ME.  J.  LISTEE  ON  THE  EAELT  STAGES  OE  INFLAMMATION. 
657 
generally  accompanied  by  a very  considerable  constriction  of  the  arteries,  and  sometimes 
by  absolute  closure  of  them.  The  contraction  usually  begins  a very  short  time  before 
the  motions  of  the  body,  so  that  the  struggle  can  commonly  be  predicted  by  the  appear- 
ance of  the  vessels ; and  dilatation  occm's  when  the  creature  becomes  quiet.  Hence  the 
changes  of  calibre  are  e\idently  under  the  control  of  the  nerves.  An  account  of  an 
inquiry  into  the  parts  of  the  nervous  system  by  which  this  control  is  exercised,  will  be 
found  at  p.  607  of  this  volume ; and  from  the  experiments  there  recorded,  it  ■will  be 
seen  that  either  extreme  constriction  or  full  dilatation  of  the  arteries  of  the  web  may 
be  induced  at  pleasure,  by  operating  upon  the  spinal  cord.  A very  good  opportunity 
is  thus  afforded  for  studying  the  effects  produced  upon  the  capillary  circulation  by 
changes  of  calibre  in  the  arteries,  "without  employing  any  means  acting  directly  upon  the 
foot.  This  is  a matter  of  very  great  importance,  for  applications  made  to  the  web  for 
the  pm’pose  of  inducmg  alterations  in  the  dimensions  of  the  vessels,  give  rise  at  the 
same  time  to  other  consequences  of  irritation,  which  complicate  such  experiments  in  a 
most  deceptive  manner,  so  as  to  have  misled,  as  I believe,  some  of  the  best  observers 
who  have  devoted  attention  to  this  subject. 
The  following  account  embodies  the  results  of  numerous  observations  in  which 
this  som’ce  of  fallacy  was  carefully  avoided,  the  variations  in  the  calibre  of  the  vessels 
beiag  generally  either  induced  by  operations  on  the  cord,  or  else  such  as  occurred  spon- 
taneously. 
In  a perfectly  healthy  state  of  the  web  with  the  heart  beating  powerfully,  when  the 
arteries  are  of  about  medium  width,  the  current  of  blood  in  them  is  so  rapid  that  the 
mdividual  corpuscles  cannot  be  discerned ; but  in  the  capillaries,  whose  aggregate  calibre 
is  very  much  greater  than  that  of  the  arterial  trunk  which  feeds  them,  the  flow  is  so 
much  slower  that  they  can  be  pretty  clearly  distinguished.  When  the  arteries  are  fully 
dilated,  if  the  heart  continues  to  act  -with  the  same  energy,  the  blood  appears  to  move 
as  rapidly  in  them  as  before,  though  of  course  ]n  much  larger  quantity ; while  in  the 
capillaries  the  flow  is  extremely  accelerated,  so  that  it  becomes  as  impossible  to  see  the 
blood-corpuscles  in  them  as  in  the  arteries.  On  the  other  hand,  when  the  arteries  are 
considerably  constricted,  the  blood  moves  more  slowly  through  the  capillaries  than 
when  the  tubes  of  supply  are  of  medium  size,  and  at  the  same  time  the  narrowed  arteries 
appear  to  filter  the  blood  more  or  less  of  corpuscles,  which  are  found  in  smaller  numbers 
in  proportion  to  the  liquor  sanguinis  in  the  capillaries : and  if  the  constriction  of  the 
arteries  is  sufficiently  great,  the  web  is  rendered  quite  pale  in  consequence  of  the  small 
number  of  corpuscles  in  it,  which  nevertheless  continue  to  move  among  the  tortuous 
capillaries,  producing  in  the  field  of  the  microscope  an  appearance  something  like  that  of 
a few  flies  playing  about  in  a room.  Finally,  if  the  arteries  are  completely  constricted, 
all  appearance  of  flow  in  the  capillaries  vanishes,  and  the  web  has  a wholly  exsanguine 
aspect.  Under  these  circumstances,  even  the  veins,  though  still  of  large  calibre,  may 
contain  little  besides  colourless  liquor  sanguinis,  which  has  continued  to  ooze  through 
the  contracted  arteries  when  the  corpuscles  have  been  completely  arrested ; and  so  incon- 
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