ME.  J.  LISTEE  ON  THE  EAELY  STAG-ES  OE  INFLAMMATION. 
695 
The  pigment-cells  in  the  common  frog  give  very  little  indication  of  the  stimulating 
properties  of  irritants,  as  is  evident  from  several  of  the  experiments  which  have  been 
recorded  in  this  section.  In  the  tree  frog,  however,  as  we  are  informed  by  the  German 
authorities,  a part  of  the  integument  subjected  to  such  influences  rapidly  assumes  a pale 
tint,  and  that  even  in  a portion  of  skin  removed  from  the  body.  I have  also  several 
times  noticed,  after  pinching  the  web  of  a common  frog,  that,  although  in  the  spot 
actually  squeezed,  the  pigment-cells  were  deprived  of  their  power  of  changing,  a pale 
ling  about  one-sixteenth  of  an  inch  in  breadth  has  gradually  formed  in  its  immediate 
\iciiiity  in  the  com’se  of  the  next  hour ; whence  it  seems  probable  that  direct  irritation 
tends  to  excite  concentration  in  the  English  species  as  well  as  in  the  continental, 
but  that  in  the  former  the  eflect  is  developed  much  more  slowly,  so  that  it  is  apt  to 
pass  unnoticed.  I further,  on  one  occasion,  saw  post  mortem  concentration  greatly 
accelerated  by  heat*.  It  is  doubtful,  however,  whether  these  results  are  due  to  direct 
action  upon  the  pigment-cells ; for  in  the  tree  frog,  as  well  as  in  the  English  kind,  the 
pale  tint  was  not  confined  to  the  precise  spot  operated  on,  but  affected  a limited  area 
of  surrounding  tissue ; whence  it  seems  likely  that  it  is  developed  through  the  medium 
of  a local  nervous  apparatus  contained  in  the  skinf.  If  this  be  true,  we  have  no  proof 
that  the  pigment-cells  are  capable  of  being  stimulated  except  by  nervous  influence, 
although  they  are,  as  we  have  seen,  peculiarly  susceptible  of  suspension  of  function 
through  the  direct  operation  of  irritants  upon  them. 
’SMth  regard  to  the  nature  of  the  change  experienced  by  the  tissues  when  temporarily 
deprived  of  power  by  irritants,  the  primary  increase  of  motion  of  the  cilia,  lapsing  into 
quiescence,  under  the  operation  of  heat,  may  suggest  the  idea  of  exertion  followed  by 
exhaustion.  But  that  the  state  of  incapacity  is  not  dependent  on  previous  action,  seems 
clear  from  the  fact  that  in  the  pigment-cells  it  is  maintained  and  aggravated  by  an  irri- 
tant continuing  in  operation  after  complete  suspension  of  function  has  been  induced,  the 
same  kind  of  eflect  being  still  produced  upon  the  tissues  which  are  unable  to  act  as  upon 
healthy  parts.  As  an  illustration  of  this,  I may  revert  to  the  results  of  immersion  of  an 
amputated  limb  in  aerated  water.  The  carbonic  acid,  as  we  have  seen,  entirely  pre- 
* This  observation  was  made  as  follows : — On  the  2nd  of  December,  1857,  having  amputated  the  legs  of 
a dark  frog,  I put  them  both  into  water  of  100°  Fahe.,  but  removed  the  left  limb  within  six  minutes  of  its 
immersion  and  placed  it  in  cold  water,  leaving  the  right  for  a quarter  of  an  hour  longer,  at  the  end  of  which 
time  it  was  considerably  paler  than  the  left,  and  the  microscope  showed  that  its  pigment  was  more  concen- 
trated. But  while  the  warm  water  had  accelerated  the  process  of  concentration  up  to  a certain  point,  it  had 
ultimately  paralysed  the  pigment-cells  and  rendered  them  incapable  of  further  change ; so  that  the  right 
limb  remained  permanently  of  the  same  colour  as  when  removed  from  it,  whereas  in  the  left,  which  had  been 
subjected  for  a much  shorter  time  to  the  noxious  agency,  concentration  continued  to  advance,  so  that  in 
twenty-five  minutes  that  leg  was  as  pale  as  the  other,  and  after  an  horn’  more  it  was  a good  deal  the  lighter 
of  the  two.  I may  further  mention  that  rigor  mortis  was  already  carried  to  the  extreme  degree  in  the 
muscles  of  the  right  limb  when  taken  out  of  the  water,  but  in  the  left  this  change  did  not  commence  till 
about  twelve  hours  later.  At  this  period  the  pigment  in  the  left  limb  still  showed  signs  of  retaining  its 
fimctions,  while  that  in  the  right  had  a dirty,  indistinct  appearance,  indicating  that  it  had  lost  its  vitality, 
t See  the  paper  ‘ On  the  Cutaneous  Pigmentary  System,’  before  referred  to,  p.  640, 
MDCCCLVIII.  4 Y 
