610 
ME.  J.  LISTEE  ON  THE  PAETS  OE  THE  NEETOHS  SYSTEM 
spinal  system,  as  distinguished  from  the  sympathetic,  does  influence  the  contractions  of 
the  arteries  of  the  frog’s  foot.  Here,  however,  as  in  the  case  of  the  divided  sciatic 
nerve,  the  effects  were  not  permanent.  Six  hours  later  the  arteries  on  the  left  side 
appeared  smaller  than  they  had  been,  though  still  bearing  marks  of  the  operation  by 
remaining  constant  in  cahbre,  whereas  those  of  the  right  foot  exhibited  very  frequent 
variations,  from  pretty  full  dilatation  to  almost  absolute  closure.  Xext  day  the  same 
state  of  things  continued,  the  vessels  of  the  left  foot  being  constant  in  size  for  four 
minutes  together,  while  in  the  right  foot  an  artery  exhibited  about  eight  distinct  varia- 
tions of  calibre  per  minute  as  observed  by  micrometer ; but  after  three  days  more  they 
had  become  both  small  and  variable  in  the  left  foot,  and  seemed  to  have  quite  recovered. 
On  the  application  of  galvanism  to  the  cord,  however,  both  legs  were  thrown  into 
violent  spasm,  showing  that  communications  still  existed  between  the  left  limb  and  the 
nervous  centre ; and  it  appeared  probable  that  the  branches  which  remained  undivided 
had  come  after  a while  to  supply  more  or  less  perfectly  the  place  of  those  which  had 
been  cut.  A similar  explanation  seemed  applicable  to  the  speedy  recovery  of  con- 
tractility in  the  vessels  after  cutting  the  sciatic,  other  nerv'es  in  the  hmb  supplying  the 
place  of  the  divided  trunk. 
In  another  experiment,  performed  on  the  11th  of  April,  the  roots  of  the  nerves 
on  the  right  side  were  divided  within  the  spinal  canal,  beginning  at  the  head  and  pro- 
ceeding gradually  backwards.  No  enlargement  of  the  vessels  of  the  webs  occuiTed  until 
the  roots  of  the  sciatic  plexus  were  cut,  when  full  dilatation  of  the  arteries  of  the  right 
foot  took  place,  one  which  had  a few  minutes  previously  varied  from  1 to  2 degrees  of 
the  eyepiece  micrometer  being  now  in  diameter,  and  remaining  so  for  ten  mmutes 
together.  Half  an  hour  later,  however,  I was  astonished  to  find  the  artery  again  con- 
tracted to  2°,  and  not  quite  constant  in  calibre.  But  next  day,  on  dissecting  the  animal. 
I found  that  some  branches  of  considerable  size  between  the  cord  and  the  sciatic  plexus 
remained  entire. 
This  experiment,  while  confirming  the  proof  of  the  influence  exerted  by  the  cord  over 
the  arteries  of  the  feet,  convinced  me  how  difficult  it  is  to  make  sure  of  diHcling  all  the 
roots  of  the  nerves  for  the  hind  legs  within  the  spinal  canal ; the  operation  being  a very 
delicate  one,  while  the  parts  are  obscured  by  the  bleeding  which  occurs  in  the  liHng 
animal.  At  the  same  time  the  speedy  recovery  of  function  after  partial  division  of  the 
roots,  pointed  out  a ready  source  of  fallacy  in  such  experiments.  Had  I deferred  the 
examination  of  the  web  for  half  an  hour  in  this  case,  there  would  have  been  no  evidence 
of  any  effect  produced  on  the  vessels  by  the  operation,  and  yet,  had  it  not  been  for 
dissecting  the  frog,  I should  not  have  doubted  that  all  the  roots  had  been  severed. 
Dilatation  of  the  vessels  of  the  webs  having  been  found  to  follow  dhision  of  the  roots 
of  the  spinal  nerves,  it  appeared  important,  in  order  to  complete  the  eHdence  on  the 
])oint  at  issue,  to  observe  the  occurrence  of  contraction  m the  arteries  on  irritation  of 
the  cerebro-spinal  centre.  For  this  purpose,  on  the  14th  I laid  open  the  cranium  of  a 
frog  under  chloroform  and  thrust  a very  fine  needle  into  the  cerebral  hemispheres, 
while  one  of  the  feet  was  stretched  under  the  microscope : no  effect  was,  however,  pro- 
