The  symptoms  may  indicate  a  morbid  process  limited  to  one-half  of  the
cord,  but  this  does  not  materially  modify  the  diagnostic  method.  Almost
any  process  may,  in  rare  cases,  be  thus  limited.  System  degenerations  and
acute  inflammations  are  least  frequently  unilateral,  and  they  never  reach  a
considerable  degree  of  intensity  on  one  side  without  some  affection  of  the
other  side.  On  the  other  hand,  tumors  and  foci  of  chronic  myelitis  are
often  one-sided,  and  still  more  often  affect  one-half  of  the  cord  first  and
then  the  other..
Indication  of  disease  outside  the  cord,  irritation  of  certain,  nerve  roots,
causing  severe  local  pain,  often  precedes  the  symptoms  of  compression  and
is  an  important  aid  to  diagnosis.  It  shows  the  existence  of  a  morbid  pro-
cess outside  the  cord  before  the  cord  is  involved.  But  we  cannot  use  even
this  indication  except  in  dependence  on  the  mode  of  onset.  A  disease,  as
a  growth  outside  the  cord,  may,  as  we  have  seen,  not  only  compress  the  cord,
and  cause  slow  loss  of  power ;  it  may  excite  inflammation  and  cause  rapid
palsy.
