EEGIJLATING  THE  CONTEACTIONS  OP  THE  AETEEIES. 
616 
grade  stream  of  the  blood  in  it  for  about  a second,  and  then  absolute  obliteration  of 
cabbre.  At  O'*  49“  the  artery  allowed  single  corpuscles  to  pass  through  it  with  con- 
siderable difficulty.  At  1 o’clock  the  arteries  of  the  web  were  still  small,  but  I noticed 
that  they  were  undergoing  very  remarkable  oscillations  in  calibre,  just  as  occurred  on 
one  occasion  in  the  frog  last  operated  on,  but  in  the  present  case  they  were  more 
striking.  I noted  the  variations  for  some  time,  and  give  in  the  following  Table  a speci- 
men of  those  which  occurred  during  one  minute : — 
h m s _ „ 
At  1 2 57  the  diameter  of  the  artery  was  1|- 
At  1 3 9 the  diameter  of  the  artery  was  1 
At  1 3 20  the  diameter  of  the  artery  was  ^ 
At  1 3 25  the  diameter  of  the  artery  was  0 
At  1 3 38  the  diameter  of  the  artery  was 
At  1 3 45  the  diameter  of  the  artery  was  0 
At  1 3 50  the  diameter  of  the  artery  was  ^ 
These  oscillations  continued  for  upwards  of  half  an  hour,  but  during  the  latter  part  of 
that  time  the  extreme  degrees  of  constriction  were  not  observed. 
At  I**  43“  p.M.  I raised  the  vertebral  arches  from  the  end  of  the  spinal  canal,  and 
removed  the  posterior  half  of  the  cord  together  with  the  cauda  equina ; immediately 
after  which,  the  artery,  which  for  the  last  hour  had  not  exceeded  became  expanded 
to  2;^°,  a dimension  which  it  had  never  before  been  observed  to  attain,  except  during 
the  secondary  dilatation  that  ensued  after  the  first  division  of  the  cord  when  the  heart 
was  in  powerful  action.  All  the  other  arteries  of  the  web  became  dilated  at  the  same 
time,  and  remained  of  perfectly  constant  diameter  during  the  hour  that  I continued  to 
observe  them.  Finally,  at  2**  40“  I introduced  a needle  into  the  anterior  part  of  the 
spinal  canal  which  had  hitherto  been  undisturbed,  and  irritated  both  the  anterior 
portion  of  the  cord  and  the  brain,  but  no  effect  whatever  was  produced  upon  the 
vessels. 
The  constriction  of  the  arteries,  which  resulted  in  this  case  from  irritation  of  the 
posterior  half  of  the  cord  isolated  from  the  rest,  and  the  permanent  dilatation  which 
ensued  on  remo’val  of  the  same  part,  prove  that  this  portion  of  the  cerebro-spinal  axis  cer- 
tainly contains  a nervous  centre  for  regulating  the  contractions  of  the  arteries  of  the  feet. 
The  frequently  alternating  contractions  and  dilatations  which  occurred  in  this  animal,  as 
well  as  in  the  last,  after  iiTitation  of  the  posterior  half  of  the  cord,  are  curious,  and  may 
perhaps  be  considered  analogous  to  rapid  action  of  the  heart  under  the  influence  of 
stimulus.  The  fact  that  the  arterial  contractions  so  constantly  observed  to  result  from 
irritation  of  the  anterior  part  of  the  cord,  while  it  retains  its  connexion  through  the  rest 
of  the  cord  with  the  roots  of  the  nerves  of  the  hind  legs,  fail  to  occur  after  removal  of 
the  posterior  two-thirds  of  the  cord,  has  been  confirmed  by  subsequent  experiments 
upon  other  frogs.  It  appears  to  imply  that  if  the  brain  and  anterior  part  of  the  cord 
discharge  the  functions  of  a nervous  centre  for  the  arteries  of  the  feet,  they  do  not  exert 
MDCCCLVIII.  4 M 
