680 
ME.  J.  LISTEE  THE  EAELY  STAGES  OE  rNTTAAnf  ATTOV. 
I thus  became  for  the  first  time  aware  that  the  pigment  is  capable  of  Tariations,  and 
my  attention  having  been  directed  to  the  subject,  I soon  found  that  similar  changes 
occur  spontaneously,  and  give  rise  to  alterations  in  the  colour  of  the  skin,  which  is  paler 
in  proportion  as  the  colouring  matter  is  more  completely  collected  into  round  spots. 
For  some  weeks  I supposed  myself  to  have  been  the  first  discoverer  of  this  curious  fact, 
till  I was  referred  by  Dr.  Shaepey  to  the  recent  labours  of  the  Germans  on  the  subject. 
They,  however,  as  I afterwards  found,  had  taken  an  entirely  erroneous  view  of  the 
phenomenon,  attributing  the  round  form  of  the  masses  of  pigment  to  contraction  of  the 
branching  offsets  of  stellate  cells ; whereas  it  tmmed  out  that  the  chromatophorous  cells 
do  not  alter  in  form,  but  that  the  colourless  fluid  and  dark  molecules  which  constitute 
their  contents  are  capable  of  remarkable  variations  in  relative  distribution,  the  mole- 
cules being  sometimes  all  congregated  in  the  central  parts  of  the  cells,  the  offeets  con- 
taining merely  invisible  fluid,  while  at  other  times  the  colouring  particles  are  diffused 
throughout  their  complicated  and  dehcate  branches ; and  between  these  extremes  any 
intermediate  condition  may  be  assumed.  It  further  appeared  that  concentration  of  pig- 
ment takes  place  in  obedience  to  nervous  influence,  while  diffusion,  though  also  an 
active  vital  process,  tends  to  occur  when  the  pigment-cells  are  liberated  from  the  action 
of  the  nerves.  But  for  further  particulars  on  this  subject,  I beg  to  refer  the  reader  to 
the  immediately  preceding  paper  in  this  volume. 
The  contrast  between  the  pigment  in  the  area  on  which  the  mustard  had  acted  and 
that  of  surrounding  parts  in  the  case  last  alluded  to,  at  once  struck  me  as  probably  the 
result  of  a direct  action  exercised  upon  the  tissues  by  the  irritant.  It  seemed  possible, 
however,  that  it  might  be  a secondary  effect  of  the  state  of  the  blood  in  the  congested 
vessels;  and  in  order  to  ascertain  which  was  the  truth,  I performed,  on  the  1 4th  of 
October,  the  following  experiment : — 
Having  cut  out  a piece  of  the  web  of  a healthy  frog,  I placed  a small  portion  of 
mustard  upon  its  centre  when  all  the  blood  had  escaped  from  it.  After  a while  the 
spots  of  pigment  seen  through  the  thin  margin  of  the  mustard,  presented  a stellate 
form,  while  in  the  rest  of  the  piece  they  were  still  of  a rounded  figure.  Hence  it  was 
clear  that  the  change  in  the  disposition  of  the  pigment  was  the  result  of  the  direct 
action  of  the  mustard  upon  the  tissues  of  the  web. 
A new  field  of  investigation  was  thus  opened  before  me,  promising  to  throw  great 
light  upon  the  nature  of  inflammation. 
To  explain  the  effects  of  irritants  upon  the  pigmentary  tissue  proved,  however,  to  be  a 
matter  of  considerable  difficulty.  Tincture  of  cantharides  and  croton  oil,  which  hap- 
pened to  be  among  the  first  substances  which  I employed  Avith  reference  to  tliis  subject, 
resembled  mustard  in  causing  diffusion  of  the  pigment.  Taking,  in  the  first  instance, 
the  same  view  of  this  change  as  the  German  authorities,  I attributed  it  to  the  relaxation 
of  contractile  cells,  and  regarded  its  occurrence,  in  consequence  of  irritation,  as  an  indi- 
cation of  loss  of  power  in  the  tissues,  a view  which  was  in  harmony  Avith  the  natm’e  of 
the  derangement  of  the  blood  in  a congested  part.  Croton  oil,  curiously  enough, 
acted  very  slowly  on  the  Aveb,  not  producing  any  change  on  either  pigment  or  blood 
