PIG-MENTAET  SYSTEM  OE  THE  FEOG. 
639 
From  the  part  taken  by  the  second  pair  of  nerves  in  bringing  about  the  changes  in  the 
tint  of  the  skin  under  the  influence  of  light,  and  also  from  the  darkening  of  the  hind 
legs  observed  to  occur  after  dividing  within  the  canal  the  roots  of  the  branches  which 
supply  them*,  we  learn  that  the  cerebro-spinal  axis  is  chiefly,  if  not  exclusively,  concerned 
in  regulating  the  functions  of  the  pigment-cells.  Considering  that  those  functions  have 
probably  a close  afiinity  with  the  processes  of  secretion  and  nutrition,  it  is  interesting  to 
find  that  they  are  thus  subject  to  the  control  of  the  spinal  system. 
The  circumstance  before  alluded  to,  that  a dark  frog  always  becomes  pale  after  death, 
is  mentioned  both  by  von  Wittich  and  Haeless,  but  without  any  discussion  of  its 
cause.  This  'post  mo7'tem  concentration  takes  place  in  a limb  in  spite  of  amputation,  and 
therefore  cannot  be  due  to  the  agency  of  any  ganglia  contained  in  the  head  or  trunk. 
Neither  can  it  be  the  result  of  failure  in  action  on  the  part  of  such  ganglia ; for  if  the 
ch’culation  be  artificially  arrested  m a part  of  a living  frog  without  interfering  with  the 
nerves  leading  to  it,  a similar  change  in  the  pigment  to  that  which  results  from  death 
comes  on  before  the  nerves  have  become,  so  far  as  we  can  judge,  at  all  impaired  in  their 
functions.  This  was  proved  by  the  following  experiment: — On  the  7th  of  June,  1858, 
having  tied  the  right  femoral  artery  of  a moderately  dark  frog  in  the  middle  of  its  course, 
I di\ided  it  below  the  ligature,  and  also  cut  through,  in  the  same  situation,  all  the  soft 
parts  of  the  thigh  except  the  sciatic  nerve  with  a little  adherent  muscle.  The  operation 
was  completed  at  noon,  when  the  animal  was  put  into  a dark  place ; and  at  1^  40“  p.m. 
the  body  generally  was  darker,  but  the  right  leg  from  the  wound  downwards  was  decidedly 
paler  than  before ; the  animal,  however,  still  moved  it  freely.  At  G*"  20“  p.m.  the  general 
surface  was  as  dark  as  ever,  but  the  right  foot  presented  the  extreme  degree  of  pallor ; 
yet  the  creature  still  moved  the  leg  both  spontaneously  and  when  the  toes  were  pinched, 
showing  that  the  motor  and  sensor  nerves  retained  their  functions.  Sensation,  however, 
was  not  so  acute  as  in  the  left  foot ; in  the  latter  a touch  sufficed  to  induce  movements 
in  the  body  generally,  whereas  in  the  former  a pinch  was  necessary  to  produce  the  same 
effect.  At  10**  15“  p.m.  the  same  contrast  in  colour  continued,  but  no  movement  could 
be  induced  in  any  manner  in  the  pale  limb,  although  obscure  indications  of  a certain 
amount  of  sensibility  remaining  in  it  were  still  elicited  by  forcible  pinching. 
In  this  case,  concentration  of  the  pigment  came  on  in  the  limb  in  consequence  of  arrest 
of  the  circulation  through  it,  several  hours  before  its  nerves  concerned  in  sensation  and 
motion  had  lost  their  powers,  and  therefore  at  a time  when  we  cannot  doubt  that  the 
ganglia  in  the  trunk  had  full  opportunity  for  acting  on  the  pigment-cells,  which,  as  we 
know  from  experiments  before  mentioned,  are  capable  of  being  influenced  through  the 
I have  little  doubt,  however,  that  this  is  due  to  the  calorific,  not  the  luminous  rays.  That  heat  does  pro- 
duce such  an  effect,  was  lately  demonstrated  to  me  by  Professor  Goonsin  upon  a living  chameleon,  which, 
when  held  in  broad  daylight  before  a dull-red  fire  for  a short  time,  grew  much  darker  on  the  side  that  was 
warmed,  but  retained  elsewhere  its  former  pale  green  colour. 
* See  p.  635. 
MDCCCLVIII.  4 P 
