238 
ME.  J.  L.  CLAEKE  ON  THE  INTIMATE  STErCTEEE  OE  THE  BEATS'. 
cate  manner  (fig.  12,  Plate  XIII.),  and  then  crossing  in  several  bundles  the  anterior  grey 
substance,  they  reach  the  commissure  in  front  of  the  spinal  canal,  where  they  decussate 
by  platting  with  those  of  the  opposite  side.  Now,  when  in  a transverse  and  shghtly 
oblique  section  through  the  commencement  of  the  pyramids,  these  fibres  were  examined 
with  a sufficiently  high  magnifying-power,  I found  that  they  were  not  prolonged  into 
the  anterior  fissure  and  laid  as  an  entirely  separate  structure  along  the  inner  border  of 
the  opposite  anterior  column,  but  that  they  plunged  into  its  very  substance  and  be- 
came incorporated  with  it,  sometimes  near  its  inner  margin,  and  sometimes  more  exter- 
nally. By  the  extension  upwards  of  this  process,  the  anterior  columns,  aheady  swollen 
by  the  incorporated  fibres  of  the  previous  decussation,  continue  to  increase  in  bulk, 
and  project  above  the  level  of  the  olivary  columns  as  two  rounded  chords.  It  is  known 
that  the  outer  portion  of  each  pyramid  consists  of  parallel  fibres,  which  are  nowhere 
involved  in  the  decussation,  and  were  supposed  by  Eosexthal  to  be  derived  from  the 
lateral  column  of  the  same  side.  These  belong  to  the  outer  and  sometimes  deeper 
portion  only  of  the  anterior  column,  which  has  not  been  traversed  by  the  obhque  decus- 
sating fibres*. 
(14.)  In  addition  to  the  fibres  that  proceed  from  the  lateral  columns  and  posterior 
grey  substance,  there  are  other  decussating  fibres  that  enter  into  the  composition  of  the 
anterior  pyramids.  These  are  derived  from  the  anterior  grey  substance,  and  belong  to 
the  system  which  constitutes  the  anterior  commissure  of  the  medulla  spinalis.  They  are 
less  numerous,  however,  and  most  distinctly  seen  at  the  lower  parts  of  the  pyramids  (see 
figs.  11  and  19). 
(15.)  The  anterior  pyramids,  therefore,  are  composed  of  four  orders  of  fibres: — 
1.  Decussating  fibres  from  the  lateral  columns,  forming  their  chief  bulk. 
2.  Decussating  fibres  from  the  posterior  columns  and  posterior  grey  substance,  parti- 
cularly at  their  upper  portion. 
3.  Decussating  fibres  from  the  anterior  commissure. 
4.  Non-decussating  fibres  of  the  anterior  columns,  separate  on  their  outer  side,  and  on 
their  inner  side  incorporated  with  those  that  form  the  decussation. 
(16.)  Before  the  fourth  month  of  foetal  life,  as  Tiedemann  has  well  observed  f,  the 
pyramids  are  broad  and  fiat,  and  have  some  resemblance,  externally,  to  the  pyramidal 
anterior  columns  in  the  medulla  oblongata  of  fishes  (fig.  1,  Plate  XII.).  At  the  fifth 
month  they  begin  to  project  above  the  level  of  the  surface,  and  continue  until  bii-th  to 
increase  in  depth,  in  consequence  of  the  continual  reinforcement  they  receive  from  the 
decussating  fibres  of  the  lateral  columns  chiefly.  The  decussation  may  be  observed  from 
the  fourth  to  the  sixth  week.  I have  repeatedly  tried  to  make  transparent  sections  of 
the  foetal  medulla,  but  owing  to  its  soft  and  friable  texture,  have  not  succeeded. 
* This  non-decussating  portion  of  the  pyramids  is  correctly  described  by  Dr.  Siiakpet  (Quaix’s  ‘Anatomy’) 
and  by  IVIr.  Solly  (Human  Brain)  as  belonging  to  the  anterior  column,  but  they  considered  it  as  the  whole 
of  that  column. 
t Anatomie  de  Cerveau,  traduit  par  Jouedax  (1823),  p.  145. 
