Another  important  ascending  tract,  s,  e,  t,  c,  lies  in  the  median
portion  of  the  posterior  white  column  and  is  named  the  median
posterior  tract.  It  commences  in  the  lower  portion  of  the
cord  and  gradually  increases  in  size  upwards.  Its  degenera-
tion follows  not  only  section  of  the  posterior  column,  but
section  of  the  dorsal  roots  of  the  spinal  nerves:  sections  of
these  roots  in  the  sacral,  lumbar,  thoracic,  and  cervical  nerves
cause  degenerations  in  the  areas  marked  respectively  s,  e,t,  c;
hemisection  of  the  cord  is  followed  above  the  section  by  de-
generations in  this  tract  cori-esponding  to  all  tue  spinal  nerves
which  join  the  cord  below  the  section.  The  posterior  median
tract  is  lost  as  a  distinct  tract  in  the  medulla  oblongata:  its
fibres  are  nearly  all  small.  The  ascending  antero-lateral  tract,
ac.al,  contains  many  fibres  which  undergo  degeneration  after
section  of  the  cord  on  the  same  side,  mixed  with  many  fibres


THE  SPINAL   OOBD  AND  REFLEX  ACTIONS.       599

which  do  not  degenerate.  It  resembles  the  cerebellar  tract
and  diifers  from  the  median  posterior  in  only  undergoing  de-
generation after  section  of  the  cord  itself,  and  not  of  the  pos-
terior roots  also.  The  upward  ending  of  its  fibres  is  still
uncertain,  but  is  probably  in  the  cerebellum :  the  origin  of  the
fibres  is  in  the  gray  matter  of  the  cord.
Allowing  for  all  the  tracts  of  degeneration  above  described
it  will  be  seen  that  considerable  portions  of  the  white  col-
umns of  the  cord  (left  unshaded  in  Fig.  175)  are  un-
affected: at  the  most,  trifling  degenerations  extending  a  little
way  above  or  below  the  point  of  cross-section  are  found.
