PiaMENTAEY  SYSTEM  OP  THE  FEOG. 
637 
stellate,  and  two  minutes  later  two  contiguous  cells  were  sketched  in  that  state.  About 
a minute  after  this  it  was  evident  that  diffusion  was  taking  place,  and  it  continued  to 
develope  itself  during  the  next  ten  minutes,  at  the  end  of  which  time  the  rays  of  the 
stellate  pigment  had  shot  out  complicated  offsets.  Within  the  following  five  minutes, 
however,  it  was  arrested  by  post  mortem  concentration,  which  gradually  carried  the 
pigment  back  to  the  angular  state.  This  experiment,  therefore,  furnished  confirmation 
of  the  view,  that,  in  the  ordinary  circumstances  of  the  animal,  the  influence  of  the  cen- 
tral organs  of  the  nervous  system  is  required  for  the  maintenance  as  well  as  the  deve- 
lopment of  concentration  of  the  pigment  in  the  hmbs. 
Supposing  this  to  be  established,  it  would  follow  that  the  accommodation  of  the  tint 
of  the  skin  to  that  of  surrounding  objects  is  certainly  not  the  result  of  direct  action  of  the 
rays  of  hght  upon  the  pigment-cells,  but  a reflex  phenomenon ; and  it  was  an  interesting 
question  whether  the  afferent  nerves  concerned  were  the  optic  pair,  or  branches  in  the 
skin  sensitive  to  luminous  impressions.  With  a view  to  determining  this  point,  I com- 
pletely removed  the  eyes  of  a pale  ffog  on  the  13th  of  September,  1858,  at  1 p.m.,  and 
then  placed  it  in  a dark  cupboard.  During  the  first  hour  after  the  operation  it  became 
even  paler  than  before,  no  doubt  in  consequence  of  the  injury  which  had  been  inflicted  *, 
assuming  apparently  the  lightest  possible  shade ; and  this  continued  with  very  httle 
change  till  night,  although  the  animal  was  still  kept  in  the  dark.  Next  morning  it  was 
decidedly  darker,  and  the  tint  was  still  deeper  at  2**  25“  p.m.  The  glass  containing  the 
frog  was  now  placed  in  a bright  light,  and  surrounded  on  all  sides  by  white  objects ; 
but  this  change  produced  no  difference  in  the  colour  of  the  skin,  which  continued  till 
7*“  30“  P.M.  of  a peculiar  dingy  hue.  It  was  then  put  back  into  the  dark  place,  and  at 
llh  4Qm  p g^jqj  exactly  the  same.  On  the  following  day,  at  8 a.m.,  the  animal 
seemed  a httle  paler,  and  was  even  lighter  at  10  a.m.,  though  still  in  the  dark;  so  that 
it  was  evident  that  no  difference  whatever  was  produced  upon  its  colour  by  admission  or 
exclusion  of  light.  But  that  the  nervous  system  generally  was  in  a state  quite  disposed 
for  acting  upon  the  pigment-cells  when  subjected  to  appropriate  irritation,  was  shown 
by  the  following  circumstances.  At  the  hour  last  mentioned,  the  animal,  having  escaped 
from  the  vessel  in  which  it  was  contained,  struggled  violently  during  my  attempts  to 
secure  it,  and  in  the  short  time  thus  occupied  changed  to  almost  the  palest  possible  tint. 
It  was  then  placed  at  once  in  the  bright  light,  as  before,  but,  in  spite  of  this,  was  within 
ten  minutes  already  decidedly  darker,  and,  half  an  horn’  later,  was  almost  coal  black, 
though  still  subject  to  the  full  influence  of  white  light.  Just  after  this  observation  was 
made,  the  frog  again  escaped,  and  having  again  struggled  considerably  before  it  was 
replaced  in  the  glass,  it  was  seen  to  be  mthin  four  minutes  as  pale  as  when  first  observed 
in  the  morning,  but  after  the  lapse  of  another  half  hour  it  was  again  almost  as  dark  as 
ever,  and  continued  so  till  2^  30“  p.m.,  though  all  the  while  exposed  to  the  same  light. 
The  observ'ations  were  continued  for  two  more  days,  during  which  period  the  same 
complete  indifference  to  the  brightness  or  obscurity  of  surrounding  objects  was  still 
e\inced. 
* Probably  from  the  irritation  of  tbe  optic  nerves. 
