IIR.  J.  L.  CLAEKE  ON  THE  INTIMATE  STEHCTUEE  OF  THE  BEAIN. 
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grey  cornu  or  column  [f)  from  the  corpus  restiforme  {a).  At  the  commencement  of  the 
fom-th  ventricle,  the  corpora  restiformia  («,  figs.  5 and  6)  diverge  to  form  its  lateral 
boundaries ; and  between  them  and  the  posterior  median  sulcus,  they  enclose  the  poste- 
rior pyramids,  or  corpora  pyramidalia  posteriora  {hh),  which  diverge  in  a similar  manner 
at  the  point  of  the  calamus  scriptorius. 
(4.)  At  the  lower  part  of  the  medulla  oblongata,  the  gelatinous  substance^  or  extre- 
mity of  the  posterior  cornu,  reaches  the  surface,  and  enlarging  as  it  ascends,  forms  a pro- 
minent column  (fig.  6y,  Plate  XII.),  which  near  the  olivary  body  becomes  overlaid  and 
concealed  by  a new  system  of  transverse  arciforni  fibres.  This  grey  column,  which  was  first 
described  and  called  by  Eolando  the  tuberculo  cinereo — tubercule  cendree, — is  generally 
believed  to  appear  on  the  surface  of  the  human  medulla  only ; but  this  opinion  is  erro- 
neous, for  I have  found  it  comparatively  larger,  or  at  least  more  prominent,  in  some 
Mammalia,  as  the  Cat,  in  which  its  upper  part,  where  the  arciform  fibres  commence, 
presents  a well-marked  and  rounded  head  or  projection.  The  anterior  (or  as  seen  in 
fig.  6,  the  lower)  border  of  the  tuberculo  cinereo  rests  on  the  lateral  column  which  sepa- 
rates it  from  that  which  encloses  the  olivary  body. 
(5.)  Of  the  Arciform  Fibres. — The  fibres  so  named  appear  in  part  on  the  lateral  sur- 
faces of  the  medulla,  and  have  been  somewhat  differently  described  by  Santoeini,  Mala- 
CAEXE,  Eolaxdo,  Eosenthal,  Sollt,  and  John  Eeid.  In  Man  they  vary  considerably  in 
distinctness,  and  sometimes  apparently  in  arrangement;  but  in  a great  number  of 
specimens  which  I carefully  examined,  their  general  course  in  each  was  found  to  be 
alike.  Those  on  the  right  side  of  the  human  medulla  are  represented  in  fig.  6 *.  These 
arciform  fibres  may  be  divided  into  two  layers — superficial  and  deep.  The  deep  layer 
will  be  described  further  on,  as  part  of  the  upper  region  of  the  medulla.  The  superficial 
layer,  for  convenience  of  description,  may  in  turn  be  divided  into  two  sets.  The  first  set 
are  entirely  transverse  in  their  course,  and  appear  externally  as  an  arched  and  flattened 
band  extending  round  each  side  of  the  medulla  from  the  anterior  sulcus  to  the  posterior 
columns,  especially  the  restiform  body,  through  which  they  interlace  in  a very  intricate 
manner  (see  fig.  36,  Plate  XVII.  x') ; but  if  this  band,  in  a properly-prepared  transverse 
section,  be  examined  with  a sufficient  magnifying-power,  it  may  be  seen  to  be  partly 
formed  by  the  convexities  of  many  subordinate  series  of  arcs,  of  many  sizes,  and  pro- 
ceeding from  different  depths  and  regions  of  the  medulla.  In  three  situations  these  are 
most  numerous  and  conspicuous.  One  series  issue  from  the  substance  of  the  anterior 
pyramid,  particularly  from  its  posterior  portion,  where  a vesicular  network  or  plexus 
and  small  groups  of  cells  are  developed,  and  whence  a kind  of  radiation  may  be  observed 
(see  fig.  36,  Plate  XVII.) : the  next  series  issue  also  from  the  back  of  the  pyramid,  and 
turn  round  the  surface  of  the  olivary  body  to  the  lateral  column,  where  they  join  the 
* It  may  be  proper  to  state,  that  in  any  single  medulla  they  are  seldom  seen  so  distinctly  as  they  are 
represented  in  this  figure,  which  was  drawn  from  picked  and  well-marked  specimens  that  had  been  mace- 
rated for  some  hours  in  spirit  and  then  treated  with  a mixture  of  spirit  and  acetic  acid,  after  the  membranes 
were  removed. 
2 I 
MDCCCLVIII. 
