622 
ME.  J.  LISTEE  OIs  THE  PAETS  OE  THE  KEETOES  SYSTEM 
than  the  tip  of  it ; but  allowing  for  a certain  amount  of  contraction,  it  may  be  reckoned 
as  one-fifth. 
This  case  shows  that  the  extremity  of  the  cord  acts  as  a nervous  centre  for  the  arteries. 
But  the  experiment  of  the  2nd  of  June  proved  that  after  the  removal  of  the  greater  part 
of  the  posterior  half  of  the  cord,  the  vessels  still  remained  under  the  control  of  the 
nervous  system*.  Hence  it  is  clear  that  the  nervous  centre  for  the  arteries  is  not  con- 
fined to  any  limited  region  of  the  cord. 
This  experiment  also  indicates,  in  a very  striking  manner,  how  small  a piece  of  the 
cord  will  suffice  to  regulate  the  calibre  of  the  arteries,  and  how  little  effect  may  be  pro- 
duced, even  in  the  first  instance,  by  the  removal  of  a large  portion  which  also  possesses 
that  function.  For  it  was  shown,  by  the  absence  of  contraction  in  the  vessels  when  the 
anterior  segment  was  irritated,  and  still  more  conclusively  by  the  absence  of  dilatation 
when  the  anterior  segment  and  the  brain  were  removed,  that  the  posterior  segment  was 
the  only  part  capable  of  acting  on  the  arteries  after  the  removal  of  the  middle  segment ; 
or,  in  other  words,  that  this  operation  deprived  the  arteries  of  the  influence  of  the  whole 
cerebro-spinal  axis,  except  the  posterior  fifth  of  the  cord.  Yet,  although  the  heart  was 
acting  powerfully  at  the  time,  the  dilatation  produced  by  this  procedure  was  only  mode- 
rate in  amount,  and  very  transient.  Hence  it  follows  that  the  mere  fact  of  the  speedy 
return  of  the  arteries  to  their  former  state  of  contraction,  after  removal  of  an  anterior 
portion  of  the  cerebro-spinal  axis,  as  seen  in  the  experiment  of  April  16th,  1857t,  is  no 
ground  whatever  for  believing  that  such  a portion  does  not  act  as  a nervous  centi'e  for 
the  arteries.  This  being  clearly  understood,  the  invariable  occurrence  of  contraction, 
when  the  posterior  part  of  the  brain  or  the  anterior  half  of  the  cord  was  mitated,  in  the 
experiments  of  April  14th  and  16th,  1867J,  must  be  regarded  as  strong  presumptive 
evidence,  if  not  absolute  proof,  that  they  as  well  as  the  posterior  half  of  the  cord  pre- 
side over  the  arterial  contractions  in  the  feet,  although,  as  shown  at  page  615,  they 
appear  to  exert  their  influence  only  through  those  roots  of  nerves  which  take  origin 
from  the  posterior  regions  of  the  cord.  On  the  other  hand,  the  cerebral  hemispheres 
seem  to  take  no  part  in  this  function,  so  far  at  least  as  it  is  safe  to  draw  any  inference 
from  the  negative  evidence  derived  from  a single  experiment  performed  upon  them,  \iz. 
that  mentioned  at  page  610. 
The  fact  that  the  removal  of  a large  portion  of  the  cord  is  followed  by  only  temporary 
dilatation  of  the  arteries,  provided  that  a part  remains  which  furnishes  roots  of  nerves 
for  the  posterior  extremities,  is  in  harmony  with  the  transient  effects  which  were  seen  to 
be  produced  upon  the  vessels  by  partial  division  of  the  roots  of  the  nerves  within  the 
spinal  canal  in  the  experiments  of  April  8th  and  11th,  1857  In  both  these  cases  the 
arteries  of  the  webs  appeared  to  recover  their  contractile  power  completely,  although  the 
leg  remained  nearly,  if  not  entirely,  paralysed ; which  seems  to  indicate  that  a few  fibres  of 
the  nerves  for  the  blood-vessels  of  a part  can  supply  the  place  of  the  rest  more  perfectly 
than  is  the  case  with  the  ordinary  nerves  of  sensation  and  motion.  This  peculiaiity  of 
* Vide  page  616.  f Vide  page  613.  X Vide  pp.  611,  613.  § Vide  pp.  609,  610. 
