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ME.  J.  L.  CLAEKE  ON  THE  INTIMATE  STEUCTHEE  OF  THE  BEAIN. 
border  of  the  transverse  commissm-e,  which  forms  the  roof  of  the  central  canal.  Beyond 
this  point,  therefore,  the  canal  is  laid  open  and  expanded  into  the  fourth  ventricle,  in 
the  floor  of  which  the  vesicular  column  or  nucleus  of  the  hypoglossal  ner^  e ascends  on 
each  side  of  the  mesial  line  (see  fig.  5,  Plate  XII.).  Fig.  33,  Plate  XVI.  represents  a 
transverse  section  of  the  medulla  of  the  Sheep  at  the  point  of  the  calamus  scriptorius, 
where  the  transverse  commissure  bridges  over  the  upper  exti'emity  of  the  canal.  A 
similar  section  of  the  human  medulla  is  represented  in  fig.  29,  Plate  XVI.,  where  the 
commissure  is  broken  and  the  canal  laid  open.  Fig.  30  represents  another  transverse 
section  about  two  lines  higher  up  (between  the  points  a and  r in  fig.  5,  Plate  XII.),  in 
which  the  hypoglossal  nuclei  [t)  are  seen  in  the  floor  of  the  fourth  ventricle.  Xear  this 
point,  the  divergent  extremities  of  the  pyriform  vagal  nuclei  appear  to  terminate  , 
abruptly ; but  in  a transverse  section,  immediately  beyond  them,  they  are  found  to  sink 
beneath  two  new  masses  of  vesicular  substance — the  auditory  ganglia  (w,  figs.  31,  32,  36) 
— which  now  begin  to  make  their  appearance  at  the  sides  of  the  medulla,  and  to  which, 
in  part,  they  appear  to  contribute.  Here  they  gradually  diminish,  and  become  one  of 
the  nuclei  or  centres  of  origin  of  the  glossopharyngeal  nerves.  At  the  last  or  highest 
roots  of  these  nerves,  which  adjoin  the  anterior  division  of  the  auditory,  there  remains 
but  a small  group  of  their  peculiar  oval  cells,  to  which  separate  dirisions  of  the  nerves 
find  their  way ; while  the  rest  of  each  nucleus  becomes  so  gradually  blended  'vrith  the 
lower  angle  of  the  auditory  ganglion,  as  not  to  be  distinguished  from  it. 
(46.)  The  arciform  fibres  which  connect  together  the  anterior  and  posterior  portions 
of  the  medulla  and  its  two  lateral  halves,  are  very  numerous,  and  extremely  intiicate 
in  their  course  and  arrangement.  They  have  been  divided  into  two  layers,  siiperjidal 
and  deep  ; but  both  these  layers  are  continuous,  or  intimately  connected  rrith  each  other 
at  difierent  points.  The  course  of  the  superficial  fibres  has  been  ah’eady  described 
(§5  and  6).  At  the  back  of  the  medulla  they  issue  from  the  posterior  columns,  but 
chiefly  from  the  restiform  nucleus,  and  are  then  collected  into  a broad  band  (a’,  fig.  36. 
Plate  XVII.),  which  proceeds  transversely  over  the  surface  of  the  gelatinous  substance. 
On  reaching  the  lateral  column  they  separate  into  bundles,  which  join  the  deep  set  and 
reunite  with  each  other  to  form  a plexus  through  the  column  of  cells  at  the  back  of  the 
olivary  body  {g\  figs.  29  and  32,  Plate  XVI.).  The  remaining  fibres  of  the  band  advance 
round  the  surface  of  the  olive  and  front  of  the  pyramid  to  the  anterior  sulcus,  as 
already  stated. 
(47.)  The  deep  layer  of  arciform  fibres  maybe  said  to- arise,  posteriorly,  from  the  post- 
pyramidal  and  restiform  ganglia,  in  which  they  are  partly  continuous  with  the  super- 
ficial layer,  either  directly  or  through  the  medium  of  cells  (figs.  31,  32,  33,  36). 
Collecting  into  bundles  that  join  in  an  intricate  plexus,  they  traverse  fii’st  the  caput  corniis 
posterions,  in  which,  with  the  horizontal  roots  of  the  vagus  and  glossopharyngeal  nerves, 
they  again  interlace  around  the  longitudinal  fasciculi  which  ascend,  as  akeady  stated, 
to  the  large  root  ol  the  trifacial  nerve.  On  arriving  at  the  lateral  column,  the  plexus 
expands  through  its  whole  breadth,  but  may  be  described  in  two  parts. — the  lateral 
