These  muscles  are  innervated  from  the  facial  nucleus  in  the  pons,  while  the
muscles  at  the  back  of  the  neck  are  for  the  most  part  innervated  from  the
spinal  cord.  It  seems  far  more  probable  that  such  an  associated  spasm
depends  on  the  cortical  centres  than  that  it  is  due  to  the  lower  centres.  So,
too,  in  the  cases  in  which  there  is  spasm  in  those  muscles  on  both  sides
of  the  neck  that  are  concerned  in  turning  the  head  to  one  side.  On  the
other  hand,  such  an  association  as  that  of  the  muscles  which  incline  the  head
toward  one  shoulder,  or  strong  spasm  limited  to  one  sterno-mastoid  and
the  opposite  deltoid  muscles,  seems  more  intelligible  on  the  assumption  that
the  over-acting  nerve  cells  are  those  of  the  lower  centres ;  since  these
muscles  do  not  commonly  act  together  without  others.  We  have  seen
reason  to  believe  that  the  over-action  that  causes  facial  spasm  is  sometimes
in  the  cortical  centres,  and  sometimes  in  the  lower  centres.  At  the
same  time  we  must  not  lay  too  much  weight  on  the  significance  of  functional
association.  It  is  probable  that  the  connection  of  the  cells  of  the  lower
centres  is  determined  by  this  association,  and  the  manner  in  which  the  spasm
spreads  among  the  muscles  of  both  sides  is  often  more  intelligible  on  the
assumption  that  the  over-acting  cells  are  those  of  lower  centres.  It  may  be
noted,  moreover,  that  there  is  reason  to  believe  that  structures  in  the  lower
part  of  the  pons  Varolii  influence  the  associated  action  of  rotation  of  the
head  (see  p.  611).  There  is  one  important  difference  between  the  spasm  in
torticollis  and  that  which  occurs  in  the  same  muscles  as  a  consequence  of
cortical  disease.  In  the  latter  a  conjugate  deviation  of  the  eyes  is  usually
associated  with  that  of  the  head,  and  this  is  true,  also,  of  the  spasm  that
results  from  irritation  of  the  structures  in  the  pons.  In  torticollis,  however
widely  the  spasm  spreads,  the  muscles  of  the  eyeball  are  never  implicated.
