the  greater  difference 
which  subsists  between  the  young  and  adult  states  of  their  jaws  and  dentition  ;  but  the 
relation  that  is  preserved  equally  proves  the  identity  of  the  young  with  the  adult  Orang, 
as  between  the  young  and  the  adult  Chimpanzee. 
PLATE  LVII. 
Side  view  of  the  skull  of  a  human  idiot,  natural  size. 
PLATE  LVIIL 
Base  of  the  same  skull,  natural  size. 
This  figure  and  the  preceding  were  taken,  by  the  liberal  permission  of  Edward  Stan- 
ley, Esq.,  F.R.S.,  from  the  specimen  in  his  Museum  of  Human  and  Pathological  Ana- 
tomy at  St.  Bartholomew's  Hospital. 
* 
