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ME.  J.  L.  CLAEKE  ON  THE  INTBIATE  STEECTUEE  OE  THE  BEATS'. 
lie,  and  with  some  of  which  they  are  certainly  continuous.  Around  the  front  of  the 
pjuamid  there  are  several  other  more  or  less  triangular  masses  of  the  same  kind  (A'), 
that  send  backwards  a number  of  similar  prolongations,  from  the  cells  of  which  a multi- 
tude of  fibres  radiate  through  the  pyramid  and  partly  disappear,  probably  to  become 
longitudinal.  Between  these  prolongations  some  of  the  fibres  form  loops  within  the 
border  of  the  masses ; and  between  the  masses  themselves,  fibres  proceeding  from  the 
substance  of  the  pyramid  run  outwards  to  the  arciform  band  at  the  sm’face,  where  they 
turn  right  and  left,  and  form  a series  of  loops  by  re-entering  at  different  parts. 
(38.)  These  vesicular  masses,  therefore,  appear  to  generate  new  fibres  in  the  pp-amids, 
and  new  arciform  fibres,  the  latter  of  which  not  only  connect  together  different  parts  of 
the  pyramids  themselves,  but,  as  we  shall  see  further  on,  the  system  to  which  they  belong 
establishes  the  most  intimate  communication  between  all  parts  of  the  medulla. 
(39.)  The  hypoglossal  and  spinal-accessory  rmclei  continue  to  increase  as  they  ascend, 
until,  by  the  divergence  and  diminution  of  the  posterior  pyramids,  they  are  exposed  on 
the  surface  of  the  fourth  ventricle.  The  first  or  hypoglossal  consist  of  two  cyhndi-ical 
columns,  which  are  separated  from  each  other  by  the  raphe,  and  situate  at  fii’st  in  front 
and  at  the  sides  of  the  canal  (see  figs.  23,  28,  29,  30,  31,  32,  33).  They  contain  a 
great  number  of  oval,  pyriform,  fusiform  and  stellate  cells,  of  large  size,  and  precisely 
similar  to  those  of  the  anterior  cornu,  and  are  surrounded  on  their  outer  sides  by  the 
network  of  the  lateral  column,  which,  as  already  shown,  is  also  studded  with  cells  of  all 
shapes  and  dimensions.  The  processes  of  the  hypoglossal  cells  are  both  longitudinal  and 
transverse:  of  the  latter  kind  (fig.  35,  Plate  XVII.),  some  extend  backwards  towards 
the  spinal-accessory  nucleus ; others  towards  the  raphe,  with  the  fibres  of  which  they  are 
sometimes  continuous ; a third  set  run  outwards,  and  escape  through  the  network  of 
the  lateral  column,  as  do  those  of  the  anterior  cornu  of  the  medulla  spinalis;  but  the 
greater  number  are  continuous  in  front  with  the  fibres  of  their  oum  nerve. 
(40.)  In  Mammalia  generally  the  nucleus  and  cells  have  nearly  the  same  appearance 
as  in  Man.  In  Birds  the  nucleus  is  rather  more  posterior,  and  its  cells  ai’e  smaller,  but 
in  other  respects  similar  (see  fig.  18). 
(41.)  The  spinal-accessory  nucleus  on  each  side  is  developed  in  great  part  from  the 
posterior  and  lateral  portion  of  the  central  grey  substance.  It  appears  at  first  as  a 
narrow  tract  (r,  fig.  28,  Plate  XVI.),  which  extends  from  the  bottom  of  the  posterior 
fissure,  obliquely  outwards  and  forwards,  along  the  side  of  the  canal,  over  which,  ultli 
its  opposite  fellow,  it  forms  a kind  of  slanting  roof.  It  is  overlaid  by  the  root  of  the 
posterior  pyramid,  from  which  it  is  afterwards  partly  developed,  and  rests,  as  it  extends 
forwards,  on  the  back  of  the  hypoglossal  nucleus.  Its  anterior  extremity  dirides  into 
two  portions  or  horns,  which  partially  enclose  some  longitudinal  bundles  of  the  lateral 
column,  and  are  continuous  with  the  roots  of  its  nerve.  As  the  nucleus  ascends  the 
medulla,  its  posterior  portion  enlarges  at  the  expense  of  the  posterior  pyramid,  on  the 
base  and  inner  side  of  which  it  gradually  encroaches.  The  majority  of  its  cells  are  oval, 
pyriform  or  fusiform,  and  turned  with  their  longer  axes  in  the  direction  of  the  nerve ; 
