ME.  J.  L.  CLAEKE  ON  THE  INTIMATE  STEUCTUEE  OE  THE  BEAIN. 
241 
Man*.  Fig.  13,  Plate  XII.  represents  a transverse  section  of  the  medulla  oblongata  of 
the  Sheep,  just  below  the  olivary  bodies ; and  fig.  14,  a similar  section  through  their 
lower  ends. 
(24.)  When  nearly  the  whole  of  the  anterior  cornu  has  become  resolved  into  network, 
at  its  root  in  front  of  the  central  canal,  a new  and  gradually  increasing  column  of 
vesicular  substance  begins  to  make  its  appearance.  This  is  the  ganglion  or  nucleus  of 
the  hypoglossal  nerve,  the  lowest  fibres  of  which  run  out  through  the  remains  of  the 
cornu,  like  roots  of  the  spinal  nerves  (see  fig.  13,  Plate  XII.).  Behind  the  canal,  and  on 
each  side,  another  vesicular  column  may  be  observed,  at  the  same  time,  in  process  of 
formation.  This  is  the  nucleus  or  ganglion  of  the  spinal-accessory  nerve. 
(25.)  Fig.  23,  Plate  XIV.  represents  a transverse  section  of  the  human  medulla  ob- 
longata, a little  below  the  olivary  bodiesf . Here  it  will  be  seen  that  the  vesicular  mass 
within  the  restiform  body  [a)  has  increased  to  a considerable  extent,  and  that  the  net- 
work developed  in  the  posterior  pyramid  (b)  has  spread  through  the  whole  of  its  sub- 
stance, while  the  cells  in  both  are  multiplied  in  an  equal  proportion,  and  lie  scattered 
at  large,  or  collected  into  small  groups  about  bundles  of  longitudinal  fibres.  The 
caput  cornus  posterioris,  or  tuberculo  cinereo,  also  [f),  has  enlarged  its  dimensions, 
and  is  now  crowded  -with  cells,  of  which  some  are  exceedingly  small ; they  are  similar, 
in  structure  and  shape,  to  those  in  the  posterior  columns,  and,  like  them,  are  connected 
with  a number  of  fibres  which  they  appear  to  generate  from  their  processes.  The 
central  canal  is  more  or  less  pervious,  but  generally  compressed  at  the  sides,  sometimes 
to  a mere  line.  It  is  lined  with  a very  distinct  layer  of  columnar  epithelium,  like  that 
which  I first  pointed  out  in  the  spinal  chordj.  Behind  it  on  each  side  is  the  com- 
mencement of  the  spinal-accessory  nucleus,  the  cells  of  which,  apparently  in  different 
states  of  development,  are  pyriform,  round,  triangular  or  variously  fusiform : they  are 
mostly  pale  and  exceedingly  delicate,  but  many  are  partially  or  entirely  filled  with 
granules  of  yellow  or  dark  browm  pigment : this  is  especially  the  case  with  the  fusiform 
variety,  which  are  frequently  ranged  in  linear  series  by  the  junction  of  their  processes, 
Avhich  would  seem  to  be  forming  into  nerve-fibres.  Between  the  posterior  parts  of  the 
* I am  inclined  to  think  that  some  of  the  smallest  of  these  cells  belong  to  connective  and  fibrous  tissue. 
In  every  part  of  the  grey  substance  of  the  spinal  chord  they  are  also  very  abundant,  and  often  resemble 
exactly  the  formative  cells  of  elastic  tissue. 
t I have  chosen  this  part  for  an  enlarged  representation,  because  it  is  the  point  where  nearly  all  the  most 
important  changes  in  the  structure  of  the  medulla  are  seen  at  the  same  time,  and  in  close  connexion  with 
each  other.  The  details  were  observed  under  powers  varying  from  150  to  420  diameters,  without  any  pressure 
or  displacement  whatever. 
4 It  is  stated  by  Stillixg,  in  his  new  work  on  the  spinal  chord  (Neue  Untersuchungen,  Erste  Lieferung, 
p.  20),  that  I was  directed  by  Bowman  to  the  discovery  of  these  cells.  This  is  a mistake,  but  one,  perhaps, 
which  might  not  unreasonably  foUow  from  the  wording  of  my  own  note  (Philosophical  Transactions,  1851, 
Part  II.  p.  614).  However,  the  fact  is,  that  having  observed  this  structure  around  the  spinal  canal,  with- 
out deciding  on  its  nature,  I pointed  it  out  to  Mr.  Bowman,  who  then  suggested  that,  perhaps,  it  was 
epithelium,  which  om  further  examination  in  a perfectly  fresh  state,  I found  to  be  the  case. 
MDCCCLVIII.  2 K 
