682 
ME.  J.  LISTEE  ON  THE  EAELY  STAGES  OE  INELA3OIATI0N. 
of  its  legs  in  that  fluid  for  half  a minute,  and  then  wrapped  both  it  and  the  other  leg 
in  damp  lint.  After  some  hours  the  limb  which  had  not  been  treated  with  chloroform 
was  quite  pale,  while  the  other,  having  lost  the  faculty  of  post  moHerm  concentration, 
remained  as  dark  as  before.  The  appearance  presented  by  the  pigment  in  the  two  feet 
is  shown  in  Plate  XLVII.  figs.  1 and  2. 
Mechanical  violence  proved  similar  in  its  effects  on  the  pigment,  which,  in  the  area 
pinched,  retained  the  same  appearance  as  before,  except  that  in  parts  where  the 
pressure  operated  most  severely  the  cells  seemed  sometimes  to  have  suffered  rupture. 
Fig.  2,  Plate  XL VIII.  is  a camera  lucida  sketch  of  part  of  a spot  which  had  been  com- 
pressed by  means  of  padded  forceps,  with  an  adjoining  uninjured  portion  of  the  web. 
The  pigment  was  fully  diffused  before  the  experiment  was  performed,  and  remained  so 
afterwards  in  the  area  squeezed,  while  it  became  concentrated  elsewhere,  and  this  was 
the  condition  of  things  when  the  drawing  was  made.  The  concomitant  differences  in 
tint  between  the  blood  in  the  affected  and  the  sound  parts  in  consequence  of  the  accumu- 
lation of  closely  packed  red  discs  in  the  former,  are  also  strikingly  shown  in  the  sketch*. 
The  galvanic  shock,  too,  produced  no  effect  apparent  to  the  eye  upon  the  pigment  of 
the  parts  in  which  it  caused  stagnation  of  the  blood,  but  experiments  afterwards  made 
showed  me  that,  like  ammonia,  it  exerted  a paralysing  agency  both  upon  the  concen- 
trating and  the  diffusive  powers ; and  the  same  results  ensued  on  the  application  of  dry 
heat  in  the  cases  mentioned  in  the  last  section. 
From  these  and  other  similar  facts  it  appeared  that  mustard,  croton  oil,  and  cantha- 
rides  are  exceptional  as  regards  the  diffusion  to  which  they  give  rise,  the  usual  com'se 
being  that  irritants,  when  applied  so  as  to  produce  stagnation  of  the  blood,  suspend  at 
the  same  time  both  the  functions  of  the  pigment-cells. 
It  afterwards  turned  out  that  mustard  was,  in  reality,  no  exception  to  this  general 
rule.  Subsequent  experiments  showed  that  diffusion  takes  place  to  very  different  degrees 
in  different  instances  under  the  action  of  this  substance,  but  that  in  all  cases,  after 
reaching  a certain  point,  it  becomes  incapable  of  advancing  fm-ther  in  the  mitated  part, 
however  much  it  may  increase  in  the  body  generally,  in  case  of  the  animal  changmg  to 
a darker  colour.  These  differences  depend  partly  upon  the  strength  of  the  mustaiff. 
the  diffusion  being  least  when  the  irritant  is  most  potent.  Thus,  on  one  occasion, 
when  a solution  of  the  volatile  oil  in  spirit  of  udne  was  applied  to  a web  in  which 
the  pigment  was  fully  concentrated,  congestion  was  very  rapidly  developed,  without 
any  alteration  in  the  appearance  of  the  chromatophorous  cells.  That  the  diffusion  is 
* Much  more  gentle  pressure,  if  long  continued,  may  give  rise  to  similar’  results,  as  I happened  to  notice 
in  the  following  manner.  Being  desirous  of  watching  the  process  of  post  mortem  concentr-ation  of  the 
pigment,  I amputated  a leg  of  a dark  frog,  and,  having  stretched  out  the  foot  over  a glass  plate,  put  a small 
piece  of  thin  glass  upon  part  of  one  of  the  wehs,  and  applied  a high  power  of  the  microscope  to  it.  I was 
disappointed  to  find,  however,  that  the  change  I wished  to  observe  did  not  take  place ; hut  on  lookmg  at 
other  parts  of  the  web,  found  that  immediately  beyond  the  edge  of  the  slip  of  thin  glass,  the  pigment  was 
on  all  sides  considerably  concentrated,  although  remaining  fully  diflused  where  the  glass  covered  it ; an 
effect  which  I could  attribute  only  to  the  gentle  squeezing  to  which  the  two  plates  subjected  the  part  of  the 
web  that  lay  between  them. 
