But  the  disease  is  rarely  limited  to  the  lower  segment  of  the  motor  path.
We  have  seen  that  the  pyramidal  tracts  are  commonly  degenerated,  and  it
is  probable  that  the  degeneration  often  extends  through  their  entire  extent,
and  involves  the  motor  cells  of  the  cortex,  as  it  has  been  proved  to  do  in
several  cases.*  Hence  the  upper  segment  is  often  degenerated  as  well  as
the  lower  segment.  In  the  presence  of  that  complete  degeneration  of  the
lower  segment  which  causes  the  atonic  atrophy  of  the  muscles,  the  degenera-
tion of  the  upper  segment  seems  to  cause  no  symptoms.  The  loss  of  power
that  it  would  produce  is  also  caused  by  the  degeneration  of  the  lower  seg-
ment, and  the  latter  abolishes  the  myotatic  irritability,  excess  of  which  is
the  characteristic  indication  of  disease  of  the  upper  segment.  Hence  the
degeneration  of  the  upper  segment  of  the  motor  path  to  the  muscles  that
present  atonic  atrophy  cannot  be  recognized  during  life.
What  is  the  relation  between  the  degeneration  of  the  two  segments,
between  the  lateral  sclerosis  and  the  affection  of  the  ganglion  cells  ?  We
cannot  assume  (as  some  have  been  inclined  to  do)  that  the  affection  of  the
upper  segment  is  secondary  to  that  of  the  lower,  because  a  primary  lesion
of  the  gray  matter,  such  as    occurs  in  cornual  myelitis,   does  not  cause


*  At  the  same  time  we  have  seen  that  the  nutritional  stability  of  the  fibres  is  less  in  the
lower  than  in  the  upper  parts,  and  that  their  isolated  degeneration  may  be  greatest  in  the
lower  parts.     This  may  sometimes  also  be  the  case  in  progressive  muscular  atrophy.
