ME.  J.  L.  CLAEKE  ON  THE  INTIMATE  STEUCTUEE  OF  THE  BEAIN. 
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and  the  deep.  The  lateral  portion,  in  front  of  the  caput  corn'ds,  communicates  freely, 
as  above  described,  with  the  superficial  layer,  and  then  sends  forward  a series  of  bundles 
that  traverse  and  surround  the  olivary  body.  Its  outer  bundles  [y)  sweep  round  the 
front  of  the  sac,  and  having  sent  in  their  course  some  fibres  between  its  convolutions  and 
into  the  white  nucleus,  they  separate  behind  the  pyramid  into  a series  of  other  bundles 
which  are  gradually  larger  from  before  backwards,  and  communicate  to  form  a small 
plexus,  of  which  the  finer  or  anterior  portion,  containing  some  cells,  proceeds  inwards 
and  forwards  through  the  pyramid  to  the  vesicular  nuclei  (A,  A')  along  its  border, 
while  the  coarser  and  posterior  portion  is  continuous  vdth  the  transverse  olivary  com- 
missui’e.  The  inner  bundles  {x")  of  the  lateral  plexus  pierce  the  olive  from  behind,  and 
winding  through  different  parts  of  its  substance,  join  the  fibres  of  the  white  nucleus, — 
those  internally  at  an  angle, — and  then  proceed  with  them  to  form  the  principal  part  of 
the  transverse  commissure.  The  bundles  of  this  commissure  thus  formed  communicate 
with  each  other  by  lateral  branches,  as  a plexus,  and  proceed  to  the  edge  of  the  raphe 
behind  the  anterior  sulcus.  Here  they  diverge  and  decussate  freely  in  such  a manner, 
that  each  exchanges  fibres  with  several  others  of  the  opposite  side  that  lie  at  different 
depths  of  the  commissure,  while  the  most  divergent  fibres  from  the  anterior  side  of  each 
bundle  decussate  to  be  continuous  with  the  arciform  band  which  runs  round  the  opposite 
pyramid  (see  fig.  36  B). 
(48.)  The  deep  portion  of  the  plexus  from  the  lateral  column,  directly  continuous  with 
the  other,  or  olivary  portion,  sweeps  inwards  to  the  remaining  posterior  part  of  the  raphe, 
its  main  bundles  curving  backwards  and  becoming  shorter  as  they  approach  the  posterior 
surface  (see  figs.  29  and  30,  Plate  XVI.).  Across  the  raphe  they  all  diverge  (see  fig.  34, 
Plate  XVII.),  and  like  those  of  the  olivary  commissure,  exchange  fibres  with  bundles 
of  the  opposite  side  at  a distance  corresponding  to  their  divergence,  the  central  fibres 
of  each  crossing  nearly  in  straight  lines.  Many  of  the  most  divergent,  on  reaching  the 
opposite  side,  take  an  antero-posterior  direction  along  the  raphe,  passing  out  singly  and  at 
intervals  into  different  parts  of  the  plexus  and  the  longitudinal  fasciculi  enclosed  in  its 
meshes.  At  the  summit  of  the  medulla,  the  number  of  antero-posterior  fibres  is  very 
much  increased  by  considerable  bands,  which,  proceeding  from  the  auditory  ganglia,  and 
decussating  at  acute  angles  as  they  enter  the  raphe,  run  down  it  parallel  to  each  other ; 
and  these  bands  at  their  decussation  are  crossed  by  that  of  two  other  bundles  proceeding 
from  the  centre  of  the  lateral  columns  and  the  remains  of  the  glossopharyngeal  nucleus 
(fig.  36,  Plate  XVII.). 
(49.)  The  raphe,  therefore,  is  the  seat  of  a wonderfully  complicated  decussation 
between  fibres  from  all  parts  of  the  opposite  halves  of  the  medulla,  chiefiy  through  the 
system  of  arciform  fibres,  which  at  the  same  time  connect  together  all  the  parts  of  each 
separate  half.  The  arciform  plexus  is  everywhere  interspersed  with  circular,  oval,  pyri- 
form, fusiform,  triangular  or  stellate  cells,  of  different  sizes,  which  give  origin  to  some 
of  its  fibres,  and  frequently  lie  against  the  longitudinal  fasciculi,  vdth  the  fibres  of  which 
they  are  probably  also  continuous,  for  many  of  their  processes  run  in  the  same  direction. 
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