If  after  this  has  occurred,  the  spinal  cord  is  destroyed  by  pushing  a  wire
down  the  vertebral  canal,  the  arterial  blood  pressure  will  again  fall,
indicating  that  the  vascular  tone  which  had  been  reacquired  after  sec-
tion of  the  pathway  between  the  main  and  the  subsidiary  centers  must
have  been  brought  about  by  the  development  in  the  subsidiary  centers
of  an  independent  power  of  reflex  tonic  action.  This  experiment  there-
fore demonstrates  that  in  the  intact  animal  the  subsidiary  centers  do  not
by  themselves  discharge  tonic  impulses.  In  other  words,  the  subsidiary
centers  ordinarily  serve  •  merely  as  transfer  stations  for  the  tonic  im-
pulses coming  from  the  chief  center,  but  when  these  impulses  no  longer


236  THE    CIRCULATION   OP    THE   BLOOD

arrive,  then  a  hitherto  dormant  power  of  tonic  activity  becomes  devel-
oped in  the  subsidiary  centers.
Independent  Tonicity  of  Blood  Vessels

Even  after  complete  disconnection  of  the  spinal  cord  from  the  blood
vessels,  as  by  cutting  of  the  splanchnic  nerve  to  the  abdomen  or  abla-
tion of  that  portion  of  the  lower  spinal  cord  from  which  the  fibers  to
the  hind  limb  arise,  the  disconnected  blood  vessels,  although  at  first
completely  dilated,  may  later  reacquire  an  independent  tone  of  their
own,  indicating  therefore,  that  they  must  possess  some  neuromuscular
mechanism  which  can  act  independently  of  the  nerve  centers,  and  which
may  be  stimulated  to  activity  by  the  presence  of  hormones  in  the  blood.
The  hormone  was  at  one  time  thought  to  be  cpinephrine  (see  page  745).
