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ME.  J.  L.  CLAEKE  ON  THE  INTIMATE  STEHCTHEE  OF  THE  BEAIN. 
deep  layer  in  a plexus  which  contains  a peculiar  group  of  cells,  to  be  described  further  on 
(see  figs.  30  to  32,  and  fig.  36):  the  third  series  are  continued  backwards  from  the  same 
plexus  and  from  the  front  of  the  tuberculo  cinereo,  around  the  suirface  of  which  they 
turn  to  spread  out  and  interlace  with  the  fibres  of  the  posterior  columns  (same  figs.);  but 
while  crossing  the  tiiberculo  cinereo,  which  they  entirely  overlay,  a few  turn  oft  and  take  a 
downward  course  along  its  surface,  as  represented  in  fig.  6,  Plate  'Kll.f.  When  this  first 
set  or  band  of  arciform  fibres  (A',  fig.  36,  Plate  XVII.)  are  traced  round  the  sm’face  of  the 
pyramid  towards  the  bottom  of  the  anterior  sulcus,  they  are  found  to  embrace  closely 
the  side  of  the  pyramid,  where  in  the  upper  part  of  the  medulla  is  developed  a layer  of 
cells  from  which  some  take  their  origin  (A).  Passing  backwards  in  a slightly  oblique 
or  ascending  direction,  they  may  be  seen  in  a good  transverse  section,  made  in  a corre- 
sponding plane,  to  have  the  following  destinations: — some,  as  aPeady  stated,  sweep 
round  and  into  the  back  of  the  anterior  pyramid,  amongst  the  vesicular  groups  or 
plexus  located  there ; some  pass  deeply  backwards  to  the  lateral  and  posterior  columns 
of  the  same  side ; others  cross  obliquely  to  those  of  the  opposite  side,  decussating  through 
the  ra])]ie  with  fibres  from  the  opposite  band  (B,  fig.  36) ; below  the  olivaiy  bodies, 
these  latter  fibres  are  not  numerous;  for  those  that  proceed  from  the  posteiior  and 
lateral  columns  to  decussate  through  the  now  commencing  raphe,  run  for  the  most  part 
into  the  substance  of  the  opposite  pyramid,  and  but  sparingly  round  its  periphery ; but 
in  the  upper  part  of  the  medulla,  as  will  be  shown  further  on,  they  are  exceedingly 
abundant  behind  the  anterior  sulcus  (see  fig.  36).  The  remaining  fibres  of  the  arciform 
band,  on  reaching  the  bottom  of  the  sulcus,  return  in  a series  of  loops  to  be  contuiuous 
with  the  band  that  embraces  the  opposite  pyramid  (fig.  36). 
(6.)  The  second  set  of  superficial  fibres  proceed  from  the  substance  of  the  anterior  pyra- 
mid, nearer  the  lower  border  of  i\iQ  pons  Varolii  (fig.  6 J,  Plate  XII.).  CurAng  down- 
wards and  backwards,  they  sweep  over  and  around  the  lower  end  of  the  olivary  body  to 
re-ascend  obliquely  with  the  first  set  to  the  upper  extremity  of  the  restiform  column. 
These  fibres  are  sometimes  very  numerous,  and  often  appear  like  a distinct  fasciculus  of 
the  pyramid.  As  the  former  set  were  traced  from  the  bottom  of  the  anterior  sulcus  to 
the  centred  and  posterior  parts  of  the  medulla,  it  is  probable  that  these  have  similar 
connexions  with  corresponding  portions  of  the  pons.  Although,  for  the  salve  of 
clearness  and  facility  of  comprehension,  this  arciform  system  of  fibres  has  been  divided 
into  separate  layers,  it  may  be  well  to  observe,  that  there  is  no  distinct  line  of  sepa- 
ration between  them,  and  that  they  all  intermingle  and  communicate  in  an  intricate 
manner. 
(7.)  In  most  of  the  Mammalia  the  arciform  fibres  are  very  conspicuous,  but  less 
intricate  in  their  arrangement  than  those  of  the  human  medulla.  As  in  Man,  they  are 
connected  with  the  anterior  pyramid  and  olivary  body,  as  well  as  with  the  lateral 
column,  and  ascend  obliquely  backwards,  in  a broad  quadrilateral  band,  to  the  posterior 
pyramids  and  restiform  columns.  Fig.  7,  Plate  XII.  represents  their  arrangement  in  the 
Sheep.  In  the  Cheetah  or  hunting  Leopard  (fig.  9),  they  appear  to  cover  nearly  the 
