638 
ME.  J.  LISTEE  OX  THE  CTTAXEOrS 
These  facts  indicated  pretty  clearly  that  the  eyes  are  the  only  channels  through  which 
the  rays  of  light  gain  access  to  the  nervous  system  so  as  to  induce  changes  of  colour 
in  the  skin.  But  for  the  sake  of  confirmation  I thought  it  worth  while  to  perform  the 
following  experiment.  Two  very  dark  frogs  haAoug  been  obtained,  I put  a hood  of 
black  cloth  on  the  head  of  one  of  them,  tearing  the  body  and  limbs  uncovered,  an  aper- 
ture being  made  in  the  cloth  below  the  throat  for  the  purposes  of  respiration,  and  then 
placed  them  both  in  the  same  glass  vessel  exposed  to  white  light.  The  stniggles  of  the 
animal  while  the  covering  was  being  adapted  and  secm’ed,  had  the  effect  of  making  it 
grow  much  paler,  so  that  it  was  of  about  medium  tint  when  introduced  into  the  glass ; 
while  the  other,  which  was  from  the  fivst  the  darker  of  the  two,  still  retained  its  original 
coal-black  appearance.  Half  an  hour  after  this  had  been  done,  the  contrast  between 
them  was  much  diminished,  partly  in  consequence  of  the  dark  one  haring  become 
slightly  paler,  but  much  more  from  the  paler  having  grorvn  darker.  After  another  half 
hour  they  were  of  precisely  the  same  colour,  and  when  another  similar  period  had 
elapsed,  that  which  was  the  darker  to  begin  rvith  was  distinctly  the  paler  of  the  two. 
being  much  lighter  than  at  first,  though  still  considerably  darker  than  medium.  A 
hood  was  now  placed  upon  this  animal,  and  that  upon  the  other  rr  as  removed,  and  both 
rvere  replaced  in  the  same  light  as  before.  This  procedure  occupied  about  ten  minutes, 
and  rvithin  seven  minutes  of  its  completion  the  creature  which  had  the  head  uncovered 
rvas  already  the  paler  of  the  two,  haring  grorvn  decidedly  lighter  in  coloru- ; while  that 
on  which  the  cap  had  been  last  placed  seemed  somewhat  darker ; and  after  another 
hour,  while  the  latter  rvas  still  of  much  the  same  dark  shade,  the  former,  rrith  the  head 
exposed,  was  very  much  paler,  being  about  midway  betrveen  the  medium  and  the  palest 
possible  tint.  An  experiment  of  the  same  kind  rvas  performed  upon  another  pair  of 
frogs  with  very  similar  results,  the  details  of  which  it  is  not  necessary  to  mention.  I 
afterwards  found  that  the  presence  of  the  hood  tends  to  check  dififesion,  or  even  in  some 
cases  to  give  rise  to  concentration  of  the  pigment,  probably  by  making  the  animal 
struggle  to  throw  it  off ; so  that  in  one  instance  a frog  which  was  put  in  a perfectly  dark 
place,  immediately  after  the  cap  had  been  put  on,  grew  much  paler  in  the  course  of  two 
hom's.  This  circumstance  prevents  the  skin  from  becoming  as  dark  on  the  application 
of  the  hood  as  it  would  do  if  the  head  could  be  covered  rvithout  excitmg  the  animal. 
This,  however,  only  renders  the  facts  above  mentioned  more  striking,  so  that  they  afford 
of  themselves  sufficient  proof  that  the  direct  action  of  light  upon  the  integument  is 
incapable  of  affecting  the  pigmentary  functions;  and  thus  the  conclusion  before  arrived* 
at  receives  complete  confirmation  from  these  experiments. 
There  is  of  course  nothing  new  in  the  fact  that  other  functions  besides  rision  may  be 
excited  in  a reflex  manner  through  the  optic  nerves ; the  contraction  of  the  pupil,  and 
the  sneezing  experienced  by  many  persons  on  coming  suddenly  into  bright  sunshine, 
being  well-knorvn  examples  of  such  phenomena.  On  the  other  hand,  the  vierv  that  the 
cutaneous  nerves  are  sensitive  to  luminous  impressions  was  destitute  of  any  support  from 
analogy 
* In  the  chameleon,  a part  exposed  to  the  sun  becomes  dark,  while  the  rest  of  the  body  remains  unaflected. 
