JIE.  J.  L.  CLAEKE  OJf  THE  INTIMATE  STEUCTIJEE  OE  THE  BEAIN. 
247 
Above,  it  blends  as  a flattened  band  with  the  trapezium,  close  to  the  origin  of  the  facial 
nerve,  which  arises  from  the  side  of  the  latter  (fig.  8)  ; and  helow,  it  is  continuous  with  a 
distinct  fasciculus  of  the  lateral  column  [q).  From  its  side  and  that  of  the  olivary  body, 
the  broad  band  of  arciform  fibres  crosses  the  medulla  to  reach  the  posterior  columns ; 
but  within,  it  is  traversed  by  a network  or  plexus,  formed  by  the  interlacement  of  these 
fibres  with  those  remaining  from  the  anterior  cornu,  and  enclosing  the  longitudinal 
bundles  of  the  lateral  column.  Amongst  this  network  lie  the  cells,  which  are  larger 
than  those  of  the  olivary  body  and  more  irregular  in  shape.  They  are  oval  in  different 
degrees,  or  pyriform,  fusiform,  crescentic,  club-shaped,  triangular,  or  variously  stellate, 
and  give  off  processes  which  nearly  encircle  the  longitudinal  bundles,  and  contribute  to 
form  the  meshes.  All  these  appearances  may  be  very  distinctly  observed  in  the  Sheep, 
Ox,  or  Cat 
(36.)  In  Man  a similar  structure  was  found,  but  owing  to  the  difference  in  the  shape 
of  the  medulla,  it  lies  behind,  instead  of  at  the  side,  of  the  olivary  bodies,  and  is  not  so 
prominent  externally  (see  figs.  23  and  28  ^) ; the  cells  also  are  rather  less  than  those  of 
the  Mammalia. 
Of  the  Structural  Changes  in  the  uf^er  gmrt  of  the  Medulla. 
(37.)  Through  the  lower  fourth  of  the  olivary  bodies,  the  decussation  diminishes  in 
breadth,  and  at  length  becomes  the  raphe,  the  fibres  of  which  are  connected  chiefly  with 
the  borders  of  the  pyramids  as  part  of  the  arciform  system.  But  now  there  begins  to  be 
developed  within  the  pyramids  themselves,  a new  vesicular  system,  which  increases  as  it 
ascends,  and  forms  the  chief  part  of  the  grey  substance  of  i\\e])ons  VaroUi.  Each  pyramid 
in  front  and  at  its  sides  is  pierced  by  blood-vessels  of  considerable  size.  One  of  these 
enters  from  the  anterior  sulcus,  and  running  posteriorly,  sends  off  from  its  side  across 
the  back  of  the  pyi’amid  a number  of  nearly  horizontal  branches,  which  communicate 
with  those  of  another  vessel  that  enters  through  the  fissure  on  its  outer  side  in  front 
of  the  olivary  body,  as  well  as  udth  each  other  by  lateral  branches  and  with  those  that 
enter  in  front  of  the  pyramid.  The  transverse  network  at  the  back  of  the  latter  con- 
tains a numher  of  scattered  cells,  which  are  small,  but  uniform  in  size,  nucleated,  and 
pigmentary ; in  shape  they  are  oval,  pyriform,  more  or  less  triangular,  or  very  long  and 
fusiform;  they  send  out  processes  which  either  encircle  the  longitudinal  bundles,  or 
run  along  the  small  fissures  towards  the  sides  of  the  pyramid ; so  that  they  form  a com- 
plete network  of  nearly  single  fibres.  But  at  the  inner  side  of  the  pyramid,  near  the 
bottom  of  the  anterior  sulcus  and  against  the  arciform  fibres  where  they  are  continuous 
with  the  raphe,  in  the  process  of  pia  mater  round  the  artery  and  its  first  branches  from 
which  the  transverse  network  proceeds,  there  is  formed  a large  vesicular  mass  (see  A, 
fig.  36,  Plate  XYII.),  which  sends  out  tapering  or  pointed  prolongations  across  tlie 
back  of  the  pyramid,  where  the  processes  of  their  cells  become  continuous  with  those 
of  the  network  just  described ; but  the  cells  at  the  base  of  the  mass  are  turned  with 
their  longer  axes  in  the  direction  of  the  course  of  the  arciform  fibres,  against  which  they 
