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ME.  J.  LISTEE  OX  THE  CETAXEGUS 
sciatic  trunk.  Hence  it  appears  that  ]iOst  mortem  concentration  is  the  result  of  the  cessa- 
tion of  the  flow  of  the  blood  through  the  vessels,  and  that  it  is  a pui’ely  local  phenome- 
non developed  in  some  manner  quite  independent  of  the  central  organs  of  the  nervous 
system. 
The  period  at  which  it  occurs  varies  a good  deal  in  different  cases.  This  seems  to 
depend  partly  upon  whether  the  blood  is  retained  in  the  vessels  or  not.  Thus  in  one 
instance  in  which  a piece  of  web  was  cut  out,  so  as  to  ensure  complete  escape  of  the 
vital  fluid,  the  process  was  already  considerably  advanced  within  nine  minutes ; whereas 
in  the  case  above  related,  in  which  the  blood  was  retained  in  the  limb  by  a ligatui'e, 
concentration  did  not  commence  till  full  a quarter  of  an  hour  after  amputation.  The 
season  of  the  year  also  seems  To  have  a great  effect.  In  a cold  room,  in  the  depth  of 
winter,  I have  known  some  hours  elapse  before  the  pigment  began  to  change  in  an 
amputated  limb : this  is  probably  owing  to  greater  languor  in  all  the  Htal  processes 
during  the  period  in  which  the  creature  naturally  hibernates. 
The  dead  frog,  if  previously  healthy,  assumes  after  a while  a nearly  uniform  pale 
colour,  concentration  being  carried  to  the  extreme  degree  in  all  parts.  It  does  not,  how- 
ever, remain  in  this  condition ; for  when  a variable  time  has  passed,  the  skin  becomes 
again  somewhat  darker,  and  on  microscopic  examination  the  pigment  is  found  prett}' 
uniformly  angular  or  stellate.  Nor  are  these  the  only  changes  to  which  the  pigment 
is  liable  after  death,  as  I first  became  aware  in  April  1858,  when  examining  an  ampu- 
tated limb  with  reference  to  the  post  mortem  contractions  of  the  arteries,  the  blood 
being  retained  in  the  vessels.  In  that  case,  after  complete  concentration  followed  by 
slight  diffusion  had  taken  place,  irregular  changes  began  to  appear ; some  tracts  of  the 
web  under  observation  becoming  affected  with  more  or  less  full  diffusion  of  the  pigment, 
while  in  others  it  became  more  concentrated.  Then  after  the  lapse  of  some  hom-s  its 
state  was  found  reversed,  being  concentrated  in  parts  where  it  had  been  diffused,  and 
dee  versa.  These  curious  variations  continued  till  so  late  as  the  tenth  day  after  ampu- 
tation, becoming  more  frequent  after  the  first  few  days ; so  that  sometimes  a considerable 
alteration  was  observable  within  half  an  hour. 
These  facts  appeared  to  me  of  great  importance,  as  proHng  the  contmuance  of  Hral 
actions  for  a much  longer  time  than  had  been  preHously  supposed  possible  in  a severed 
portion  of  the  body.  They  seem  also  valuable  mth  reference  to  the  influence  of  the 
nervous  system  over  the  pigmentary  functions ; for  the  cffcumstance  that  considerable 
patches  of  the  web  usually  had  the  pigment  in  the  same  condition  tln-oughout  at  one 
time  implies  that  a large  number  of  pigment-cells  were  acting  in  concert,  and  therefore 
probably  under  the  control  of  the  nervous  system,  although,  as  the  leg  had  been  ampu- 
tated, they  were  of  course  freed  from  the  influence  of  the  central  ganglia.  Hence  we  are 
led  to  suspect  the  existence  in  the  limb  of  an  apparatus,  probably  ganglionic  in  struc- 
ture, coordinating  the  actions  of  the  pigment-cells,  just  as  'vve  know  that  the  muscular 
contractions  in  the  mammalian  intestine  are  harmonized  by  a local  mechanism  of  that 
