684 
ME.  J.  LISTEE  0'S  THE  EAELT  STAGES  OE  ENTLA^OIATION. 
part  where  it  had  caused  a great  degree  of  pigmentary  diffusion,  is  strictly  analogous 
to  the  healthy  state  of  the  circulation  in  the  reticular  ring  round  the  congested  area  in 
the  last  experiment. 
Cantharides  also  presents  a parallel  case.  Its  action  is  even  more  slow  than  that  of 
croton  oil ; and  on  referring  to  notes  taken  at  an  early  period  in  this  investigation.  I find 
that  in  one  instance,  when  two  hours  and  a half  had  elapsed  after  the  apphcation  of  a 
small  drop  of  the  tincture  to  the  web,  though  difiFusion  of  the  pigment  had  become 
apparent  in  the  area  on  which  it  had  acted,  no  change  of  the  blood  had  yet  been 
observed ; and  an  hour  and  a half  later,  the  red  corpuscles,  though  abnormally  adhe- 
sive as  compared  with  those  in  surrounding  parts  of  the  web,  were  still  moving  slowly 
through  the  vessels. 
Hence  it  appears  that  diflfusion  of  the  pigment  may  be  produced  by  either  of  these 
three  substances  without  the  blood  undergoing  any  material  derangement,  and  therefore 
that  its  occurrence  under  their  influence  is  to  a great  extent,  if  not  entirely,  independent 
of  the  inflammatory  process.  On  the  other  hand,  it  has  been  demonstrated,  as  regards 
mustard,  that  when  stagnation  of  the  blood  has  been  developed  through  its  action,  the 
state  of  the  pigment-cells  is  the  same  as  is  induced  by  irritants  generally,  viz.  a com- 
plete suspension  of  functional  activity ; and,  from  analogy,  we  may  be  pretty  sure  that 
this  is  also  true  of  croton  oil  and  cantharides,  although  their  slow  operation  renders  it 
dififtcult  to  obtain  absolute  proof  upon  the  point. 
In  a physiological  point  of  view,  it  is  an  interesting  question,  what  is  the  cause  of  the 
diffusion  of  the  pigment  induced  by  these  three  irritants.  I have  shown  elsewhere* 
that  concentration  is  the  invariable  result  of  the  action  of  the  nerves  upon  the  chroma- 
tophorous  cells,  and  that  diffusion  takes  place  whenever  they  are  liberated  from  nervous 
influence.  Also  in  the  tree-frog  of  the  Continent,  which  is  much  more  liable  to  changes 
in  the  colour  of  the  integument,  in  consequence  of  direct  irritation,  than  our  own  spe- 
cies, the  invariable  experience  of  the  German  observers  was,  that  concentration  followed 
the  application  of  a local  stimulus,  while  secondary  diffusion  sometimes  occuiTed  in  the 
irritated  spot,  depending  apparently  upon  exhaustion  f.  From  these  facts,  diffusion 
ensuing  on  irritation  cannot  well  be  regarded  as  an  increased  action  excited  by  the 
stimulus,  but  rather  as  an  evidence  of  diminished  vigour.  With  croton  oil  and  can- 
tharides, which  have  not  an  irritating  vapour,  the  diflfusion  is  exactly  limited  to  the 
extent  of  the  irritant,  shomng  that  it  is  due  to  a direct  action  on  the  tissues ; and 
the  most  probable  explanation  of  its  occurrence  appears  to  be  that  mustard,  croton 
oil,  and  cantharides  have  the  peculiarity  among  irritants  of  affecting  the  neiwes  of  the 
pigment-cells  in  the  part  they  act  on,  somewhat  more  rapidly  than  the  cells  them- 
selves, and,  paralysing  the  former  while  the  latter  still  retain  their  powers  more  or 
less  intact,  permit  diffusion  to  go  on  unrestrained  by  nervous  influence,  till  the  fiu'ther 
operation  of  the  irritant  completely  suspends  the  pigmentary  functions.  It  may  be  ob- 
jected to  this  view,  that  diffusion  occurs  on  the  application  of  these  substances  to  an 
amputated  limb,  but,  from  evidence  given  elsewhere  ;J;,  it  is  probable  that  the  pigment- 
* See  the  paper  “ On  the  Pigmentary  System,”  page  641.  t See  page  627.  f See  page  640. 
