240  ME.  J.  L.  CLAEKE  ON  THE  INTIIVIATE  STErCTHEE  OE  THE  BEAIN. 
fissure,  a small  conical  eminence  may  be  observed  to  arise  from  the  posterior  border  of 
the  central  grey  mass,  from  which  a network  of  hlood-vessels  and  radiating  fibres  extend 
directly  backwards  through  the  posterior  pyramid  [b,  figs.  12  and  19).  Within  the 
eminence  and  amongst  the  network,  are  developed  a number  of  cells,  which  for  the  most 
part  are  circular,  oval,  or  pyriform,  and  give  off  two  processes  or  more,  in  both  a trans- 
verse and  longitudinal  direction.  Fig.  20,  Plate  XIII.,  represents  them  magnified  420 
diameters.  Each  cell  contains  a distinct  nucleus  and  nucleolus,  with  a mass  of  yellow 
pigment-granules  accumulated  on  one  side,  or  at  one  extremity.  Further  outwards, 
from  the  root  of  the  cervix  cornus,  a larger  and  gradually  increasing  eminence  arises  in 
the  same  way,  and  projects  through  the  base  of  the  restiform  body,  towards  the  surface 
of  which  it  sends  forth  a number  of  radiating  fibres  (see  h,  figs.  12  and  19) ; but  its  cells, 
though  precisely  similar  in  structure  and  shape,  are  superior  in  size,  to  those  of  the 
former  (fig.  21,  Plate  XIII.).  The  fibres  of  both  are  delicate  tubules  of  small  average 
diameter,  and  for  the  most  part  without  double  contours.  They  radiate  in  tapering 
and  irregular  bundles,  which  interlace  with  those  of  the  restiform  body,  where  they 
become  longitudinal  in  part,  and  are  partly  continuous  at  the  surface  'with  the  super- 
ficial layer  of  arciform  fibres. 
(22.)  These  new  productions  of  grey  substance  will  be  distinguished  respectively  as 
the  jiost-pyramidal  and  restiform  nuclei,  or  ganglia.  They  exist  in  all  the  Mammalia, 
and  to  a certain  extent  in  Birds.  In  some  of  the  former  animals — -in  the  Cat,  for 
instance, —the  restiform  nuclei  are  very  largely  developed  and  nearly  fill  the  white 
column. 
(23.)  Coincident  with  these  changes,  others  are  taking  place  towards  the  sides  and 
front  of  the  medulla ; for  the  anterior  cornu  becomes  gradually  dmded  and  subdivided 
into  smaller  portions  by  longitudinal  fasciculi  of  various  sizes,  around  which  they  mostly 
communicate  (see  fig.  11) ; and  from  these  subdivisions  fibres  run  outwards  in  great 
numbers.  By  the  extension  of  this  process,  the  whole  of  the  cornu  is  at  length  resolved 
into  network,  which  spreads  through  the  lateral  column.  At  the  same  time  the  fibres  of 
the  network,  formerly  described  as  arising  from  the  side  of  the  posterior  cornu  and  of  the 
central  grey  substance^  continue  to  increase  in  number.  At  first  they  are  crossed, 
interlaced  and  very  much  obscured  by  those  of  the  decussation  proceeding  from  the 
bundles  enclosed  in  the  meshes  which  they  form ; but  as  the  decussation  diminishes,  the 
network,  still  further  developed,  comes  clearly  into  view,  and,  in  continuity  with  that  of 
the  anterior  cornu,  appears  in  the  form  of  a wing  -with  its  point  directed  forwards  (see 
figs.  13,  16  and  19).  The  network  contains  a multitude  of  cells  which  differ  in  shape 
and  size.  Many  are  variously  fusiform,  and  appear  at  intervals  in  the  course  of  fibres 
like  mere  dilatations,  which  are  sometimes  exceedingly  small ; others  are  o'sul,  triangular, 
semilunar,  or  irregular  in  form,  and  send  out  their  processes  in  different  dfrections 
between  the  bundles  of  the  lateral  column,  which  they  often  closely  embrace.  This 
arrangement  is  distinctly  seen  in  Mammalia,  Birds,  and  in  Fishes ; in  the  fii'st  of  these 
classes  the  network  is  rather  coarser,  and  the  cells  larger  and  more  abundant  than  in 
