624 
ME.  J.  LISTEE  ON  THE  PAETS  OE  THE  KEEVOUS  STSTE3I 
sion  of  the  nerves  connecting  the  vessels  with  the  nervons  centre  which  presides  over 
their  contractions.  It  has  been  shown  by  recent  discovery  that  sensation  and  voluntary 
motion  are  abolished  in  parts  whose  nerves  have  been  divided,  until  repair  has  been 
effected  by  a process  of  fresh  formation  of  the  nerve-fibres.  But  the  control  of  the  flow 
of  the  nutrient  fluid  is  not  allowed  to  be  interrupted  in  this  manner,  but  continues  to  be 
exercised  more  or  less  perfectly,  notwithstanding  nearly  absolute  severance  of  nervous 
connexion. 
Allusion  has  been  more  than  once  made  to  the  circumstance  that  arteries  do  not 
dilate  so  fully  when  the  heart  is  very  feeble  as  when  it  is  in  powerful  action.  This  was 
strikingly  illustrated  in  the  case  of  the  frog  which  was  the  subject  of  operation  on 
April  16,  1857.  Immediately  after  the  experiments  recorded  at  page  614,  the  heart 
having  ceased  to  cause  movement  of  blood  in  the  web,  I induced  complete  constriction 
of  the  arteries  by  irritating  with  a needle  the  posterior  part  of  the  cord,  and  then 
thoroughly  cleared  the  spinal  canal  of  its  contents.  The  arteiy  under  special  observa- 
tion did  not,  however,  become  dilated  to  a greater  diameter  than  1^°,  although  dming 
the  earlier  experiments,  when  the  heart  was  acting  vigorously,  it  had  been  observed  to 
attain  sometimes  a calibre  of  3°.  The  heart  never  recovered  its  power,  and  the  vessel 
maintained  this  medium  width  as  long  as  I continued  to  examine  the  animal,  namely, 
three  hours. 
From  this  and  other  similar  observations,  I infer  that  full  dilatation  of  the  arteries  is 
a merely  passive  phenomenon  as  respects  the  parietes  of  the  vessels.  Contraction  is 
effected  by  the  muscular  fibre-ceUs  of  their  circular  coat,  on  the  relaxation  of  which 
the  elasticity  of  the  arteries  tends  to  make  them  expand  to  a certain  degree,  beyond 
which  they  do  not  dilate,  except  in  so  far  as  they  are  distended  by  the  blood. 
It  was  observed  by  Whaeton  Jones*,  that  section  of  the  sciatic  nerve  in  the  thigh  of 
a frog  was  followed  after  a time  by  oedema  of  the  limb  and  exfoliation  of  the  epidermis. 
If  this  were  dependent  on  the  dilatation  of  the  arteries  produced  by  the  division  of  the 
nerve,  the  fact  would  have  a very  important  bearing  upon  the  cause  of  iaflammatoiy 
effusion.  I find,  however,  that  neither  oedema  nor  exfoliation  results  from  permanent 
full  dilatation  produced  by  operations  upon  the  cord  or  the  roots  of  the  spinal  neiwes ; 
while,  on  the  contrary,  both  took  place  in  the  case  of  division  of  the  sciatic,  given  in 
the  early  part  of  this  paper,  in  which  it  will  be  remembered  that  the  arteries  reco- 
vered their  contractility  completely  within  half  an  hour,  and  presented,  during  the  next 
twenty-four  hours,  precisely  similar  appearances  with  those  in  the  other  foot.  Hence 
it  is  evident  that  the  phenomena  in  question  are  not  due  to  vascular  relaxation,  but  to 
some  other  circumstances  attending  the  operation  performed  upon  the  tliigh. 
It  remains  to  be  added,  that,  in  a healthy  state  of  the  web,  no  change  in  the  proper- 
ties of  the  blood  was  ever  observed  to  accompany  the  constriction  of  the  arteries  on 
irritation  of  the  cord,  or  the  dilatation  which  followed  the  destruction  of  the  nervous 
* Medico- Cliir.  Trans,  loc.  cit. 
