614 
ME.  J.  LISTEE  ON  THE  PAETS  OF  THE  NEETOES  SYSTEM 
artery  was  found  quite  constricted  and  the  web  exsanguine.  At  6^  36“  10“  the  artery 
had  somewhat  dilated,  and  measured  but  the  blood  was  moving  very  slowly  through 
the  vessels,  the  heart  being  evidently  exceedingly  enfeebled.  At  6^  40“  the  portion  of 
the  cord  was  detached  from  the  roots  of  the  nerves  which  sprung  from  it  and  removed 
from  the  canal,  immediately  after  which  the  artery  was  found  dilated  to  If*^,  hut  the 
blood  had  ceased  to  move  in  consequence  of  the  feebleness  of  the  heart. 
The  experiments  upon  this  animal  show  that  if  the  brain  and  anterior  third  of  the 
cord  act  at  all  as  nervous  centres  for  the  arteries  of  the  feet,  they  are  certainly  not  the 
only  parts  which  possess  that  function;  and  also,  that  irritation  of  any  part  of  the 
anterior  half  of  the  cord  gives  rise  to  contraction  of  the  arteries  of  the  webs,  followed 
by  dilatation,  varying  much  in  extent  and  duration,  but  generally  proportioned  in  both 
respects  to  the  previous  constriction.  It  is  probable  that  the  dilatation  would  have  been 
greater  after  the  last  operations,  had  the  heart  been  working  more  powerfully ; for  it 
will  hereafter  appear  that  a certain  amount  of  distenduig  force  on  the  part  of  the  blood 
is  necessary  for  the  vessels  becoming  fully  expanded. 
On  the  18th  of  April,  having  put  a large  frog  under  the  influence  of  chloroform,  I 
removed  a vertebral  arch  opposite  the  junction  of  the  middle  and  posterior  thfrds  of  the 
scapulae,  and  then  cut  across  the  cord  in  that  situation,  ^.  e.  rather  more  than  a line 
anterior  to  its  middle ; a slight  retraction  of  the  two  segments  proved  that  the  division 
had  been  thoroughly  effected.  This  was  at  10  o’clock  a.m.  A few  minutes  later  the 
arteries  had  recovered  from  the  effects  of  the  irritation ; one  selected  for  special  obseiwa- 
tion,  having  measured  If'^  just  before  the  operation,  now  varied  occasionally  between 
1-1°  and  If®,  and  the  circulation  was  rapid  through  the  vessel.  The  next  vertebral  arch 
posteriorly  having  been  removed,  the  cord  was  divided  as  far  back  as  it  was  exposed,  at 
10''  23“  50“ ; immediately  after  which  the  web  was  found  exsanguine  in  consequence  of 
complete  closure  of  all  the  arteries,  which  continued  almost  m the  same  condition  for 
ten  minutes,  at  the  end  of  which  time  the  artery  selected  was  still  so  small  as  to  trans- 
mit single  corpuscles  with  difficulty.  At  10''  35“  the  portion  of  cord  included  between 
the  points  of  section  was  detached  from  the  roots  of  the  nerves  connected  with  it  and 
removed  from  the  canal.  It  measured  nearly  a line  in  length,  and  the  posterior  seg- 
ment thus  shortened  proved  afterwards  to  be  only  a very  small  fraction  more  than  half 
the  lengtli  of  the  cord.  The  vessels  afterwards  relaxed  slowly,  so  that  at  10''  37“  the 
corpuscles  were  passing  a little  more  freely  through  the  artery.  At  IT'  15“  the  artery 
measured  1^°,  but  transmitted  the  blood  in  a very  languid  stream ; and  at  noon  the 
evidences  of  circulation  were  so  equivocal,  that  I suspected  the  creatm-e,  which  was  weak 
to  begin  with,  to  be  dead,  though  this  afterwards  proved  to  be  a mistake.  At  0''  45“  p.m. 
the  same  state  of  things  continued,  and  the  artery  still  measured  1^°,  haring  remained 
unaltered  in  calibre  for  the  last  hour  and  a half;  but  I determmed  to  try  the  effect  of 
irritating  the  posterior  segment  of  the  cord,  and  introduced  the  point  of  a needle  a short 
distance  into  its  anterior  extremity  and  withdrew  it  immediately,  keeping  my  eye  o-^  er 
the  microscope.  The  effect  upon  the  artery  was  immediate  constriction,  causing  a retro- 
